'''Torium''' simvolu Th və nüvə yükü 90 olan kimyəvi elementdir. Torium gümüşü rəngli, havaya çıxarıldızda qara rəngli torium- dioksid əmələ gətirən metaldır. Yüksək ərimə temperaturuna malikdir. Torium elektrolizedici aktinoiddir və +4 oksidləşmə dərəcəsi göstərir. Olduqca reaktivdir və xırdalandıqda havada pirofor xassə göstərir. Torium zəif radioaktivdir, belə ki, onun bütün nuklidləri qeyri stabildir. Ən uzunömürlü nuklidi isə torium- 232- dir (yarımçevrilmə periodu 14,05 milyard ildir və kainatın yaşı haqqında məlumat verir). Alfa parçalanması çox yavaş gedir və termium seriyası qurğuşun- 208 ilə bitir. Torium təbiətdə geniş miqdarda yayılmış iki mühüm radioaktiv elementdən biridir (digəri urandır).{{efn|[[Bismuth]] is very slightly radioactive, but its half-life (1.9{{e|19}} years) is so long that its decay is negligible even over geological timespans.}} Yer qabığında urandan üç- dörd dəfə çox yayıldığı ehtimal olunur və əsasən monazit (monazit nadir torpaq metallarının fosfatlarının izomorf qarışığıdır) qumlarından nadir torpaqların çıxarılması zamanı əldə olunur. Torium, 1829-cu ildə gənc Norveç mineraloqu Morten Esmark tərəfindən aşkar edilmiş və İsveç kimyaçısı Yens Yakob Bertselius tərəfindən roma allahı Torun şərəfinə torium kimi adlandırılmışdır. Ancaq torium ilk dəfə yarım əsrdən sonra, 1885-ci ildə tətbiq olunmağa başladı. xx əsrin ikinci yarısında toriumun tətbiqi tamamilə onun radioaktivalik və digər ziyanlı xassələrinə əsaslandığından onun məişətdə tətbiqi qadağan olundu. Torium TIG qaynaqlama elektrodlarında (volfram ilə 1-2 % nisbətində bir qarışıq) ərinti kimi istifadə olunurdu.<ref>{{cite book|title=Manufacturing Technology|url=https://books.google.com/books?id=fSHZAgAAQBAJ&pg=PA389|year=2013|publisher=Tata McGraw-Hill Education|isbn=978-1-259-06257-5|pages=389–}}</ref> Yüksək optik və elmi qurğularda toriumdan istifadə edilir. Torium və uran radioaktivliyi ilə yanaşı digər xassələrinə görə də kommersiyada tətbiqi olan iki elementdir. Torium qaz mantiyasında və ərinti şəklində işıq mənbəyi kimi istifadə olunur. Torium nüvə reaktorlarında nüvə yanacağı kimi uranı əvəz edə biləcəyinə baxmayaraq hələ nüvə yanacağı uran olan reaktorlardan yalnız bəziləri torium reaktorları ilə əvəz olunmuşdur. == Ümumi xüsusiyyətlər == Torium orata sərtlikli, paramaqnit xassəli, parlaq gümüşü rəngli aktinoid metaldır. Dövri cədvəldə sağ tərəfində aktinium, sol tərəfində protaktinium, və yuxarısında isə lantanid serium yerləşir. Saf torium soyuqda digər metallar kimi səthi mərkəzli kubik quruluşunda olur.<ref name=Wickleder6163 /> Torium yüksək tamperaturda (1360°C-dən yuxarı) həcm mərkəzli kubik, yüksək təzyiqdə isə (≈100 GPa) həcm mərkəzli tetraqonal quruluşda olur. Toriumun xassələri nümunənin saflıq dərəcəsindən olduqca asılıdır. Əsasən ən böyük təmizlik dərəcəsi torium-dioksiddə (ThO₂) olur. Ən təmiz torium nümunələri əsasən toriumun bir hissəsinin onda birini əhatə edir.<Ref name = Wickleder6163 /> Onun sıxlığının eksperimental ölçülməsi aşağıdakı nəticələri verdi:11.5 & nbsp; & nbsp; 11.66 & nbsp; qr/sm³. Kristal qəfəsinin parametrləri əsasında alınan nəticədən (11.7& nbsp qr/sm³) bir qədər aşağı olduğundan proses zamanı metalda yaranan mikroskopik boşluqlar ilə əlaqələndirilir.<ref name=Wickleder6163>Wickleder et al., pp. 61–3.</ref> Bu nəticələr aktinidlərin nümayəndələri olan aktinium (10.1 qr/sm³) və protaktinium (15.4 qr/sm³) arasında aralıq qiymətlərdir.<Ref name = Wickleder6163 /> Thorium's melting point of 1750 °C is above both that of actinium (1227 °C) and that of protactinium (~1560 °C). In the beginning of [[period 7 |period 7]], from [[francium]] to thorium, the melting points of the elements increase (following the trend in the other periods): this is because the number of delocalised electrons that each atom contributes increases from one in francium to four in thorium, and there is a greater attraction between these electrons and the metal ions as their charge increases from one in francium to four in thorium. After thorium, there is a new smooth trend downward in the melting points of the early actinides from thorium to [[plutonium]] where the number of f electrons increases from about 0.4 to about 6, due to the itinerance of the f-orbitals, increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds in the metal resulting in increasingly complex crystal structures and weakened metallic bonding.<ref name="Yu. D. Tretyakov" /><ref name=Johansson/> (The f-electron count for thorium is listed as a non-integer due to a 5f–6d overlap.)<ref name=Johansson>Johansson B., Abuja R., Eriksson O. & Wills J. M. (1995). "Anomalous fcc crystal structure of thorium metal." ''Physical Review Letters.'' '''75'''(2), pp. 280–283 (282), {{doi|10.1103/PhysRevLett.75.280}}</ref> Among the actinides, thorium has the highest melting and boiling points and second-lowest density (second only to actinium). Its boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points, behind only [[osmium]], [[tantalum]], [[tungsten]], and [[rhenium]].<ref name=Wickleder6163 /> Thorium metal has a [[bulk modulus]] (measure of resistance to compression of a material) of 54 [[gigapascal|GPa]], about the same as that of [[tin]] (58.2 GPa). In comparison, that of aluminium is 75.2 GPa; copper 137.8 GPa; and mild steel 160–169 GPa.<ref>{{cite book |last=Gale |first=W. F. |last2=Totemeier |first2=T. C. |title=Smithells Metals Reference Book |year=2003 |url=https://books.google.com/books?id=zweHvqOdcs0C |publisher=Butterworth-Heinemann |isbn=978-0-08-048096-1 |location= |language=en |via=|pp=15-2–15-3}}</ref> Thorium's hardness is similar to that of soft [[steel]], so heated pure thorium can be rolled in sheets and pulled into wire.<ref name="Yu. D. Tretyakov">{{cite book|editor=Yu. D. Tretyakov|title = Non-organic chemistry in three volumes| place =Moscow|publisher = Academy|date = 2007|volume = 3|series = Chemistry of transition elements|isbn = 5-7695-2533-9}}</ref> While thorium is nearly half as dense as [[uranium]] and plutonium, it is harder than either of them.<ref name="Yu. D. Tretyakov" /> Thorium becomes [[superconductor|superconductive]] below 1.4 [[kelvin|K]].<ref name=Wickleder6163 /> Thorium can also form [[alloy]]s with many other metals. Addition of small amounts of thorium improves the mechanical strength of [[magnesium]], and thorium-[[aluminium]] alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. With [[chromium]] and uranium, it forms [[eutectic mixture]]s, and thorium is completely [[miscibility|miscible]] in both solid and liquid [[state of matter|states]] with its lighter [[congener (chemistry)|congener]] cerium.<ref name=Wickleder6163 /> == Isotopes == {{Main article|Isotopes of thorium}} All but two elements up to [[bismuth]] (element 83) have an isotope that is practically stable for all purposes ("classically stable"), with the exceptions being [[technetium]] and [[promethium]] (elements 43 and 61). On the other hand, all the elements from [[polonium]] (element 84) onward are noticeably radioactive. The isotope ²³²Th is one of the three nuclides beyond bismuth (the other two being [[uranium-235|²³⁵U]] and [[uranium-238|²³⁸U]]) that have half-lives measured in billions of years; its half-life is 14.05 billion years, about three times the [[age of the earth]], and even slightly longer than the generally accepted [[age of the universe]] (about 13.8 billion years). As such, ²³²Th still occurs naturally today: four-fifths of the thorium present at Earth's formation has survived to the present.<ref name=NUBASE>{{cite journal |date=2003 |title=The NUBASE evaluation of nuclear and decay properties |url=http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf |journal=[[Nuclear Physics A]] |volume=729 |pages=3–128 |doi=10.1016/j.nuclphysa.2003.11.001 |bibcode=2003NuPhA.729....3A|last1=Audi |first1=G. |last2=Bersillon |first2=O. |last3=Blachot |first3=J. |last4=Wapstra |first4=A. H. }}</ref><ref>{{cite journal|date=2003 |title=Atomic weights of the elements. Review 2000 (IUPAC Technical Report) |url=http://www.iupac.org/publications/pac/75/6/0683/pdf/ |journal=[[Pure and Applied Chemistry]] |volume=75 |issue=6 |pages=683–800 |doi=10.1351/pac200375060683|last1=De Laeter |first1=John R. |last2=Böhlke |first2=John Karl |last3=De Bièvre |first3=P. |last4=Hidaka |first4=H. |last5=Peiser |first5=H. S. |last6=Rosman |first6=K. J. R. |last7=Taylor |first7=P. D. P. }}</ref><ref>{{cite journal |author=Wieser, M. E. |date=2006 |title=Atomic weights of the elements 2005 (IUPAC Technical Report) |url=http://iupac.org/publications/pac/78/11/2051/pdf/ |journal=[[Pure and Applied Chemistry]] |volume=78 |issue=11|doi=10.1351/pac200678112051 |access-date=2017-07-27}}</ref> It is the only isotope of thorium occurring in significant quantities in nature today, and thus thorium is usually considered to be a [[mononuclidic element]].<ref name="NUBASE" /> Its stability is attributed to its closed [[nuclear shell]]: it has one at 142 neutrons.<ref>{{cite web |url=http://www.personal.soton.ac.uk/ab1u06/teaching/phys3002/course/05_shell.pdf |title=Chapter 5: Nuclear Shell Model |last=Belyaev |first=Alexander |date=2014–2015 |website=www.personal.soton.ac.uk |publisher=[[University of Southampton]] |access-date=9 May 2017}}</ref><ref>{{cite book |last=Nagy |first=Sándor |date=2009 |title=Radiochemistry and Nuclear Chemistry |volume=2 |publisher=EOLSS Publications |page=374 |isbn=9781848261273}}</ref><ref>{{cite book |last=Griffin |first=H. C. |date=2010 |title=Handbook of Nuclear Chemistry |publisher=Springer Science & Business Media |page=668 |isbn=9781441907196 |editor1-last=Vértes |editor1-first=Attila |editor2-last=Nagy |editor2-first=Sándor |editor3-last=Klencsár |editor3-first=Zoltán |editor4-last=Lovas |editor4-first=Rezso György |editor5-last=Rösch |editor5-first=Frank |chapter=Chapter 13: Natural Radioactive Decay Chains}}</ref> Thorium has a characteristic terrestrial isotopic composition, with [[standard atomic weight|atomic weight]] 232.0377(4). Thorium is one of only three significantly radioactive elements (the others being protactinium and uranium) that occur in large enough quantities on Earth for this to be possible.{{CIAAW2016}} Thorium nuclei are susceptible to [[alpha decay]] because the strong nuclear force is not strong enough to overcome the electromagnetic repulsion between their protons.<ref name="beiser">{{cite book |title=Concepts of Modern Physics|url=http://phy240.ahepl.org/Concepts_of_Modern_Physics_by_Beiser.pdf|year=2003|publisher=McGraw-Hill|isbn=0-07-244848-2|chapter=Chapter 12|pages=432–434|edition=6|author1=Arthur Beiser}}</ref> The alpha decay of ²³²Th decay initiates the so-called 4''n'' [[decay chain]] which includes isotopes with a [[mass number]] divisible by 4 (hence the name; it is also called the thorium series after its progenitor). This chain of consecutive alpha and [[beta decay]]s begins with the decay of ²³²Th to ²²⁸Ra and terminates at stable ²⁰⁸Pb.<ref name="NUBASE" /> Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of [[thallium]], [[lead]], bismuth, polonium, [[radon]], [[radium]], and actinium.<ref name=NUBASE /> As such, natural thorium samples can be chemically purified to extract its useful daughter nuclides, such as ²¹²Pb, which is used in [[nuclear medicine]] for [[cancer therapy]].<ref>{{cite press release|url=http://us.areva.com/EN/home-2564/areva-inc-areva-med-launches-production-of-lead212-at-new-facility.html |title=AREVA Med launches production of lead-212 at new facility |publisher=[[AREVA]] |date=22 November 2013 |accessdate=1 January 2017}}</ref><ref>{{cite journal | url = http://minerals.usgs.gov/minerals/pubs/commodity/thorium/myb1-2011-thori.pdf|title=Mineral Yearbook 2012| publisher = USGS}}</ref> ²³²Th also very occasionally undergoes [[spontaneous fission]] rather than alpha decay, and has left evidence in doing so in its minerals (as trapped [[xenon]] gas formed as a fission product), but the [[partial half-life]] of this process is very large at over 10²¹ years and hence alpha decay predominates.<ref name="Wickleder535" /><ref>{{cite journal |last=Bonetti |first=R. |last2=Chiesa |first2=C. |first3=A. |last3=Guglielmetti |first4=R. |last4=Matheoud |first5=G. |last5=Poli |first6=V. L. |last6=Mikheev |first7=S. P. |last7=Tretyakova |date=1 May 1995 |title=First observation of spontaneous fission and search for cluster decay of ²³²Th |journal=Phys. Rev. C |volume=51 |issue=5 |pages=2530 |doi=10.1103/PhysRevC.51.2530}}</ref> [[File:Decay Chain Thorium.svg|thumb|300px|right|The 4''n'' [[decay chain]] of ²³²Th, commonly called the "thorium series"]] Thirty [[radioisotope]]s have been characterised, which range in mass number from 209<ref name="Ikezoe">{{cite journal|author=Ikezoe, H. |title=alpha decay of a new isotope of ²⁰⁹Th |date=1996| journal=[[Physical Review C]]| volume=54| issue=4| pages=2043–2046|doi=10.1103/PhysRevC.54.2043 |bibcode = 1996PhRvC..54.2043I |displayauthors=1 |author2=Ikuta, T. |author3=Hamada, S. |author4=Nagame, Y. |author5=Nishinaka, I. |author6=Tsukada, K. |author7=Oura, Y. |author8=Ohtsuki, T. }}</ref> to 238.<ref name="Wickleder535" /> The most stable of them (after ²³²Th) are ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,340 years, ²²⁸Th with a half-life of 1.92 years, ²³⁴Th with a half-life of 24.10 days, and ²²⁷Th with a half-life of 18.68 days. All of these isotopes occur in nature as [[trace radioisotope]]s due to their presence in the decay chains of ²³²Th, ²³⁵U, ²³⁸U, and ²³⁷[[neptunium|Np]]: the last of these is long extinct primordially in nature due to its short half-life (2.14 million years), but is continually produced in minute traces from neutron capture in uranium ores. All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes.<ref name="NUBASE" /> In deep [[seawater]]s the isotope ²³⁰Th becomes significant enough that the [[International Union of Pure and Applied Chemistry]] reclassified thorium as a binuclidic element in 2013, as it can then make up to 0.04% of natural thorium.{{CIAAW2016|name}} The reason for this is that while its parent ²³⁸U is soluble in water, ²³⁰Th is insoluble and thus precipitates to form part of the sediment, and may be observed doing so. Uranium ores with low thorium concentrations can be purified to produce gram-sized thorium samples of which over a quarter is the ²³⁰Th isotope, since ²³⁰Th is one of the daughters of ²³⁸U.<ref name="Wickleder535" /> Thorium also has three known [[nuclear isomer]]s (or metastable states), ^216m1Th, ^216m2Th, and ^229mTh. ^229mTh has the lowest known excitation energy of any isomer,<ref name=Ruchowska>{{cite journal|author=Ruchowska, E. |title=Nuclear structure of ²²⁹Th |journal=Phys. Rev. C|volume= 73|pages=044326|date=2006 |doi=10.1103/PhysRevC.73.044326|issue=4 |bibcode = 2006PhRvC..73d4326R |displayauthors=1 |last2=Płóciennik |last3=Żylicz |last4=Mach |last5=Kvasil |last6=Algora |last7=Amzal |last8=Bäck |last9=Borge |last10=Boutami |last11=Butler |last12=Cederkäll |last13=Cederwall |last14=Fogelberg |last15=Fraile |last16=Fynbo |last17=Hagebø |last18=Hoff |last19=Gausemel |last20=Jungclaus |last21=Kaczarowski |last22=Kerek |last23=Kurcewicz |last24=Lagergren |last25=Nacher |last26=Rubio |last27=Syntfeld |last28=Tengblad |last29=Wasilewski |last30=Weissman }}</ref> measured to be (7.6 ± 0.5) eV. This is so low that when it undergoes [[isomeric transition]], the emitted gamma radiation is in the [[ultraviolet]] range.<ref name=Beck>{{cite journal|author=Beck, B. R. |title=Energy splitting in the ground state doublet in the nucleus ²²⁹Th |journal=[[Physical Review Letters]]|volume= 98|pages=142501|date=6 April 2007 |doi=10.1103/PhysRevLett.98.142501 |pmid=17501268 |bibcode=2007PhRvL..98n2501B|issue=14|displayauthors=1 |last2=Becker |last3=Beiersdorfer |last4=Brown |last5=Moody |last6=Wilhelmy |last7=Porter |last8=Kilbourne |last9=Kelley }}</ref><ref>{{cite journal | journal=[[Nature (journal)|Nature]] | volume=533 | issue=7601 | pages=47–51 | date=5 May 2016 | title = Direct detection of the ²²⁹Th nuclear clock transition | first1=Lars | last1=von der Wense | first2=Benedict | last2=Seiferle | first3=Mustapha | last3=Laatiaoui | first4=Jürgen B. | last4=Neumayr | first5=Hans-Jörg | last5=Maier | first6=Hans-Friedrich | last6=Wirth | first7=Christoph | last7=Mokry | first8=Jörg | last8=Runke | first9=Klaus | last9=Eberhardt | first10=Christoph E. | last10=Düllmann | first11=Norbert G. | last11=Trautmann | first12=Peter G. | last12=Thirolf | doi=10.1038/nature17669 }}</ref><ref>{{Cite press release | url=http://www.med.physik.uni-muenchen.de/aktuelles/nature-229-thorium/index.html | title=Results on ^229mThorium published in "Nature" | publisher=[[Ludwig Maximilian University of Munich]] | date=2016-05-06 }}</ref>{{efn|Gamma rays are distinguished by their origin in the nucleus, not their wavelength; hence there is no lower limit to gamma energy derived from radioactive decay.<ref>{{cite book|last = Feynman|first = Richard|author2=Robert Leighton |author3=Matthew Sands|title = The Feynman Lectures on Physics, Vol.1|publisher = Addison-Wesley|year = 1963|location = US|pages = 2–5|isbn = 0-201-02116-1}}</ref>}} Different isotopes of thorium behave identically chemically, but have slightly differing physical properties: for example, the densities of isotopically pure ²²⁸Th, ²²⁹Th, ²³⁰Th, and ²³²Th in g/cm³ are respectively expected to be 11.5, 11.6, 11.6, and 11.7.<ref name=critical>{{cite web|publisher = Institut de Radioprotection et de Sûreté Nucléaire|title = Evaluation of nuclear criticality safety data and limits for actinides in transport|page = 15|url = http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|format = PDF|accessdate=20 December 2010 }}</ref> The isotope ²²⁹Th is expected to be [[fissionable]] with a bare [[critical mass]] of 2839 kg, although with steel reflectors this value could drop to 994 kg.<ref name=critical />{{efn|name="fissionable"|A ''fissionable'' nuclide is capable of undergoing fission (even with a low probability) after capturing a high-energy neutron. Some of these nuclides can be induced to fission with low-energy thermal neutrons with a high probability; they are referred to as ''fissile''. A ''fertile'' nuclide is one that could be bombarded with neutrons to produce a fissile nuclide. [[Critical mass]] is a mass of a ball of a material which could undergo a sustained [[nuclear chain reaction]].}} While ²³²Th is not fissionable, it is [[fertile material|fertile]] as it can be converted to fissile [[uranium-233|²³³U]] by [[neutron capture]] and subsequent [[beta decay]].<ref name=critical /><ref name=Wickleder523 /> === Radiometric dating === Two radiometric dating methods involve thorium isotopes: [[uranium–thorium dating]], involving the decay of [[uranium-234|²³⁴U]] to ²³⁰Th, and ionium–thorium dating, which measures the ratio of ²³²Th to ²³⁰Th. (The name ''ionium'' for ²³⁰Th is a remnant from the early history of radioactivity, when different isotopes were not recognised to be the same element and were given different names.) These rely on the fact that ²³²Th is a primordial radioisotope, but ²³⁰Th only occurs as an intermediate decay product in the decay chain of ²³⁸U.<ref name=uth /> Uranium–thorium dating is a relatively short-range process because of the short half-lives of ²³⁴U and ²³⁰Th relative to the age of the Earth: it is also accompanied by a sister process involving the alpha decay of ²³⁵U into ²³¹Th, which very quickly becomes the longer-lived ²³¹Pa, and this process is often used to check the results of uranium–thorium dating. Uranium–thorium dating is commonly used to determine the age of [[calcium carbonate]] materials such as [[speleothem]] or [[coral]], because while uranium is rather soluble in water, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor [[sediment]]s, from which their ratios are measured. The scheme has a range of several hundred thousand years.<ref name=uth>[http://www3.nd.edu/~nsl/Lectures/phys178/pdf/chap3_6.pdf 3–6: Uranium Thorium Dating]. ISNAP, [[University of Notre Dame]]</ref><ref>Davis, Owen. [http://www.geo.arizona.edu/Antevs/ecol438/uthdating.html Uranium-Thorium Dating]. Department of Geosciences, [[University of Arizona]]</ref> Ionium–thorium dating is a related process, which exploits the insolubility of thorium (both ²³²Th and ²³⁰Th) and thus its presence in ocean sediments to date these sediments by measuring the ratio of ²³²Th to ²³⁰Th.<ref name="rafferty2010">{{citation|title=Geochronology, Dating, and Precambrian Time: The Beginning of the World As We Know It|date=2010|url=https://books.google.com/books?id=cHvnMJUw0wAC&pg=PA150|postscript=.|last1=Rafferty|first1=John P.|series=The Geologic History of Earth|page=150|publisher=The Rosen Publishing Group|isbn=1-61530-125-9}}</ref><ref name="vertes2010">{{citation|title=Handbook of Nuclear Chemistry|date=2010|url=https://books.google.com/books?id=NQyF6KaUScQC&pg=PA800|postscript=.|last1=Vértes|first1=Attila|volume=5|page=800|edition=2nd|publisher=Springer|editor1-last=Nagy|editor2-last=Klencsár|editor3-last=Lovas|editor4-last=Rösch|editor1-first=Sándor|editor2-first=Zoltán|editor3-first=Rezso György|editor4-first=Frank|isbn=1-4419-0719-X}}</ref> Both of these dating methods assume that the proportion of ²³⁰Th to ²³²Th is a constant during the time period when the sediment layer was formed, that the sediment did not already contain thorium before contributions from the decay of uranium, and that the thorium cannot shift within the sediment layer.<ref name=rafferty2010 /><ref name=vertes2010 /> == Chemistry == {{main article|Compounds of thorium}} [[File:Thorium reactions.svg|thumb|350px|right|Reactions of thorium metal]] A thorium atom has 90 electrons, of which four are [[valence electron]]s. Three [[atomic orbital]]s are theoretically available for the valence electrons to occupy: 5f, 6d, and 7s.<ref name=Wickleder5960>Wickleder et al., pp. 59–60</ref> Despite thorium's position in the [[f-block]] of the periodic table, it has an anomalous [Rn]6d²7s² electron configuration in the ground state, as the 5f and 6d subshells in the early actinides are very close in energy, even more so than the 4f and 5d subshells of the lanthanides: thorium's 6d subshells are lower in energy than its 5f subshells, because its 5f subshells are not well-shielded by the filled 6s and 6p subshells and are destabilised. Such unusual behaviour is due to [[relativistic effects]], which are increasingly stronger near the bottom of the periodic table, specifically the relativistic [[spin–orbit interaction]]. The closeness in energy levels of the 5f, 6d, and 7s energy levels of thorium result in thorium almost always losing all four of its valence electrons and hence occurring in its highest possible oxidation state of +4. This behaviour is quite distinct from that of its lanthanide congener cerium, for whom +4 is the highest possible state but +3 also plays an important role and is more stable. Therefore, thorium is much more similar to the [[transition metal]]s zirconium and hafnium than to its true lanthanide congener cerium in its properties such as ionisation energies and redox potentials, and hence also in its chemistry: this transition-metal-like behaviour is the norm in the first half of the actinide series.<ref name=CottonSA2006/><ref name=NIST>{{cite journal |first1=W. C. |last1=Martin |first2=Lucy |last2=Hagan |first3=Joseph |last3=Reader |first4=Jack |last4=Sugan |date=1974 |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |url=https://www.nist.gov/data/PDFfiles/jpcrd54.pdf |journal=J. Phys. Chem. Ref. Data |volume=3 |issue=3 |pages=771–9 |accessdate=19 October 2013 |doi=10.1063/1.3253147}}</ref> Despite this anomalous electron configuration for gaseous thorium atoms, however, metallic thorium shows significant 5f involvement. This was first realised in 1995, when it was pointed out that a hypothetical metallic state of thorium that had the [Rn]6d²7s² configuration with the 5f orbitals above the [[Fermi level]] should be [[hexagonal close packed]] like the [[group 4 element]]s titanium, zirconium, and hafnium, and not face-centered cubic as it actually is. Indeed, the correct crystal structure can only be obtained when the 5f states are included, proving that thorium, and not protactinium, acts as the first actinide metallurgically with the clear influence of the 5f orbitals.<ref name=Johansson/> The 5f character of thorium is also clear in the rare and highly unstable +3 oxidation state, in which thorium exhibits the electron configuration [Rn]5f¹.<ref name=Greenwood1262/> Tetravalent thorium compounds are usually colourless or yellow, like those of [[silver]] or lead, as the Th⁴⁺ ion has no 5f or 6d electrons.<ref name="Yu. D. Tretyakov" /> Thorium chemistry is therefore largely that of an electropositive metal forming a single [[diamagnetic]] ion with a stable noble-gas configuration, indicating a similarity between thorium and the [[main group element]]s of the s-block.<ref name=King>{{cite book |last=King |first=R. Bruce |date=1995 |title=Inorganic Chemistry of Main Group Elements |publisher=Wiley-VCH |isbn=0-471-18602-3}}</ref>{{efn|Unlike the previous similarity between the actinides and the transition metals, the main-group similarity largely ends at thorium before being resumed in the second half of the actinide series, because of the growing contribution of the 5f orbitals to covalent bonding. Nevertheless, the one other commonly-encountered actinide, uranium, retains some echoes of main-group behaviour. The chemistry of uranium is significantly more complicated than that of thorium, but the two most common oxidation states of uranium are uranium(VI) and uranium(IV); these are two oxidation units apart, with the higher oxidation state corresponding to formal loss of all valence electrons, which is similar to the behaviour of the heavy main-group elements in the [[p-block]].<ref name=King/>}} Thorium and uranium are the most investigated of the radioactive elements because their radioactivity is slight enough to not pose major problems of handling and accessibility and they may be safely handled in a normal laboratory.<ref name=Greenwood1262>Greenwood and Earnshaw, p. 1262</ref> ===Reactivity=== Thorium is a highly [[reactivity (chemistry)|reactive]] and electropositive metal. With a [[standard reduction potential]] of −1.90 V for the Th⁴⁺/Th couple, it is somewhat more electropositive than zirconium or [[aluminium]].{{sfn|Stoll|p=6}} Finely divided thorium metal presents a fire hazard due to its [[pyrophoric]]ity and must therefore be handled carefully.<ref name=Wickleder6163 /> When heated in air, thorium [[swarf|turnings]] ignite and burn brilliantly with a white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may eventually occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.<ref name=Wickleder6163 /> Such samples slowly tarnish in air, becoming grey and finally black at the surface.<ref name=Wickleder6163 /> At [[standard temperature and pressure]], thorium is slowly attacked by water, but does not readily dissolve in most common acids, with the exception of [[hydrochloric acid]], where it dissolves leaving behind a black insoluble residue, ThO(OH,Cl)H.<ref name=Wickleder6163 /><ref name=CRC>{{cite book| author = Hammond, C. R. |title = The Elements, in Handbook of Chemistry and Physics |edition = 81st| publisher =CRC press| isbn = 0-8493-0485-7| date = 2004}}</ref> It dissolves in concentrated [[nitric acid]] containing a small amount of catalytic [[fluoride]] or [[fluorosilicate]] ions;<ref name=Wickleder6163 /><ref name="ekhyde">{{cite book| url=http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|author = Hyde, Earl K.|title =The radiochemistry of thorium| publisher = Subcommittee on Radiochemistry, National Academy of Sciences—National Research Council| date = 1960}}</ref> if these are not present, [[passivation (chemistry)|passivation]] can occur, similarly to uranium and plutonium.<ref name=Wickleder6163 /><ref name=Greenwood1264>Greenwood and Earnshaw, p. 1264</ref> === Inorganic compounds === [[File:CaF2 polyhedra.png|250px|thumb|right|Thorium dioxide has the [[fluorite]] structure. Th⁴⁺: <span style="color:silver; background:silver;">__</span>  /  O²⁻: <span style="color:#9c0; background:#9c0;">__</span>]] Most binary compounds of thorium with nonmetals may simply be prepared by heating the elements together.<ref name=Greenwood1267>Greenwood and Earnshaw, p. 1267</ref> In air, thorium burns to form the simple dioxide, ThO₂: this has the [[fluorite]] structure.<ref name=Yamashita>{{cite journal | title = Thermal expansions of NpO₂ and some other actinide dioxides | journal = J. Nucl. Mat. | volume = 245 | issue = 1 | date = 1997 | pages = 72–78 | author= Yamashita, Toshiyuki | author2= Nitani, Noriko | author3= Tsuji, Toshihide | author4= Inagaki, Hironitsu| doi = 10.1016/S0022-3115(96)00750-7 | postscript = . | bibcode=1997JNuM..245...72Y}}</ref> Thorium dioxide, a refractory material, has the highest [[melting point]] (3390 °C) of all known oxides.<ref name=Emsley>{{cite book | last = Emsley | first = John | title = Nature's Building Blocks | edition = Hardcover, First | publisher = [[Oxford University Press]] | date = 2001 | pages = 441 | isbn = 0-19-850340-7 }}</ref> It is somewhat [[hygroscopic]] and reacts readily with water and many gases,<ref name=Wickleder7077>Wickleder et al., pp. 70–7</ref> but dissolves easily in concentrated nitric acid in the presence of fluoride.<ref name=Greenwood1269>Greenwood and Earnshaw, p. 1269</ref> When heated, it emits intense blue light through [[incandescence]], which becomes white when mixed with its lighter homologue [[cerium dioxide]] (CeO₂, ceria): this is the basis for its previously common application in gas mantles.<ref name=Wickleder7077 /> Several binary thorium [[chalcogen]]ides and oxychalcogenides are also known with [[sulfur]], [[selenium]], and [[tellurium]].<ref name=Wickleder9597>Wickleder et al., pp. 95–97</ref> [[File:Kristallstruktur Uran(IV)-fluorid.png|thumb|right|Crystal structure of thorium tetrafluoride. Th⁴⁺: <span style="color:silver; background:silver;">__</span>  /  F⁻: <span style="color:#9c0; background:#9c0;">__</span>]] All four thorium tetrahalides are known, as are some low-valent bromides and iodides:<ref name=Wickleder7894>Wickleder et al., pp. 78–94</ref> the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.<ref name=Greenwood1271/> Additionally, many related polyhalide ions are also known.<ref name=Wickleder7894 /> Thorium tetrafluoride has a [[monoclinic crystal system|monoclinic]] crystal structure and is isotypic with [[zirconium tetrafluoride]] and [[hafnium tetrafluoride]], where the Th⁴⁺ ions are coordinated with F⁻ ions in somewhat distorted [[square antiprism]]s.<ref name=Wickleder7894 /> The other tetrahalides instead have dodecahedral geometry.<ref name=Greenwood1271>Greenwood and Earnshaw, p. 1271</ref> Lower iodides ThI₃ (black) and ThI₂ (gold) can also be prepared by reducing the tetraiodide with thorium metal: These do not contain Th(III) and Th(II), but instead contain Th⁴⁺ and could be more clearly formulated as [[electride]] compounds.<ref name=Wickleder7894 /> Many polynary halides with the alkali metals, [[barium]], thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.<ref name=Wickleder7894 /> For example, when treated with [[potassium fluoride]] and [[hydrofluoric acid]], Th⁴⁺ forms the complex anion {{chem|ThF|6|2-}}, which precipitates as an insoluble salt, K₂ThF₆.<ref name=ekhyde /> Thorium borides, carbides, silicides, and nitrides are [[refractory material]]s, as are those of uranium and plutonium, and have thus received attention as possible [[nuclear fuel]]s.<ref name="Greenwood1267"/> All four heavier [[pnictogen]]s ([[phosphorus]], [[arsenic]], [[antimony]], and bismuth) also form binary thorium compounds. Thorium germanides are also known.<ref name=Wickleder97101>Wickleder et al., pp. 97–101</ref> Thorium reacts with hydrogen to form the thorium hydrides ThH₂ and Th₄H₁₅, the latter of which is superconducting below the transition temperature of 7.5–8 K; at standard temperature and pressure, it conducts electricity like a metal.<ref name=Wickleder6466>Wickleder et al., pp. 64–6</ref> They are thermally unstable and readily decompose upon exposure to air or moisture.<ref name=Greenwood1267/> ===Coordination compounds === In acidic aqueous solution, thorium occurs as the tetrapositive [[aqua ion]] [Th(H₂O)₉]⁴⁺, which has [[tricapped trigonal prismatic molecular geometry]]:<ref name=Wickleder117134>Wickleder et al., pp. 117–134</ref><ref>{{cite journal |last=Persson |first=Ingmar |date=2010 |title=Hydrated metal ions in aqueous solution: How regular are their structures? |url=http://pac.iupac.org/publications/pac/pdf/2010/pdf/8210x1901.pdf |journal=Pure Appl. Chem. |volume=82 |issue=10 |pages=1901–1917 |doi=10.1351/PAC-CON-09-10-22 |accessdate=23 August 2014 |registration=yes}}</ref> at pH < 3, the solutions of thorium salts are dominated by this cation.<ref name=Wickleder117134 /> The Th⁴⁺ ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95 and 1.14 Å.<ref name=Wickleder117134/> It is quite acidic due to its high charge, slightly stronger than [[sulfurous acid]]: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent than [[iron|Fe³⁺]]), predominantly to [Th₂(OH)₂]⁶⁺ in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide Th(OH)₄ is formed and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down significantly just before the precipitation).<ref name=Greenwood1275>Greenwood and Earnshaw, p. 1275–7</ref> As a [[HSAB theory|hard Lewis acid]], Th⁴⁺ favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis.<ref name=CottonSA2006>{{cite book |last=Cotton |first=Simon |year=2006 |title=Lanthanide and Actinide Chemistry|publisher= John Wiley & Sons Ltd}}</ref> Large coordination numbers are the rule for thorium due to its large size. Thorium nitrate pentahydrate was the first known example of coordination number 11, the oxalate tetrahydrate has coordination number 10, and the borohydride (first prepared in the [[Manhattan Project]]) has coordination number 14.<ref name=Greenwood1275/> The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.<ref name="Yu. D. Tretyakov" /> Many other inorganic thorium compounds with polyatomic anions are known, such as the [[perchlorate]]s, [[sulfate]]s, [[sulfite]]s, nitrates, carbonates, phosphates, [[vanadate]]s, [[molybdate]]s, and [[chromate]]s, and their hydrated forms.<ref name=Wickleder101115>Wickleder et al., pp. 101–115</ref> They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterised, especially regarding their structural properties.<ref name=Wickleder101115 /> For example, thorium nitrate is produced by reacting thorium hydroxide with nitric acid: it is soluble in water and alcohols and is an important intermediate in the purification of thorium and its compounds.<ref name=Wickleder101115 /> Thorium complexes with organic ligands, such as [[oxalate]], [[citrate]], and [[EDTA]], are much stronger and tend to occur naturally in natural thorium-containing waters in concentrations orders of magnitude higher than the inorganic complexes.<ref name=Wickleder117134/> === Organothorium compounds === [[File:Uranocene-3D-balls.png|thumb|120px|Sandwich structure of thorocene]] [[File:Thorium half sandwich.svg|thumb|120px|Piano-stool structure of (''η''⁸-C₈H₈)ThCl₂(THF)₂]] Most of the work on organothorium compounds has focused on the [[cyclopentadienyl]]s and [[cyclooctatetraenide anion|cyclooctatetraenyls]]. Like many of the early and middle actinides (up to [[americium]], and also expected for [[curium]]), thorium forms the yellow cyclooctatetraenide complex Th(C₈H₈)₂, thorocene. It is [[isotypic]] with the better-known analogous uranium compound, [[uranocene]].<ref name=Wickleder116117>Wickleder et al., pp. 116–7</ref> It can be prepared by reacting [[potassium cyclooctatetraenide|K₂C₈H₈]] with thorium tetrachloride in [[tetrahydrofuran]] (THF) at the temperature of [[dry ice]], or by reacting thorium tetrafluoride with MgC₈H₈.<ref name=Wickleder116117 /> It is an unstable compound in air and outright decomposes in water or at 190 °C.<ref name=Wickleder116117 /> Half-sandwich compounds are also known, such as (''η''⁸-C₈H₈)ThCl₂(THF)₂, which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran.<ref name=CottonSA2006/> The simplest of the cyclopentadienyls are Th(C₅H₅)₃ and Th(C₅H₅)₄: many derivatives are known. The former (which has two forms, one purple and one green) is a rare example of thorium in the formal +3 oxidation state;<ref name=Wickleder116117/><ref name=Greenwood1278>Greenwood and Earnshaw, pp. 1278–80</ref> a formal +2 oxidation state even occurs in a derivative.<ref>{{cite journal |first1=Ryan R. |last1=Langeslay |first2=Megan E. |last2=Fieser |first3=Joseph W. |last3=Ziller |first4=Philip |last4=Furche |first5=William J. |last5=Evans |title=Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C₅H₃(SiMe₃)₂]₃Th}¹⁻ anion containing thorium in the formal +2 oxidation state |url=http://pubs.rsc.org/en/Content/ArticleLanding/2015/SC/C4SC03033H#!divAbstract |journal=Chem. Sci. |year=2015 |issue=6 |pages=517–521 |accessdate=16 July 2016 |doi=10.1039/C4SC03033H}}</ref> The chloride derivative [Th(C₅H₅)₃Cl] is prepared by heating thorium tetrachloride with [[Limiting reagent|limiting]] K(C₅H₅) used (other univalent metal cyclopentadienyls can also be used). The [[alkyl]] and [[aryl]] derivatives are prepared from the chloride derivative and have received attention due to the insight they give regarding the nature of the Th–C [[sigma bond]].<ref name=Greenwood1278/> Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH₂C₆H₅), and tetraallylthorium, Th(C₃H₅)₄, are known, but their structures have not yet been determined and they decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion [Th(CH₃)₇]³⁻, heptamethylthorate, which forms the salt [Li(tmeda)]₃[ThMe₇] (tmeda = Me₂NCH₂CH₂NMe₂). Although one methyl group is only attached to the thorium atom (Th–C distance 257.1 pm) and the other six connect the lithium and thorium atoms (Th–C distances 265.5–276.5 pm) they behave equivalently in solution. Tetramethylthorium, Th(CH₃)₄, is not known, but its [[adduct]]s are stabilised by [[phosphine]] ligands.<ref name=CottonSA2006/> == Occurrence == {{Main article|Occurrence of thorium}} ===Formation=== ²³²Th is a primordial nuclide, having existed in its current form for over ten billion years; it was forged in the cores of dying stars through the [[r-process]] and scattered across the galaxy by [[supernova]]e.<ref name="Cameron">{{cite journal | last1 = Cameron |first1 = A. G. W. | year = 1973 | title = Abundance of the Elements in the Solar System | url = http://pubs.giss.nasa.gov/docs/1973/1973_Cameron_1.pdf | journal = Space Science Review | volume = 15 | pages = 121–146 | doi = 10.1007/BF00172440 | bibcode = 1973SSRv...15..121C }}</ref> The letter "r" stands for "rapid neutron capture", and occurs in core-collapse supernovae, where heavy seed nuclei such as [[iron-56|⁵⁶Fe]] rapidly capture neutrons, running up against the [[neutron drip line]], as neutrons are captured much faster than the resulting nuclides can [[beta decay]] back toward stability. Neutron capture is the only way for stars to synthesise elements beyond iron because of the increased [[Coulomb barrier]]s that make interactions between charged particles difficult at high atomic numbers and the fact that fusion beyond ⁵⁶Fe is endothermic.<ref name=nucleosynthesis>{{cite journal |last=Roederer |first=Ian U. |last2=Kratz |first2=Karl-Ludwig |first3=Anna |last3=Frebel |first4=Norbert |last4=Christlieb |first5=Bernd |last5=Pfeiffer |first6=John J. |last6=Cowan |first7=Christopher |last7=Sneden |date=5 June 2009 |title=The End of Nucleosynthesis: Production of Lead and Thorium in the Early Galaxy |url=http://iopscience.iop.org/article/10.1088/0004-637X/698/2/1963/meta;jsessionid=DABAC204E5A01AB7C60E29E1B6D168B8.c1.iopscience.cld.iop.org |journal=The Astrophysical Journal |publisher=The American Astronomical Society |volume=698 |issue=2 |pages=1963–80 |doi=10.1088/0004-637X/698/2/1963 |access-date=18 July 2016}}</ref> Because of the abrupt loss of stability past ²⁰⁹Bi, the r-process is the only process of stellar nucleosynthesis that can create isotopes of thorium and uranium, because all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.<ref name=Cameron/><ref name="B2FH">{{cite journal |author1=E. M. Burbidge |author2=G. R. Burbidge |author3=W. A. Fowler |author4=F. Hoyle | year=1957 | title=Synthesis of the Elements in Stars | journal=[[Reviews of Modern Physics]] | volume=29 | issue=4 | page=547 | bibcode=1957RvMP...29..547B | doi=10.1103/RevModPhys.29.547 | url=https://www.pmf.unizg.hr/_download/repository/burbidge_RMP_29_547_1957.pdf}}</ref><ref>{{cite book|last=Clayton|first=Donald D.|author-link=Donald D. Clayton|title=Principles of Stellar Evolution and Nucleosynthesis|publisher=Mc-Graw-Hill|location=New York |date=1968|pages=577–91|isbn=978-0226109534}}</ref> [[Image:Elements_abundance-bars.svg|thumb|center|1100px|Estimated abundances of the 83 primordial elements in the Solar system. Thorium is one of the rarest elements.]] In the universe, thorium is among the rarest of the primordial elements: it achieves this position not only because it is one of the two elements that can be produced only in the r-process, but also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are uranium, the only other element produced only in the r-process, as well as [[thulium]], [[lutetium]], tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals.<ref name=Cameron/><ref name=nucleosynthesis/>{{efn|An even number of either protons or neutrons generally increases nuclear stability of isotopes, compared to isotopes with odd such numbers. For example, elements with odd atomic numbers have no more than two stable isotopes, while even-numbered elements have multiple stable isotopes, with tin (element 50) having the highest number of isotopes of all elements, ten.<ref name="NUBASE" /> See [[Even and odd atomic nuclei]] for more details.}} Furthermore, neutron capture by nuclides beyond ''A'' = 209 often results in nuclear fission instead of neutron absorption, reducing the fraction of nuclei that cross the gap of instability past bismuth to become actinides such as thorium.<ref name=nucleosynthesis/> In the distant past the abundances of thorium and uranium were still being enriched by the decay of extinct plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of extinct ²³⁶U to ²³²Th and the natural depletion of ²³⁵U, but these sources have long since decayed and no longer contribute.{{sfn|Stoll|2005|p=2}} On Earth, thorium is much more abundant: with an abundance of 8.1 ppm in the Earth's crust, it is one of the most abundant of the heavy elements, almost as abundant as lead (13 ppm) and significantly more abundant than tin (2.1 ppm).<ref name=Greenwood1294>Greenwood and Earnshaw, p. 1294</ref> This is because thorium is likely to form oxide minerals that do not sink into the core; as such, it is classified as a [[Goldschmidt classification|lithophile]]. Furthermore, thorium compounds are also poorly soluble in water. Thus, even though the whole of the Earth contains the same abundances of the elements as the Solar System as a whole, there is significantly more accessible thorium than there are accessible heavy platinum group metals in the crust alone.<ref name="albarede">{{cite book | title = Geochemistry: an introduction | url = https://books.google.com/books?id=doVGzreGq14C&pg=PA17 | pages = | publisher = Cambridge University Press | year = 2003 | isbn = 978-0-521-89148-6 | first =Francis | last = Albarède }}</ref> ===On Earth=== [[File:Evolution of Earth's radiogenic heat-no total.svg|thumb|right|350px|The [[radiogenic heat]] from the decay of ²³²Th (violet) is a major contributor to the [[earth's internal heat budget]]. Of the four major nuclides providing this heat, ²³²Th has grown to be the most abundant as the other ones decayed faster than thorium.<ref name=thoruranium>{{cite journal |last1=Trenn |first1=Thaddeus J. |date=1978 |title=Thoruranium (U-236) as the extinct natural parent of thorium: The premature falsification of an essentially correct theory |journal=Annals of Science |volume=35 |issue=6 |pages=581–97 |doi=10.1080/00033797800200441}}</ref><ref>{{cite journal |last1=Diamond |first1=H. |last2=Friedman |first2=A. M. |last3=Gindler |first3=J. E. |last4=Fields |first4=P. R. |date=1 October 1956 |title=Possible Existence of Cm²⁴⁷ or Its Daughters in Nature |journal=Physical Review |volume=105 |issue=2 |pages=679–80 |doi=10.1103/PhysRev.105.679}}</ref><ref>{{cite journal |last1=Rao |first1=M. N. |last2=Gopalan |first2=K. |date=2 May 1973 |title=Curium-248 in the Early Solar System |journal=Nature |volume=245 |pages=304–7 |doi=10.1038/245304a0}}</ref><ref>{{cite journal |last1=Rosenblatt |first1=David B. |date=13 February 1953 |title=Effects of a Primeval Endowment of U²³⁶ |journal=Physical Review |volume=91 |issue=6 |pages=1474–5 |doi=10.1103/PhysRev.91.1474}}</ref>]] Natural thorium is essentially isotopically pure ²³²Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe.<ref name=Wickleder535/> Its [[radioactive decay]] is the largest single contributor to the [[Earth#Heat|Earth's internal heat]]; the other major contributors are the shorter-lived primordial radionuclides, which are ²³⁸U, ⁴⁰K and ²³⁵U in descending order of their size of contribution. (At the time of the Earth's formation, ⁴⁰K and ²³⁵U contributed much more by virtue of their short half-lives, but by the same token they have also decayed more quickly, leaving the almost constant contribution from ²³²Th and ²³⁸U predominant.)<ref name=NGJuly11>{{Cite journal | last1 = Gando | first1 = A. | last2 = Gando | first2 = Y. | last3 = Ichimura | first3 = K. | last4 = Ikeda | first4 = H. | last5 = Inoue | first5 = K. | last6 = Kibe | first6 = Y. | last7 = Kishimoto | first7 = Y. | last8 = Koga | first8 = M. | last9 = Minekawa | first9 = Y. | last10 = Mitsui | first10 = T. | last11 = Morikawa | first11 = T. | last12 = Nagai | first12 = N. | last13 = Nakajima | first13 = K. | last14 = Nakamura | first14 = K. | last15 = Narita | first15 = K. | last16 = Shimizu | first16 = I. | last17 = Shimizu | first17 = Y. | last18 = Shirai | first18 = J. | last19 = Suekane | first19 = F. | last20 = Suzuki | first20 = A. | last21 = Takahashi | first21 = H. | last22 = Takahashi | first22 = N. | last23 = Takemoto | first23 = Y. | last24 = Tamae | first24 = K. | last25 = Watanabe | first25 = H. | last26 = Xu | first26 = B. D. | last27 = Yabumoto | first28 = H. | first29 = S. | last30 = Enomoto | first30 = S. | last28 = Yoshida | last29 = Yoshida | first27 = H. | title = Partial radiogenic heat model for Earth revealed by geoneutrino measurements | doi = 10.1038/ngeo1205 | journal = Nature Geoscience | volume = 4 | issue = 9 | pages = 647–651 | year = 2011 }}</ref> The other natural thorium isotopes are much shorter-lived; of them, only ²³⁰Th is usually detectable, occurring in [[secular equilibrium]] with its parent ²³⁸U, and making up at most 0.04% of natural thorium.<ref name=Wickleder535/>{{efn|Other isotopes may occur alongside ²³²Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be ²²⁸Th, which occurs in the [[decay chain]] of ²³²Th (the [[thorium series]]): the ratio of ²²⁸Th to ²³²Th would be under 10⁻¹⁰.<ref name=Wickleder535>Wickleder et al., pp. 53–5</ref> If uranium is present, tiny traces of several other isotopes will also be present: ²³¹Th and ²²⁷Th from the decay chain of ²³⁵U (the [[actinium series]]), and slightly larger but still tiny traces of ²³⁴Th and ²³⁰Th from the decay chain of ²³⁸U (the [[uranium series]]).<ref name=Wickleder535 /> ²²⁹Th is also been produced in the decay chain of ²³⁷Np (the [[neptunium series]]): while all primordial ²³⁷Np is [[extinct radionuclide|extinct]], it is still produced today as a result of nuclear reactions in uranium ores.<ref>{{cite journal |last=Peppard |first=D. F. |last2=Mason |first2=G. W. |first3=P. R. |last3=Gray |first4=J. F. |last4=Mech |date=December 1952 |title=Occurrence of the (4''n'' + 1) Series in Nature |journal=J. Am. Chem. Soc. |volume=74 |issue=23 |pages=6081–4 |doi=10.1021/ja01143a074}}</ref> ²²⁹Th is mostly produced as a [[decay product|daughter]] of artificial ²³³U, itself produced from [[neutron irradiation]] of ²³²Th, due to its extreme rarity in nature.<ref name=Wickleder535 />}} On Earth, thorium is not a rare element as was previously thought, having a crustal abundance comparable to that of lead and [[molybdenum]], twice that of arsenic, and thrice that of tin.<ref name=Wickleder556>Wickleder et al., pp. 55–6</ref> Thorium only occurs as a minor constituent of most minerals.<ref name=Wickleder556 /> Soil normally contains about 6 [[parts per million]] (ppm) of thorium.<ref>[http://www.atsdr.cdc.gov/tfacts147.pdf THORIUM] [[Agency for Toxic Substances and Disease Registry]]. July 1999.</ref> In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), [[zirconium]](IV), hafnium(IV), and cerium(IV), but also with scandium, yttrium, and the trivalent lanthanides which have similar [[ionic radius|ionic radii]].<ref name=Wickleder556 /> Because of thorium's radioactivity, minerals containing significant quantities of thorium are often [[metamictization|metamict]], their crystal structure having been partially or totally destroyed by the alpha radiation produced in the radioactive decay of thorium.<ref name=Woodhead>{{cite journal|author=Woodhead, James A.|url=http://www.minsocam.org/ammin/AM76/AM76_74.pdf |title=The metamictization of zircon: Radiation dose-dependent structural characteristics|journal= American Mineralogist|volume =76| pages =74–82|year= 1991}}</ref> An extreme example is [[ekanite]], (Ca,Fe,Pb)₂(Th,U)Si₈O₂₀, which almost never occurs in nonmetamict form due to thorium being an essential part of its chemical composition.<ref name=ekanite>{{cite journal|author=Szymański, J. T.|url=http://rruff.info/doclib/cm/vol20/CM20_65.pdf |title=A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa₂Si₈O₂₀|journal=Canadian Mineralogist|volume =20| pages =65–75|year= 1982}}</ref> [[Monazite]] is the most important commercial source of thorium because it occurs in large deposits worldwide, principally in [[India]], [[South Africa]], [[Brazil]], [[Australia]], and [[Malaysia]]. It contains around 2.5% thorium on average, although some deposits may contain up to 20% thorium.<ref name=Wickleder556/><ref name=Greenwood1255>Greenwood and Earnshaw, p. 1255</ref> Monazite is a chemically unreactive [[phosphate]] mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.<ref name=Wickleder556 /> [[Allanite]] can have 0.1–2% thorium and [[zircon]] up to 0.4% thorium.<ref name=Wickleder556 /> Thorium dioxide occurs as the rare mineral [[thorianite]]. Due to its being isotypic with [[uranium dioxide]], these two common actinide dioxides can form solid-state solutions and the name of the mineral changes according to the ThO₂ content.<ref name=Wickleder556 />{{efn|Thorianite refers to minerals with 75–100 mol% ThO₂; uranothorianite, 25–75 mol% ThO₂; thorian uraninite, 15–25 mol% ThO₂; [[uraninite]], 0–15 mol% ThO₂.<ref name=Wickleder556 />}} Thorite, or [[tetragonal crystal system|tetragonal]] [[thorium silicate]] (ThSiO₄), also has a high thorium content and is the mineral in which thorium was first discovered.<ref name=Wickleder556 /> In thorium silicate minerals, the Th⁴⁺ and {{chem|SiO|4|4-}} ions are often replaced with M³⁺ (M = Sc, Y, Ln) and phosphate ({{chem|PO|4|3-}}) ions respectively.<ref name=Wickleder556 /> Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released in significant quantities. However, the Th⁴⁺ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.<ref>{{cite book| pages=547| title =Nature's building blocks: an A-Z guide to the elements|first =John|last=Emsley| publisher=Oxford University Press| isbn = 9780199605637| date=2011}}</ref> == History == [[File:Mårten Eskil Winge - Tor's Fight with the Giants - Google Art Project.jpg|thumb|''Tor's Fight with the Giants'' (1872) by [[Mårten Eskil Winge]], depicting an artist's perception of [[Thor]], the [[Norse god]] of thunder, raising [[Mjölnir|his hammer]] in a battle against the [[Jötunn|giants]].<ref>{{Cite web|url=https://www.google.com/culturalinstitute/beta/asset/tor-s-fight-with-the-giants/3gGd_ynWqGjGfQ?hl=en|title=Tor's Fight with the Giants - Mårten Eskil Winge - Google Arts & Culture|language=en|access-date=2016-06-26}}</ref>]] === Erroneous report === In 1815, the Swedish chemist [[Jöns Jakob Berzelius]] analysed an unusual sample of [[gadolinite]] from a copper mine in [[Falun]], central Sweden. He noted impregnated traces of a white mineral, which he cautiously assumed to be an earth ([[oxide]] in modern chemical nomenclature) of an unknown element. (By that time, Berzelius had already discovered two elements, cerium and selenium, but he had made a public mistake once, announcing a new element, ''gahnium'', that turned out to be simply [[zinc oxide]].)<ref name=Lost/> Berzelius privately named the supposed tentative element "thorium" in 1817<ref>{{cite book |last=Ryabchikov |first=D. I. |last2=Gol'braikh |first2=E. K. |date=2013 |title=The Analytical Chemistry of Thorium: International Series of Monographs on Analytical Chemistry |publisher=Elsevier |page=1 |isbn=9781483156590}}</ref> and its supposed oxide "thorina" after [[Thor]], the [[Norse god]] of thunder.<ref>{{cite book |last=Thomson |first=Thomas |date=1831 |title=A System of Chemistry of Inorganic Bodies, Volume 1 |publisher=Baldwin & Cradock, London; and William Blackwood, Edinburgh., 1831 |page=475}}</ref> In 1824, after more deposits of the same mineral in Vest-Agder, Norway, were discovered, he retracted his findings, as the mineral in question proved to actually be an [[yttrium]] mineral, primarily composed of [[yttrium phosphate|yttrium orthophosphate]].<ref name=Wickleder523>Wickleder et al., pp. 52–3</ref><ref name=Lost>{{cite book|ref=Fontani|last1=Fontani|first1=Marco|last2=Costa|first2=Mariagrazia|last3=Orna|first3=Virginia|title=The Lost Elements: The Periodic Table's Shadow Side|publisher=Oxford University Press|year=2014|page=73|url=https://books.google.com/books?id=Ck9jBAAAQBAJ|isbn=978-0199383-344}}</ref><ref>{{cite journal |last=Berzelius |first=J. |date=1824 |title=Undersökning af några Mineralier. 1. Phosphorsyrad Ytterjord. |journal=Kongliga Svenska Vetenskaps-Akademiens Handlingar |volume=2 |pages=334–338 |language=sv}}</ref><ref name=Mindat/> As the yttrium in this mineral was initially mistaken as being a new element, the mineral was named kenotime by the French mineralogist [[François Sulpice Beudant]] as a rebuke of Berzelius, from the [[Greek language|Greek]] words κενός (vain) and τιμή (honour). This became "[[xenotime]]" as a misprint from the beginning, blunting the criticism.<ref name=Lost/><ref name=Mindat>[http://www.mindat.org/min-4333.html Xenotime-(Y)]. Mindat database</ref><ref name=Handbook>{{cite book|editor1=Anthony, John W. |editor2=Bideaux, Richard A. |editor3=Bladh, Kenneth W. |editor4=Nichols, Monte C. |title= Handbook of Mineralogy|publisher= Mineralogical Society of America|place= Chantilly, VA, US|url=http://rruff.geo.arizona.edu/doclib/hom/xenotimey.pdf|format=PDF|chapter=Xenotime-(Y)|isbn=0962209732 |volume=IV (Arsenates, Phosphates, Vanadates)|year=2000}}</ref> This misspelt form was later explained as being from ξενός (stranger to) and τιμή (honour), supposedly referencing the small, rare and easily overlooked crystals that xenotime occurs as.<ref>{{cite book |last1=Dana |first1=James Dwight |last2=Brush |first2=George Jarvis |date=1875 |title=A System of Mineralogy: Descriptive Mineralogy, Comprising the Most Recent Discoveries |publisher=J. Wiley |page=529 |url=https://books.google.com/books?id=CJYQAAAAIAAJ}}</ref> === Discovery === In 1828, [[Morten Thrane Esmark]] found a black mineral on [[Løvøya, Telemark|Løvøya]] island, [[Telemark]] county, Norway. He was a Norwegian [[priest]] and amateur [[mineralogist]] who studied the minerals in Telemark, where he served as [[vicar]]. He commonly sent the most interesting specimens, such as this one, to his father, [[Jens Esmark]], a noted mineralogist and professor of mineralogy and geology at the [[University of Oslo]].<ref name=snl>{{cite encyclopedia|year=2007|title=Morten Thrane Esmark|encyclopedia=[[Store norske leksikon]]|editor=Henriksen, Petter|publisher=Kunnskapsforlaget|location=Oslo|url=http://www.snl.no/Morten_Thrane_Esmark|language=Norwegian|accessdate=16 May 2009}}</ref> The elder Esmark determined that it was not any known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element.<ref name=Wickleder523 /> He published his findings in 1829, having isolated an impure sample for the first time by reducing KThF₅ with [[potassium]] metal.<ref name="Weeks" /><ref>{{cite journal|author=Berzelius, J. J. |date=1829|url=http://gallica.bnf.fr/ark:/12148/bpt6k151010.pleinepage.r=Annalen+der+Physic.f395.langFR|title=Untersuchung eines neues Minerals und einer darin erhalten zuvor unbekannten Erde (Investigation of a new mineral and of a previously unknown earth contained therein)|journal=Annalen der Physik und Chemie|volume= 16| pages =385–415|doi=10.1002/andp.18290920702|bibcode=1829AnP....92..385B|issue=7}} (modern citation: ''Annalen der Physik'', vol. 92, no. 7, pp. 385–415)</ref><ref>{{cite journal|author= Berzelius, J. J. |date= 1829|title=Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord" (Investigation of a new mineral (thorite), as contained in a previously unknown earth)|journal=Kungliga Svenska Vetenskaps Akademiens Handlingar (Transactions of the Royal Swedish Science Academy)| pages=1–30}}</ref> Berzelius reused the name of the previous supposed element discovery.<ref name="Weeks">{{cite journal | doi = 10.1021/ed009p1231|bibcode = 1932JChEd...9.1231W | title = The discovery of the elements. XI. Some elements isolated with the aid of potassium and sodium: Zirconium, titanium, cerium, and thorium | date = 1932 | last1 = Weeks | first1 = Mary Elvira |authorlink1=Mary Elvira Weeks| journal = Journal of Chemical Education | volume = 9 | issue = 7 | pages = 1231 }}</ref><ref>{{cite journal | doi = 10.1002/ange.19020153703 | title = Die eigentlichen Thorit-Mineralien (Thorit und Orangit) | date = 1902 | last1 = Schilling | first1 = Johannes | journal = Zeitschrift für Angewandte Chemie | volume = 15 | issue = 37 | pages = 921–929}}</ref> Thus, he named the source mineral [[thorite]], which has the chemical composition (Th,U)[[silicate|SiO₄]].<ref name=Wickleder523 /> Berzelius also made some initial characterisation of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and as such assumed that the atomic mass was 7.5 times that of oxygen (120 [[atomic mass unit|amu]]), while it is actually 15 times as large.{{efn|At the time, the [[rare earth element]]s, among which thorium was found and with which it is closely associated in the nature, were thought to be divalent; this is shown by the fact that the rare earths are there given [[atomic weight]] values two-thirds of their actual ones, and thorium and uranium are given values half of their actual ones.}} He determined that thorium was a very electropositive metal, that he placed ahead of cerium and behind zirconium in electropositivity.<ref name=leach2>{{cite web |url=http://www.meta-synthesis.com/webbook/35_pt/pt_database.php?PT_id=453 |title=The INTERNET Database of Periodic Tables: Berzelius' Electronegativity Table |author=Leach, Mark R. |accessdate=16 July 2016}}</ref> Berzelius did not isolate the element in its metallic state; for the first time, thorium was isolated in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger. They obtained 99% pure thorium metal by reducing thorium chloride with sodium metal.<ref name=Meister/>{{efn|The main difficulty in isolating thorium lies not in its chemical electropositivity, but in the close association of thorium in nature with the rare earth elements and uranium, which collectively are difficult to separate from each other. [[Lars Fredrik Nilson]], the discoverer of [[scandium]], had previously made an attempt to isolate thorium metal in 1882, but was unsuccessful at achieving a high degree of purity.<ref>{{cite journal |last=Nilson |first=L. F. |date=1882 |title=Über metallisches Thorium |journal=Berichte der deutschen chemischen Gesellschaft |volume=15 |issue=2 |pages=2537–47 |doi=10.1002/cber.188201502213 |language=de}}</ref>}} A simpler method leading to even higher purity was discovered in 1927 by American engineers John Marden and Harvey Rentschler, involving the reduction of thorium oxide with calcium when calcium chloride was present.<ref name=Meister>Meister, G.: [http://www.lm.doe.gov/Considered_Sites/F/Foote_Mineral_Co_-_PA_27/PA_27-3.pdf ''Production of Rarer Metals'']. Westinghouse Electric Corporation.</ref> === Initial chemical classification === In the periodic table published by Russian chemist [[Dmitri Mendeleev]] in 1869, thorium and the [[rare earth element]]s were placed outside the main body of the table, at the end of each vertical period after the [[alkaline earth metal]]s. This reflected the belief at that time that thorium and the rare earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which contained the modern [[carbon group]] (group 14), [[group 4 element|titanium group]] (group 4), cerium, and thorium, because their maximum oxidation state was +4.<ref name=leach>{{cite web |url=http://www.meta-synthesis.com/webbook//35_pt/pt_database.php |title=The INTERNET Database of Periodic Tables |author=Leach, Mark R. |accessdate=14 May 2012}}</ref><ref name="Jensen">{{cite journal |last1=Jensen |first1=William B. |date=2003 |title=The Place of Zinc, Cadmium, and Mercury in the Periodic Table |journal=Journal of Chemical Education |volume=80 |issue=8 |pages=952–961 |publisher=[[American Chemical Society]] |doi=10.1021/ed080p952 |bibcode=2003JChEd..80..952J |url=http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/091.%20Zn-Cd-Hg.pdf |accessdate=6 May 2012}}</ref> Cerium was soon removed from the main body of the table and placed in a separate lanthanide series, while thorium remained with group 4 as it had similar properties to its supposed lighter congeners in that group, such as [[titanium]] and zirconium.<ref name=Masterton/>{{efn|Thorium also appears in the 1864 table by British chemist [[John Newlands (chemist)|John Newlands]] as the last and heaviest element, as it was initially thought that uranium was a trivalent element with an atomic weight of around 120: this is half of its actual value, since uranium is predominantly hexavalent. It also appears as the heaviest element in the 1864 table by British chemist [[William Odling]] under titanium, zirconium, and [[tantalum]]. It does not appear in the periodic systems published by French geologist [[Alexandre-Émile Béguyer de Chancourtois]] in 1862, German-American musician [[Gustav Hinrichs]] in 1867, or German chemist [[Julius Lothar Meyer]] in 1870, all of which exclude the rare earths and thorium.<ref name=leach/>}} === First uses === Although thorium was discovered in 1828, it had no applications until 1885, when Austrian chemist [[Carl Auer von Welsbach]] invented the [[gas mantle]], a portable source of light which produces light from the [[incandescence]] of very hot thorium oxide, heated to extremely high temperatures by burning gaseous fuels.<ref name=Wickleder523 /> After that, many applications were found for thorium and its compounds, such as in ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid. [[File:Old thorium dioxide gas mantle - oblong shape.JPG|thumb|[[World War II]] thorium dioxide gas mantle]] === Radioactivity === In the late 19th century onward, the [[atomic theory]] underwent significant improvements, which shaped the further history of thorium. Thorium was first observed to be radioactive in 1898, independently, by the German chemist [[Gerhard Carl Schmidt]] and later that year, the Polish-French physicist [[Marie Curie]]. It was the second element that was found to be radioactive, after the 1896 discovery of radioactivity in uranium by French physicist [[Henri Becquerel]].<ref>{{cite journal|author=Curie, Marie |date=1898|title= Rayons émis par les composés de l'uranium et du thorium (Rays emitted by compounds of uranium and thorium)|journal=Comptes Rendus|volume =126| pages =1101–1103|ol=24166254M |language=French}}</ref><ref>{{cite journal|author=Schmidt, G. C. |date=1898|title=Über die vom Thorium und den Thoriumverbindungen ausgehende Strahlung (On the radiation emitted by thorium and thorium compounds) |journal=Verhandlungen der Physikalischen Gesellschaft zu Berlin (Proceedings of the Physical Society in Berlin)| volume= 17| pages= 14–16 |language=German}}</ref><ref>{{cite journal|author=Schmidt, G. C. |url=http://gallica.bnf.fr/ark:/12148/bpt6k153068.image.r=Annalen+der+Physic.f149.langFR |title=Über die von den Thorverbindungen und einigen anderen Substanzen ausgehende Strahlung (On the radiation emitted by thorium compounds and some other substances) |journal=Annalen der Physik und Chemie |volume= 65| pages =141–151|date= 1898 |language=German}} (modern citation: ''Annalen der Physik'', vol. 301, pages 141–151 (1898)).</ref> Between 1900 and 1903, British physicists [[Ernest Rutherford]] and [[Frederick Soddy]] showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of [[half-life]] as one of the outcomes of the [[alpha particle]] experiments that led to their disintegration theory of [[radioactivity]].<ref>{{cite book|last=Simmons|first=John Galbraith|title=The Scientific 100: A Ranking of the Most Influential Scientists, Past and Present|page=19|date=1996|publisher=Seacaucus NJ: Carol|isbn=0-8065-2139-2}}</ref> The biological effect of radiation was discovered in 1903;<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/themes/physics/curie/ |title=Marie and Pierre Curie and the Discovery of Polonium and Radium |last=Fröman |first=Nanny |date=1 December 1996 |website=nobelprize.org |publisher=Nobel Media AB |access-date=11 May 2017}}</ref> the danger presented by radioactivity to health and environment was the reason thorium was phased out of use in applications that did not explicitly use the radioactivity.<ref name=Wickleder523/> Since the 1930s, it has been widely acknowledged that thorium possesses a minor threat to human organisms in large quantities. === Further classification === Up to late 19th century, chemists unanimously agreed that thorium and uranium were analogous to the 5d elements hafnium and tungsten; the existence of the lanthanides in the sixth row was considered to be a one-off fluke. In 1892, British chemist Henry Bassett postulated a second extra-long periodic table row to accommodate known and undiscovered elements, considering thorium and uranium to be analogous to the lanthanides. In 1922, Danish physicist [[Niels Bohr]] published a [[Bohr model|theoretical model]] of the atom and its electron orbitals, which soon gathered wide acceptance. The model indicated that the seventh row of the periodic table should also have f-shells filling before the d-shells that were filled in the transition elements, like the sixth row with the lanthanides preceding the 5d transition metals.<ref name=leach/> The existence of a second inner transition series, in the form of the actinides, was not accepted until similarities with the electron structures of the lanthanides had been established,<ref>van Spronsen, J. W. (1969). ''The periodic system of chemical elements.'' Amsterdam: Elsevier. p. 315–316, {{ISBN|0-444-40776-6}}.</ref> such that Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.<ref name=leach/> It was only with the discovery of the first [[transuranic element]]s, which from plutonium onward have dominant +3 and +4 oxidation states like the lanthanides, that it was realised that the actinides were indeed filling f-orbitals rather than d-orbitals, with the transition-metal-like chemistry of the early actinides being the exception and not the rule.<ref>{{cite book |last=Rhodes |first=Richard |title=The Making of the Atomic Bomb |edition=25th Anniversary |date=2012 |publisher=Simon & Schuster |location=New York |isbn=1-451-67761-8 |pages=221–2, 349}}</ref> In 1945, when American physicist [[Glenn T. Seaborg]] and his team had discovered the transuranic elements americium and curium, he realised that thorium was the second member of the actinide series and was filling an f-block row, instead of being the heavier congener of [[hafnium]] and filling a fourth d-block row.<ref name=Masterton>{{cite book |last1=Masterton|first1=William L. |last2=Hurley|first2=Cecile N.|last3=Neth|first3=Edward J.|title=Chemistry: Principles and reactions|publisher=Brooks/Cole Cengage Learning|location=Belmont, CA|edition=7th|isbn=1-111-42710-0|page=173}}</ref>{{efn|The filling of the 5f subshell from the beginning of the actinide series was confirmed in 1964 when the next element, [[rutherfordium]], was first synthesised and found to behave like hafnium, as would be expected if the filling of the 5f orbitals had already finished by then.<ref>{{cite journal |doi = 10.1016/S0925-8388(98)00072-3 |title =Evidence for relativistic effects in the chemistry of element 104 |first9 = D. |last10 =Timokhin |first10 = S. N. |last11 =Yakushev |first11 = A. B. |last12 =Zvara |first12 =I. |last9 = Piguet |first8 = V. Ya. |last8 = Lebedev |first7 = D. T. |last7 = Jost |first6 = S. |last6 = Hübener |first5 = M. |last5 = Grantz |first4 = H. W. |last4 = Gäggeler |first3 = B. |last3 = Eichler |first2 = G. V. |date = 1998 |last2 = Buklanov |last1 = Türler| first1 = A. | journal = Journal of Alloys and Compounds |volume = 271–273 |page = 287| display-authors=8}}</ref> Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group-4 element".<ref name=Pershina>{{cite book |last=Kratz |first=Jens Volker |last2=Nagame |first2=Yuichiro |editor1-last=Schädel |editor1-first=Matthias |editor2-last=Shaughnessy |editor2-first=Dawn |chapter=Liquid-Phase Chemistry of Superheavy Elements |date=2014 |edition=2nd |title=The Chemistry of Superheavy Elements |publisher=Springer-Verlag |page=335 |isbn=978-3-642-37465-4 |doi=10.1007/978-3-642-37466-1}}</ref>}} === Phasing out === Despite thorium's radioactivity, the element has remained in use for a long time for applications not exploiting the effect as no suitable alternatives could be found. While a 1981 study by the [[Oak Ridge National Laboratory]] ([[Oak Ridge, Tennessee|Oak Ridge]], [[Tennessee]], United States) estimated that a dose from using a thorium mantle every weekend would be safe for a person,<ref>[https://web.archive.org/web/20131230234200/http://www.survivalunlimited.com/lanternsstoves/aladdinmantleinfo.htm Stoves]. Survival Unlimited</ref><ref name="straightdope">[http://www.straightdope.com/columns/031205.html Are camp lanterns radioactive?]. The Straight Dope (5 December 2003)</ref> <!-- here you may add a note with your figures in sieverts and rems: here's the text I removed: "A study in 1981 estimated that the dose from using a thorium mantle every weekend for a year would be 0.3–0.6 millirems (mrem), tiny in comparison to the normal annual dose of a few hundred millirems: a person actually ingesting a mantle would receive a dose of 200 mrem (2 mSv)" --> this was not the case for the dose received by people manufacturing the mantles (and thus contacting many) as well as soils around some factory sites.<ref>{{cite journal|date=1996|title=HEALTH and HAZARDOUS WASTE|url=http://www.state.nj.us/health/eoh/hhazweb/hhw_no_3.pdf|journal=A Practitioner's Guide to Patients' Environmental Exposures|volume=1|issue=3|pages=1–8|author=New Jersey Department of Health}}</ref> A major shift occurred in the 1990s, when most of these applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements have been found.<ref name=Wickleder523 /><ref name=Furuta>{{cite journal |last=Furuta |first=E. |last2=Yoshizawa |first2=Y. |last3=Aburai |first3=T. |date=December 2000 |title=Comparisons between radioactive and non-radioactive gas lantern mantles |journal=J. Radiol. Prot. |volume=20 |issue=4 |pages=423–31 |pmid=11140713}}</ref> Due to concerns, some manufacturers have switched to other materials, such as yttrium, although these are usually either more expensive or less efficient. Other manufacturers continued to make thorium mantles, but moved their factories to [[developing country|developing countries]].<ref name="straightdope" /> As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even fraudulently claiming them to be non-radioactive while in reality using significant quantities of thorium, up to 259 milligrams per mantle.<ref name="Furuta" /><ref name="Poljanc">{{cite journal|last=Poljanc|first=K.|last2=Steinhauser|first2=G.|last3=Sterba|first3=J. H.|last4=Buchtela|first4=K.|last5=Bichler|first5=M.|date=1 March 2007|title=Beyond low-level activity: on a "non-radioactive" gas mantle|journal=Sci. Total. Environ.|volume=374|issue=1|pages=36–42|doi=10.1016/j.scitotenv.2006.11.024|pmid=17270253}}</ref> === Nuclear power === {{See also|Thorium fuel cycle}} The United States explored the possibility to use ²³²Th as a source for ²³³U, which would be used in a [[nuclear bomb]], in the wake of the [[Cold War]]; they fired a test bomb in 1955.<ref name="World Nuclear Association">{{Cite web|url=http://www.world-nuclear.org/information-library/current-and-future-generation/thorium.aspx|title=Thorium - World Nuclear Association|website=www.world-nuclear.org|access-date=2017-06-21}}</ref> However, they soon recognized that while a ²³³U-fired bomb would be a very potent weapon, it bore few sustainable "technical advantages" over the contemporary method of uranium–plutonium bombs.<ref>{{Cite journal|last=Woods|first=W. K.|date=1966|title=Lrl Interest in U-233|url=https://www.osti.gov/scitech/biblio/79078|journal=|language=English|volume=|pages=|via=}}</ref> In particular, this nuclide is difficult to produce in an isotopically pure state; and the impurities, as well as its decay products, complicate handling it and make it more easily recognizable.<ref name="World Nuclear Association" /> Usage of thorium as a power source has been explored; the earliest thorium-based reactor was made in the United States: the first core at the [[Indian Point Energy Center]] ([[Buchanan, New York|Buchanan]], [[New York (state)|New York]]) in 1962.<ref>{{cite web|url=http://www.americanscientist.org/issues/feature/thorium-fuel-for-nuclear-energy/2|title=Thorium Fuel for Nuclear Energy|date=September 2003|publisher=American Scientist}}</ref> India has one of the largest supplies of thorium in the world but does not have much uranium used elsewhere, and targeted in the 1950s at achieving energy independence for the country with their [[India's three-stage nuclear power programme|three-stage nuclear power programme]].<ref>{{Citation|last=Majumdar|first=S.|title=Experience of thorium fuel development in India|url=http://www.iaea.org/inisnkm/nkm/aws/fnss/abstracts/abst_te_1319_9.html|year=1999|work=BARC|location=Vienna|publisher=IAEA|accessdate=4 March 2012}}</ref><ref name=":0">[http://www.world-nuclear.org/info/inf53.html Nuclear Power in India|Indian Nuclear Energy]. World-nuclear.org. Retrieved on 1 May 2011.</ref> On the other hand, in most countries, the progress stalled because uranium was relatively abundant and the progress of thorium-based reactors was therefore slow (in the 20th century, 3 reactors were opened in India and 12 elsewhere<ref>{{cite web |url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf |format=PDF |publisher=International Atomic Energy Agency |title=IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges |date=May 2005 |accessdate=2009-03-23}}</ref>). Large-scale research was begun in 1996 by the [[International Atomic Energy Agency]] (IAEA) to study the use of thorium reactors; a year later, the [[United States Department of Energy]] started their research on the matter. Nuclear scientist [[Alvin Radkowsky]] of [[Tel Aviv University]] in Israel, the head designer of the American first civilian [[Shippingport Atomic Power Station|nuclear power plant in Shippingport, Pennsylvania]], whose third core bred thorium,<ref name="asme-landmark">{{cite web|url=http://files.asme.org/ASMEORG/Communities/History/Landmarks/5643.pdf|title=Historic Achievement Recognized: Shippingport Atomic Power Station, A National Engineering Historical Landmark|pages=4|format=PDF|accessdate=24 June 2006}}</ref> founded a consortium to develop thorium reactors, which included other laboratories: [[Raytheon]] Nuclear Inc. ([[Cambridge, Massachusetts|Cambridge]], [[Massachusetts]], United States), [[Brookhaven National Laboratory]] ([[Upton, New York|Upton]], New York, United States) and the [[Kurchatov Institute]] ([[Moscow]], Russia).<ref name="Radkowsky">[https://books.google.com/books?id=2wsAAAAAMBAJ&pg=PA19 ''Bulletin of the Atomic Scientists'']. September/October 1997 pp. 19–20</ref> In the 21st century, thorium's potential for improving proliferation resistance and [[nuclear waste|waste]] characteristics led to renewed interest in the thorium fuel cycle.<ref>{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/te_1349_web.pdf|title=IAEA-TECDOC-1349 Potential of thorium-based fuel cycles to constrain plutonium and to reduce the long-lived waste toxicity|date=2002|publisher=International Atomic Energy Agency|accessdate=24 March 2009}}</ref><ref>{{cite news|url=http://www.abc.net.au/news/newsitems/200604/s1616391.htm|title=Scientist urges switch to thorium|last=Evans|first=Brett|date=14 April 2006|publisher=[[ABC News (Australia)|ABC News]]|archiveurl=https://web.archive.org/web/20100328211103/http://www.abc.net.au/news/newsitems/200604/s1616391.htm|archivedate=28 March 2010|accessdate=17 September 2011}}</ref><ref>{{cite news|url=https://www.wired.com/magazine/2009/12/ff_new_nukes/|title=Uranium is So Last Century — Enter Thorium, the New Green Nuke|last=Martin|first=Richard|date=21 December 2009|work=[[Wired (magazine)|Wired]]|accessdate=19 June 2010}}</ref> == Production == <div style="float: right; margin: 2px; font-size:85%; margin-left:18px; margin-bottom:18px> {| class="wikitable sortable collapsible" cellpadding="3" rules="all" style="background:#f9f9f9; border:1px #aaa solid" |+'''Lower-bound estimates of thorium reserves in [[tonne]]s, 2014'''<ref name="World Nuclear Association" /> ! Country !! data-sort-type="number"|Reserves |- | {{flag|India}} || align="right"|846,000 |- | {{flag|Brazil}} || align="right"|632,000 |- | {{flag|Australia}} || align="right"|595,000 |- | {{flag|United States}} || align="right"|595,000 |- | {{flag|Egypt}} || align="right"|380,000 |- | {{flag|Turkey}} || align="right"|374,000 |- | {{flag|Venezuela}} || align="right"|300,000 |- | {{flag|Canada}} || align="right"|172,000 |- | {{flag|Russia}} || align="right"|155,000 |- | {{flag|South Africa}} || align="right"|148,000 |- | {{flag|China}} || align="right"|100,000 |- | {{flag|Norway}} || align="right"|87,000 |- | {{flag|Greenland}} || align="right"|86,000 |- | {{flag|Finland}} || align="right"|60,500 |- | {{flag|Sweden}} || align="right"|50,000 |- | {{flag|Kazakhstan}} || align="right"|50,000 |- | Other countries || align="right" |1,725,000 |- | align="center" | ''World total'' || align="right" |6,355,000 |} </div> Worldwide production of thorium is low, at a few tens of tons per year.{{sfn|Stoll|2005|p=2}} Such low demands make working mines for extraction of thorium alone not profitable; as a result, it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals.{{sfn|Stoll|2005|p=7}} The current reliance on monazite for production is due to thorium being largely produced as a by-product; other sources such as [[thorite]] contain more thorium and could easily be used for production if demand rose.<ref>{{cite web |url=https://minerals.usgs.gov/minerals/pubs/commodity/thorium/mcs-2012-thori.pdf |title=Thorium |author=U.S. Geological Survey |date=2012 |website=www.usgs.gov |publisher=U.S. Geological Survey |access-date=12 May 2017}}</ref> Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.<ref>{{cite web|url=http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |title=An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future |author=Jayaram, K.M.V. |deadurl=yes |archiveurl=https://web.archive.org/web/20110628234922/http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/0412_1.pdf |archivedate=28 June 2011 }}</ref> The common production route of thorium constitutes concentration of thorium minerals; extraction of thorium from the concentrate; purification of thorium; and (optionally) conversion to compounds, such as thorium dioxide.<ref name=Stoll8>Stoll, p. 8</ref> <!--the requirement of extra purity for nuclear applications to be added in the purification part--> === Concentration === There are two categories of thorium minerals for thorium extraction: primary and secondary. Primary deposits occur in acidic granitic magmas and pegmatites. They are concentrated, but of a small size. Secondary deposits occur at the mouths of rivers in granitic mountain regions. In these deposits, thorium is enriched along with other heavy minerals.{{sfn|Stoll|p=6}} Initial concentration varies with the type of deposit.{{sfn|Stoll|p=8}} For the primary deposits, the source pegmatites, which are usually obtained by mining, are divided into small parts and then undergo [[Froth flotation|flotation]]. [[Alkaline earth metal]] carbonates may be removed after dissolution with by reaction with [[hydrogen chloride]]; then follow [[thickening]], filtration, and calcination. The result is concentrate of a thorium–rare earth content of up to 90%.{{sfn|Stoll|p=8}} Secondary materials (such as coastal sands) undergo gravity separation. Then follows magnetic separation with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.{{sfn|Stoll|p=8}} Industrial production in the twentieth century relied on treatment with hot, concentrated sulfuric acid in cast iron vessels, followed by selective precipitation by dilution with water, as on the subsequent steps. However, this method heavily relied on the techique specifics and the concentrate grain size; while many alternatives have been proposed, only one has proven effective economically: alkaline digestion with hot sodium hydroxide solution. This is more expensive than the original method but yields higher purity of thorium; in particular, it removes [[Phosphate|phosphates]] from the concentrate.{{sfn|Stoll|p=8}} ==== Acid digestion ==== Acid digestion is a two-stage process, involving the use of up to 93% [[sulfuric acid]] at 210–230 °C. First, 60% sulfuric acid is added, thickening the reaction mixture as products are formed. Then, fuming sulfuric acid is added and the mixture is kept at the same temperature for another five hours to reduce the volume of solution remaining after dilution. The concentration of the sulfuric acid is selected based on reaction rate and viscosity, which both increase with concentration, albeit with viscosity retarding the reaction. Increasing the temperature also speeds up the reaction, but temperatures of 300 °C and above must be avoided, because they cause insoluble thorium pyrophosphate to form. Since dissolution is very exothermic, the monazite sand cannot be added to the acid too quickly. Conversely, at temperatures below 200 °C the reaction does not go fast enough for the process to be practical. To ensure that no precipitates form to block the reactive monazite surface, the mass of acid used must be twice that of the sand, instead of the 60% that would be expected from stoichiometry. The mixture is then cooled to 70 °C and diluted with ten times its volume of cold water, so that any remaining monazite sinks to the bottom as it is so dense, while the rare earths and thorium remain in solution. Thorium may then be separated by precipitating it as the phosphate at pH 1.3, since the rare earths do not precipitate until pH 2.{{sfn|Stoll|p=8}} ==== Alkaline digestion ==== Alkaline digestion is carried out in 30–45% [[sodium hydroxide]] solution at about 140 °C for about three hours. Too high a temperature leads to the formation of poorly soluble thorium oxide and an excess of uranium in the filtrate, while too low a concentration of alkali leads to a very slow reaction. These reaction conditions are rather mild and consequently require finely grained monazite sand, with particle size under 45 μm. Following filtration, the filter cake includes thorium and the rare earths as their hydroxides, uranium as [[sodium diuranate]], and phosphate as [[trisodium phosphate]]. This crystallises trisodium phosphate decahydrate when cooled below 60 °C; uranium impurities in this product increase with the amount of [[silicon dioxide]] in the reaction mixture, necessitating recrystallisation before commercial use. The hydroxides are then dissolved at 80 °C in 37% [[hydrochloric acid]]. Filtration of the remaining precipitates followed by addition of 47% sodium hydroxide results in the precipitation of thorium and uranium at about pH 5.8. Complete drying of the precipitate must be avoided, as then air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can then liberate free [[chlorine]] from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are then neutralised by the original sodium hydroxide solution, although most of the phosphate must be removed beforehand to avoid precipitating rare earth phosphates. [[Solvent extraction]] may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in [[nitric acid]]. The presence of [[titanium hydroxide]] is deleterious as it binds thorium and prevents it from dissolving fully.{{sfn|Stoll|p=8}} === Purification === High thorium concentrations are needed in nuclear applications; particularly, concentrations of atoms with high [[neutron capture]] [[cross-section (physics)|cross-sections]] must be very low (for example, [[gadolinium]] concentrations must be lower than one part per million by weight). Previously, repeated dissolution and recrystallisation was used to achieve these high purities. Today, liquid solvent extraction procedures involving selective complexation of Th⁴⁺ is used. For example, following alkaline digestion and the removal of phosphate, the resulting nitrato complexes of thorium, uranium, and the rare earths can be separated by extraction with [[tributyl phosphate]] in [[kerosene]].{{sfn|Stoll|p=8}} == Modern applications == Non-radioactivity-related uses have been on decline since the 1950s.{{sfn|Stoll|2005|p=32}} Many applications of thorium are becoming obsolete due to environmental concerns largely stemming from the radioactivity of thorium and its decay products. Thorium is thus being phased out of many of its uses.<ref name=Wickleder523 /><ref name=Furuta/> Most thorium applications use its dioxide (sometimes called "thoria" in the industry), rather than the metal. One particular characteristic of this compound is its high melting point, 3300 °C (6000 °F) – the highest of all known oxides; only a few substances have higher melting points.<ref name="Emsley" /> In particular, that helps the compound remain solid when introduced into a flame, and when it is, it considerably increases the luminacy of the flame; this is the main reason thorium is used in gas mantles.{{sfn|Stoll|2005|p=31}} Although all substances emit energy (glow) when heated to high temperatures, light emitted by thorium is almost exclusively located in the [[visible spectrum]], which explain the brightness of thorium mantles.<ref name="Ivey">{{ cite journal | first = H. F. | last = Ivey | title = Candoluminescence and radical-excited luminescence | journal = Journal of Luminescence | year = 1974 | volume = 8 | issue = 4 | pages = 271–307 | doi = 10.1016/0022-2313(74)90001-5 }}</ref> Energy, some of it in the form of the visible light, is emitted when thorium is exposed to a source of energy itself, such as a cathode ray, heat or [[ultraviolet light]]. Generally, this effect is shared by [[cerium dioxide]], which converts ultraviolet light into visible light more efficiently, but thorium dioxide gives a higher temperature of the flame, emitting less [[infrared light]].{{sfn|Stoll|2005|p=31}} Thorium in mantles, though still common, have been being replaced with a different [[rare earth element]], [[yttrium]], since the late 1990s.<ref name="Matson2011">{{cite book|last=Matson|first=Tim|title=The Book of Non-electric Lighting: The Classic Guide to the Safe Use of Candles, Fuel Lamps, Lanterns, Gaslights & Fire-View Stoves|url=https://books.google.com/books?id=0-UZUv8eh4IC&pg=PA60|year=2011|publisher=Countryman Press|isbn=978-1-58157-829-4|page=60}}</ref> According to the 2005 review by the United Kingdom's [[National Radiological Protection Board]], "although they were widely available a few years ago, they are not any more."<ref>https://ec.europa.eu/energy/sites/ener/files/documents/139.pdf</ref> During the production of [[incandescent]] filaments, [[recrystallization]] of [[tungsten]] is signifiantly lowered by adding small amounts of thorium dioxide to the tungsten [[sintering]] powder before drawing the filaments.{{sfn|Stoll|2005|p=32}} A small addition of thorium dioxide to tungsten considerably reduced the [[work function]] of electrons with the result that the thoriated tungsten [[hot cathode|thermocathode]] emits electrons at considerably lower temperatures.<ref name=Wickleder523 /> (The effect responsible may be the formation of a thorium monoxide dipole layer, which is constantly renewed from the interior of the electrode.) Since the 1920s, thoriated tungsten wires have been used in electronic tubes and in the cathodes and anticathodes of X-ray tubes and rectifiers. Thanks to the reactivity of thorium with atmospherial oxygen and nitrogen, thorium also marks impurities in the evacuated tubes. Introduction of transistors in the 1950s significantly diminished this use, though not entirely.{{sfn|Stoll|2005|p=32}} Thorium dioxide is used in [[gas tungsten arc welding]] (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.{{sfn|Wickleder|2006|p=52}} In electronic equipment, thorium coating of tungsten wire improves the electron [[thermionic emission|emission]] of heated [[cathode]]s<!-- by ... -->.<ref name="CRC" /> <!--is this related to the para that mentions the work function?--> Nevertheless, because of safety concerns, thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and [[lanthanum]].<ref>{{cite book |last=Uttrachi |first=Jerru |date=2015 |title=Weld Like a Pro: Beginning to Advanced Techniques |publisher=CarTech Inc |page=42 |isbn=9781613252215}}</ref><ref>{{cite book |last=Jeffus |first=Larry |date=2016 |title=Welding: Principles and Applications |publisher=Cengage Learning |page=393 |isbn=9781305494695}}</ref> Thorium dioxide is a material for [[refraction (metallurgy)|heat-resistant]] ceramics, as used in high-temperature laboratory [[crucible]]s,<ref name=Wickleder523 /> as a main material or as an addition to [[zirconium dioxide]]. An alloy of 90% [[platinum]] and 10% thorium is an effective catalyst for oxidising [[ammonia]] to nitrogen oxides, but this has likewise been replaced by an alloy of 95% platinum and 5% [[rhodium]] because of its better mechanical properties and greater durability.{{sfn|Stoll|2005|p=32}} [[File:Yellowing of thorium lenses.jpg|thumb|Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)]] When added to [[glass]], thorium dioxide helps increase [[refractive index]] and decrease [[dispersion (optics)|dispersion]]. Such glass finds application in high-quality [[lens (optics)|lenses]] for cameras and scientific instruments.<ref name=CRC /> The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal.<ref>[http://www.orau.org/ptp/collection/consumer%20products/cameralens.htm Thoriated Camera Lens (ca. 1970s)]. orau.org</ref> Yellowed lenses may be restored to their original colourless state with lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced by rare earth oxides in this application, as they provide similar effects and are not radioactive.{{sfn|Stoll|2005|p=32}} Thorium tetrafluoride is used as an antireflection material in multilayered optical coatings. It has an optical transparency in the range of 0.35–12 µm, and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.<ref>{{cite book|url=https://books.google.com/books?id=_VsEiRoFnXcC&pg=PA196|page=196|title=Optical thin films: user handbook|author=Rancourt, James D.|publisher=SPIE Press|date=1996|isbn=0-8194-2285-1}}</ref> Thorium tetrafluoride was also used in manufacturing [[carbon arc lamp]]s, which provided high-intensity illumination for movie projectors and search lights.<ref name="appl">{{cite book|url=https://books.google.com/books?id=ahNFGR1jMB4C&pg=PA81|page=81|title=Encyclopedia of Chemical Processing and Design: Thermoplastics to Trays, Separation, Useful Capacity|author=McKetta, John J. |publisher=CRC Press|date=1996|isbn=0-8247-2609-X}}</ref> ==Potential use for nuclear energy== {{Main article|Thorium fuel cycle}} Thorium has been suggested as a potent [[nuclear power]] source and a possible replacement to the currently used uranium and plutonium. India, which has little uranium but much thorium, is a big proponent of development of thorium-based power technologies and has prioritised developing a thorium fuel cycle.<ref name=":0" /> The main nuclear power source in a reactor is the [[spontaneous fission]] of a certain nuclide; of the synthetic fissile{{efn|name="fissionable"}} nuclei, ²³³U and ²³⁹Pu can be [[breeder reactor|bred]] from neutron capture by the naturally occurring quantity nuclides ²³²Th and ²³⁸U (note that ²³⁵U occurs naturally and is also fissile).<ref name="Ronen">Ronen Y., 2006. A rule for determining fissile isotopes. ''Nucl. Sci. Eng.'', 152:3, pages 334–335. [http://www.ans.org/pubs/journals/nse/a_2588]</ref><ref name="anucene">{{Cite journal | last1 = Ronen | first1 = Y. | title = Some remarks on the fissile isotopes | doi = 10.1016/j.anucene.2010.07.006 | journal = Annals of Nuclear Energy | volume = 37 | issue = 12 | pages = 1783–1784 | year = 2010 | pmid = | pmc = }}</ref>{{efn|The thirteen fissile actinide isotopes with half-lives over a year are ²²⁹Th, ²³³U, ²³⁵U, [[neptunium-236|²³⁶Np]], ²³⁹Pu, [[plutonium-241|²⁴¹Pu]], [[americium-242m|^242mAm]], [[curium-243|²⁴³Cm]], [[curium-245|²⁴⁵Cm]], [[curium-247|²⁴⁷Cm]], [[californium-249|²⁴⁹Cf]], [[californium-251|²⁵¹Cf]], and [[einsteinium-252|²⁵²Es]]. Of these, only ²³⁵U is naturally occurring, and only ²³³U and ²³⁹Pu can be bred from naturally occurring nuclei with single neutron capture.<ref name=Ronen /><ref name=anucene />}}. In the thorium fuel cycle, the fertile isotope ²³²Th is bombarded by [[slow neutron]]s, undergoing neutron capture to become ²³³Th, which undergoes two consecutive beta decays to become first [[protactinium-233|²³³Pa]] and then the fissile ²³³U:<ref name="Wickleder523" /> :{{nuclide|thorium|232}} + n → {{nuclide|thorium|233}} + {{math|γ}} {{overunderset|→|''β''⁻|21.8 min}} {{nuclide|protactinium|233}} {{overunderset|→|''β''⁻|27.0 days}} {{nuclide|uranium|233}} {{Thorium Cycle Transmutation}} ²³³U is fissile and hence can be used as a nuclear fuel in much the same way as the more-commonly used ²³⁵U or [[plutonium-239|²³⁹Pu]]. When ²³³U undergoes nuclear fission, the neutrons emitted can strike further ²³²Th nuclei, restarting the cycle.<ref name=Wickleder523 /> This closely parallels the uranium fuel cycle in [[fast breeder reactor]]s where ²³⁸U undergoes neutron capture to become ²³⁹U, beta decaying to first ²³⁹Np and then fissile ²³⁹Pu.<ref>{{cite web|title=Information Paper 15|url=http://www.world-nuclear.org/info/inf15.html|publisher=World Nuclear Association|accessdate=15 December 2012}}</ref> === Advantages === Thorium is more abundant than uranium and hence can satisfy world energy demands for longer. ²³²Th also absorbs neutrons more readily than ²³⁸U, and not only does ²³³U have a higher probability of fission upon neutron capture (92.0%) than ²³⁵U (85.5%) or ²³⁹Pu (73.5%),<ref>{{cite web |url=http://www.nndc.bnl.gov/chart/reColor.jsp?newColor=sigf |title=Interactive Chart of Nuclides |publisher= Brookhaven National Laboratory |accessdate=2013-08-12}}</ref> it also releases more neutrons upon fission on average.<ref name="Greenwood1259" /> While a single neutron capture by ²³⁸U would produce transuranic waste along with the fissile ²³⁹Pu, ²³²Th only produces this waste after five captures, forming ²³⁷Np. This number of captures does not happen for 98–99% of the ²³²Th nuclei because the intermediate products ²³³U or ²³⁵U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in [[MOX fuel|mixed oxide fuels]] to minimise the generation of transuranics and maximise the destruction of plutonium.<ref name="wnn-20130621">{{cite news |url=http://www.world-nuclear-news.org/ENF_Thorium_test_begins_2106131.html |title=Thorium test begins |publisher=World Nuclear News |date=21 June 2013 |accessdate=21 July 2013}}</ref> Thorium fuels also result in a safer and better-performing [[reactor core]]<ref name="Wickleder523" /> because thorium dioxide has a higher melting point, higher [[thermal conductivity]], lower [[coefficient of thermal expansion]] and is more stable chemically than the now-common fuel [[uranium dioxide]], which can further oxidise to [[triuranium octoxide]] (U₃O₈).<ref name="Thorium Fuel Cycle – Potential Benefits and Challenges">{{cite web|url=http://www-pub.iaea.org/MTCD/publications/PDF/TE_1450_web.pdf|format=PDF|publisher=International Atomic Energy Agency|title=IAEA-TECDOC-1450 Thorium Fuel Cycle-Potential Benefits and Challenges|date=May 2005|accessdate=23 March 2009}}</ref> === Disadvantages === The used fuel is difficult and dangerous to reprocess because many of the daughters of ²³²Th and ²³³U are strong gamma emitters.<ref name="Greenwood1259">Greenwood and Earnshaw, p. 1259</ref> Additionally, all ²³³U production methods other than mercury fluorescence always result in significant impurities of the very dangerous [[uranium-232|²³²U]], either from parasitic knock-out (n,2n) reactions on ²³²Th, ²³³Pa, or ²³³U that result in the loss of a neutron, or from double neutron capture of ²³⁰Th, an impurity in natural ²³²Th:<ref name="Intro2WMD" /> :{{nuclide|thorium|230}} + n → {{nuclide|thorium|231}} + {{math|γ}} {{overunderset|→|''β''⁻|25.5 h}} {{nuclide|protactinium|231}} {{overunderset|→|''α''|3.28 × 10{{su|p=4}} y}} {{shy}} {{nuclide|protactinium|231}} + n → {{nuclide|protactinium|232}} + {{math|γ}} {{overunderset|→|''β''⁻|1.3 d}} {{nuclide|uranium|232}} While ²³²U by itself is not particularly harmful, it quickly decays to produce significant quantities of the strong gamma emitter ²⁰⁸Tl. (While ²³²Th also follows the same decay chain, its much longer half-life means that the quantities of ²⁰⁸Tl produced by it are essentially negligible.)<ref name=Stoll30>Stoll, p. 30</ref> These impurities of ²³²U make ²³³U very easy to detect and very dangerous to work on, and the impracticality of their separation limits the possibilities of [[nuclear proliferation]] using ²³³U as the fissile material.<ref name="Intro2WMD">{{cite book |title= Introduction to Weapons of Mass Destruction: Radiological, Chemical, and Biological |last= Langford |first= R. Everett |year= 2004 |publisher= [[John Wiley & Sons]] |location= [[Hoboken, New Jersey]] |isbn=0471465607 |page=85 |accessdate=10 October 2012 |url= http://www.amazon.com/dp/0471465607/}} ''"The US tested a few uranium-233 bombs, but the presence of uranium-232 in the uranium-233 was a problem; the uranium-232 is a copious alpha emitter and tended to 'poison' the uranium-233 bomb by knocking stray neutrons from impurities in the bomb material, leading to possible pre-detonation. Separation of the uranium-232 from the uranium-233 proved to be very difficult and not practical. The uranium-233 bomb was never deployed since plutonium-239 was becoming plentiful."''</ref> Additionally, ²³³Pa has a relatively long half-life of 27 days and a high [[cross section (physics)|cross section]] for neutron capture. Thus it is a [[neutron poison]]: instead of rapidly decaying to the useful ²³³U, a significant amount of ²³³Pa converts to ²³⁴U and consumes neutrons, degrading [[neutron economy|the reactor efficiency]]. To avoid this, ²³³Pa is extracted from the active zone of thorium [[molten salt reactor]]s during their operation, so that it only decays to ²³³U.<ref name=b1>Groult, Henri (2005) [https://books.google.com/books?id=dR2DA50PUV4C&pg=PA562 Fluorinated materials for energy conversion], Elsevier, pp. 562–565, {{ISBN|0-08-044472-5}}.</ref> The need to irradiate ²³²Th with neutrons and process it come before these advantages become real, and this requires more advanced technology than the presently used fuels based on uranium and plutonium; nevertheless, advances are being made in this area.<ref name=Wickleder523 /> Another common criticism centres around the low commercial viability of the thorium fuel cycle:<ref>{{cite web|url=https://www.theguardian.com/environment/2011/jun/23/thorium-nuclear-uranium|title=Don't believe the spin on thorium being a greener nuclear option|author=Rees, Eifion |date=23 June 2011|publisher=The Guardian}}</ref><ref name="SovacoolValentine2012">{{cite book|author1=Sovacool, Benjamin K. |author2=Valentine, Scott Victor |title=The National Politics of Nuclear Power: Economics, Security, and Governance|date=2012|publisher=Routledge|isbn=978-1-136-29437-2|page=226}}</ref><ref>[http://www.ne.anl.gov/pdfs/NuclearEnergyFAQ.pdf Nuclear Energy FAQs]. [[Argonne National Laboratory]]</ref> some entities like the [[Nuclear Energy Agency]] go further and predict that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which [[Trevor Findlay]] predicts will persist "in the coming decades".<ref name="FindlayFindlay2011">{{cite book|author=Findlay, Trevor |title=Nuclear Energy and Global Governance: Ensuring Safety, Security and Non-proliferation|date=2011|publisher=Routledge|isbn=978-1-136-84993-0|page=9}}</ref> Furthermore, though the isotopes produced in the thorium fuel cycle are mostly not transuranic, some of them are still very dangerous, such as ²³¹Pa, which has a long half-life of 32760 years and is a major contributor to the long-term [[radiotoxic]]ity of spent nuclear fuel.<ref name=b1/> == Hazards == === Radiological === [[File:PSM V74 D233 Thorium radioactive incandescent gas mantle placed above plant seeds.png|thumb|right|Experiment on the effect of radiation (from an unburned thorium gas mantle) on the germination and growth of [[timothy-grass]] seed; from ''[[Popular Science]]'', 1909.]] Natural thorium decays very slowly compared to many other radioactive materials, and the emitted [[alpha radiation]] cannot penetrate human skin. As a result, owning and handling small amounts of thorium, such as those in a gas mantle, is considered safe, although usage of such items may pose some risks.<ref name="epa">{{citation|title=Thorium: Radiation Protection|url=http://www.epa.gov/radiation/radionuclides/thorium.html|archiveurl=https://web.archive.org/web/20061001225000/http://www.epa.gov/radiation/radionuclides/thorium.htm|deadurl=yes|publisher=U.S. EPA|accessdate=27 February 2016|archivedate=1 October 2006}}</ref> Exposure to an aerosol of thorium, such as contaminated dust, can lead to increased risk of [[cancer]]s of the [[lung]], [[pancreas]], and [[blood]], as lungs and other internal organs can be penetrated by alpha radiation.<ref name="epa" /> Exposure to thorium internally leads to increased risk of [[liver]] diseases.<ref name="arpansa" /> The decay products of ²³²Th include more dangerous radionuclides such as radium and radon. Although relatively little of those products is created as the result of the faint decay of thorium, a proper assessment of the radiological toxicity of ²³²Th must include the contribution of its daughters, some of which are dangerous [[gamma radiation|gamma]] emitters,<ref>[http://gonuke.org/ComprehensiveTeachingToolkits/Radiation%20Protection/ChSCC_RP/Columbia%20Basin%20RPT-111/Supplementary%20materials/natural-decay-series.pdf Natural Decay Series: Uranium, Radium, and Thorium]. Argonne National Laboratory, EVS: Human Health Fact Sheet, August 2005</ref> and which are built up quickly following the initial decay of ²³²Th due to the absence of long-lived nuclides along the decay chain.{{sfn|Stoll|2005|p=35}} As the dangerous daughters of thorium have much lower melting points than thorium dioxide (except of course ²²⁸Th, which is chemically identical to its parent ²³²Th), they would be volatilised every time the mantle is heated for use. During burning, significant fractions of the thorium daughters ²²⁴Ra, ²²⁸Ra, ²¹²Pb, and ²¹²Bi are released in the first hour of use alone.<ref>{{cite journal |title=Radioactivity released from burning gas lantern mantles |first=J. W. |last=Luetzelschwab |first2=S. W. |last2=Googins |date=April 1984 |journal=Health Phys. |volume=46 |issue=4 |pages=873–81 |pmid=6706595}}</ref> Most of the radiation dose by a normal user arises from inhaling the radium, resulting in a radiation dose of up to 0.2 [[sievert|millisieverts]] per use, about a third of the dose sustained during a [[mammogram]].<ref>{{cite journal |last=Huyskens |first=C. J. |last2=Hemelaar |first2=J. T. |last3=Kicken |first3=P. J. |date=October 1985 |title=Dose estimates for exposure to radioactivity in gas mantles |journal=Sci. Total Environ. |volume=45 |pages=157–64 |pmid=4081711}}</ref> Some [[nuclear safety]] agencies make recommendations about use of thorium mantles and have raised some safety concerns regarding their [[Gas mantle#Safety concerns|manufacture]] and disposal, because while the radiation dose from one mantle is not a serious problem, that from many mantles gathered together in factories or landfills is.<ref name="arpansa">[http://web.archive.org/web/20071014211034/http://arpansa.gov.au/RadiationProtection/Factsheets/is_lantern.cfm Radioactivity in Lantern Mantles]. Australian Radiation Protection and Nuclear Safety Agency</ref> === Biological === Thorium is odourless and tasteless.<ref>https://www.atsdr.cdc.gov/toxprofiles/tp147.pdf</ref> The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,<ref name="Schneckenstein">Merkel, B. et al. (1988) [http://web.archive.org/web/20130108094057/http://www.geo.tu-freiberg.de/~merkel/schneckenstein.PDF ''Untersuchungen zur radiologischen Emission des Uran-Tailings Schneckenstein''] (PDF; 4 MB), [[Technische Universität Bergakademie Freiberg|TU Bergakademie Freiberg]] and [[Technische Universität Dresden|TU Dresden]] (in German).</ref> precipitating out before entering the body as the hydroxide.{{sfn|Stoll|2005|p=34}} (Some thorium compounds are chemically moderately [[toxic]], especially in the presence of strong complex-forming ions such as [[citrate]] that carry the thorium into the body in soluble form.){{sfn|Stoll|2005|p=35}} If thorium is ingested, 0.4% of it and 90% of its dangerous daughters are leached into the body.<ref name="Poljanc" /> Out of the thorium that does remain in the body, three quarters of it accumulates in the [[skeleton]]. While absorption through the skin is possible, it is not a likely means of thorium exposure.<ref name="epa" /> Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.{{sfn|Stoll|2005|p=35}} Tests on the thorium uptake of workers involved in monazite processing showed thorium levels above recommended limits in their bodies, but no adverse effects on health were found at those moderately low concentrations. No chemical toxicity has yet been observed in the [[tracheobronchial tract]] and the lungs from exposure to thorium.{{sfn|Stoll|2005|p=34}} People who work with thorium compounds are at a risk of [[dermatitis]].{{Why|date=July 2017}} It can take as much as thirty years after the ingestion of thorium for symptoms to manifest themselves.<ref name="nbb">{{cite book| pages=544–5| title =Nature's building blocks: an A-Z guide to the elements|first =John|last=Emsley| publisher=Oxford University Press| isbn = 9780199605637| date=2011}}</ref> ===Chemical=== Powdered thorium metal is pyrophoric and often ignites spontaneously in air.<ref name="Wickleder6163" /> In the 1956 [[Sylvania Electric Products explosion]], the burning of thorium metal powder [[sludge]] led to nine injuries, some severe, as well as one death from thorium poisoning.<ref name="AP">{{cite news |author=[[Associated Press]] |url=https://news.google.com/newspapers?nid=1129&dat=19560703&id=eSgNAAAAIBAJ&sjid=BmwDAAAAIBAJ&pg=4985,347025 |title=Nine Injured In Atomic Lab Blasts |newspaper=[[Pittsburgh Post-Gazette]] |date=July 3, 1956 |page=2 }}</ref><ref>{{cite news |author=Associated Press |url=https://news.google.com/newspapers?nid=888&dat=19560703&id=n6RSAAAAIBAJ&sjid=M3YDAAAAIBAJ&pg=7412,1311234 |title=No Radiation Threat Seen In A-laboratory Blast |newspaper=[[St. Petersburg Times]] |date=July 3, 1956 |page=2 }}</ref><ref name="Newsday">{{cite news|last=Harrington|first=Mark|url=http://www.newsday.com/columnists/glenn-gamboa/2.1091/sad-memories-of-56-sylvania-explosion-1.451270|title=Sad Memories of '56 Sylvania Explosion|work=[[New York Newsday]]|date=August 17, 2003|archiveurl=https://web.archive.org/web/20120204183944/http://www.newsday.com/columnists/glenn-gamboa/2.1091/sad-memories-of-56-sylvania-explosion-1.451270|archivedate=February 4, 2012}}</ref> === Exposure routes === Thorium exists in very small quantities almost everywhere on Earth: the average human contains about 100 [[microgram]]s of thorium and typically consumes three micrograms per day.<ref name="nbb" /> Most thorium exposure occurs through dust inhalation; some thorium comes with food and water.{{Cn|date=July 2017}} Exposure is raised for people who live near thorium deposits, radioactive waste disposal sites, those who live near or work in uranium, phosphate, or tin processing factories, and for those who work in gas mantle production industries.<ref name="ATSDR">[http://www.atsdr.cdc.gov/toxfaqs/tfacts147.pdf Thorium ToxFAQs] – Agency for Toxic Substances and Disease Registry</ref> Thorium is especially common in the [[Tamil Nadu]] coastal areas of India, where residents may be exposed to a naturally occurring radiation dose ten times higher than the worldwide average.<ref>{{cite web|url=http://www.dae.gov.in/iandm/minesback.htm|title=Compendium Of Policy And Statutory Provisions Relating To Exploitation Of Beach Sand Minerals |publisher=Government Of India |accessdate=2008-12-19| archiveurl= https://web.archive.org/web/20081204114125/http://www.dae.gov.in/iandm/minesback.htm| archivedate= 4 December 2008 <!--DASHBot-->| deadurl= no}}</ref> == Notes == {{notes|30em}} == References == {{reflist|30em}} == Bibliography == * {{cite book|author = Golub, A. M. |title = Общая и неорганическая химия (General and Inorganic Chemistry)|date = 1971|volume = 2}} * {{cite book |last1=Greenwood |first1=Norman N. |authorlink1=Norman Greenwood |last2=Earnshaw |first2=Alan |year=1997 |title=Chemistry of the Elements |edition=2nd |page= |pages= |publisher=[[Butterworth-Heinemann]] |isbn=0-08-037941-9}} * {{cite book |last1=Wickleder|first1 = Mathias S.|first2 = Blandine |last2 = Fourest|first3 = Peter K.|last3 = Dorhout|ref=Wickleder et al.|contribution = Thorium|title = The Chemistry of the Actinide and Transactinide Elements|editor1-first = Lester R.|editor1-last = Morss|editor2-first = Norman M.|editor2-last = Edelstein|editor3-first = Jean|editor3-last = Fuger|edition = 3rd|date = 2006|volume = 3|publisher = Springer|location = Dordrecht, the Netherlands|pages = 52–160|url = http://radchem.nevada.edu/classes/rdch710/files/thorium.pdf|doi = 10.1007/1-4020-3598-5_3}} * {{cite book |first=Wolfgang |last=Stoll |chapter=Thorium and Thorium Compounds |doi=10.1002/14356007.a27_001 |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=[[Wiley-VCH]] |year=2005 |isbn=978-3-527-31097-5 |ref=harv}} == Further reading == * {{cite web|author=Brett W. Jordan|author2=Rod Eggert|author3=Brent Dixon|author4=Brett Carlsen|date=October 2014|url=http://econbus.mines.edu/working-papers/wp201407.pdf|title=Thorium: Does Crustal Abundance Lead to Economic Availability?|publisher=[[Colorado School of Mines]] Working Paper 2014-07}} == External links == {{Commons}} * [http://www.ithec.org/ International Thorium Energy Committee – iThEC] * [http://www.itheo.org/ International Thorium Energy Organisation – IThEO.org] * [http://www.euronuclear.org/info/encyclopedia/d/decaybasinnatural.htm European Nuclear Society – Natural Decay Chains] * [http://www.atsdr.cdc.gov/tfacts147.html ATSDR CDC ToxFAQs: health questions about thorium] * [http://www.world-nuclear.org/info/inf62.html FactSheet on Thorium], [[World Nuclear Association]] * [http://www.periodicvideos.com/videos/090.htm Thorium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)