Editing Thorium (section)

=== Abundance ===
In the universe, thorium is among the rarest of the primordial elements at rank 77th in cosmic abundance<ref name=Cameron/><ref>{{Cite web |last=Helmenstine |first=Anne |date=28 June 2022 |title=Composition of the Universe – Element Abundance |url=https://sciencenotes.org/composition-of-the-universe-element-abundance/ |access-date=13 June 2024 |website=Science Notes and Projects |language=en-US |archive-date=24 May 2024 |archive-url=https://web.archive.org/web/20240524071239/https://sciencenotes.org/composition-of-the-universe-element-abundance/ |url-status=live }}</ref> because it is one of the two elements that can be produced only in the r-process (the other being uranium), and also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are [[thulium]], [[lutetium]], tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals, as well as uranium.<ref name="Cameron" /><ref name="nucleosynthesis" />{{efn|[[Even and odd atomic nuclei|An even number of either protons or neutrons]] generally increases nuclear stability of isotopes, compared to isotopes with odd numbers. Elements with odd atomic numbers have no more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having ten.<ref name="NUBASE" />}} In the distant past the abundances of thorium and uranium were enriched by the decay of plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of <sup>236</sup>U to <sup>232</sup>Th and the natural depletion of <sup>235</sup>U, but these sources have long since decayed and no longer contribute.{{sfn|Stoll|2005|p=2}}

In the Earth's crust, thorium is much more abundant: with an [[Abundance of elements in Earth's crust|abundance]] of 8.1&nbsp;g/[[tonne]], it is one of the most abundant of the heavy elements, almost as abundant as lead (13&nbsp;g/tonne) and more abundant than tin (2.1&nbsp;g/tonne).{{sfn|Greenwood|Earnshaw|1997|p=1294}} This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a [[Goldschmidt classification|lithophile]] under the [[Goldschmidt classification]], meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though the [[Refractory metals|refractory elements]] have the same relative abundances in the Earth as in the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.<ref name="albarede">{{cite book |title= Geochemistry: an introduction |page= 17 |publisher= [[Cambridge University Press]] |year= 2003 |isbn= 978-0-521-89148-6 |first= F. |last= Albarède}}</ref>

[[File:Evolution of Earth's radiogenic heat-no total.svg|thumb|upright=1.25|alt=Heat produced by the decay of K-40, Th-232, U-235, U-238 within the Earth over time|The [[radiogenic heat]] from the decay of <sup>232</sup>Th (violet) is a major contributor to the [[earth's internal heat budget]]. Of the four major nuclides providing this heat, <sup>232</sup>Th has grown to provide the most heat as the other ones decayed faster than thorium.<ref name="thoruranium">{{cite journal |last1=Trenn |first1=T. 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=581–597 |doi=10.1080/00033797800200441}}</ref><ref>{{cite journal |last1=Diamond |first1=H. |last2=Friedman |first2=A. M. |last3=Gindler |first3=J. E. |display-authors=3 |last4=Fields |first4=P. R. |date=1956 |title=Possible Existence of Cm<sup>247</sup> or Its Daughters in Nature |journal=Physical Review |volume=105 |issue=2 |pages=679–680 |doi=10.1103/PhysRev.105.679|bibcode=1957PhRv..105..679D}}</ref><ref>{{cite journal |last1=Rao |first1=M. N. |last2=Gopalan |first2=K. |date=1973 |title=Curium-248 in the Early Solar System |journal=Nature |volume=245 |issue=5424 |pages=304–307 |doi=10.1038/245304a0|bibcode=1973Natur.245..304R|s2cid=4226393 }}</ref><ref>{{cite journal |last1=Rosenblatt |first1=D. B. |date=1953 |title=Effects of a Primeval Endowment of U<sup>236</sup> |journal=Physical Review |volume=91 |issue=6 |pages=1474–1475 |doi=10.1103/PhysRev.91.1474|bibcode=1953PhRv...91.1474R}}</ref>]]