Editing Thorium (section)

==Chemistry==
{{Main|Thorium compounds}}
A thorium atom has 90 electrons, of which four are [[valence electron]]s. Four [[atomic orbital]]s are theoretically available for the valence electrons to occupy: 5f, 6d, 7s, and 7p. The 7p orbitals are not occupied in the ground state of Thorium, however, due to being greatly destabilized.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=59–60}} Despite thorium's position in the [[f-block]] of the periodic table, it has an anomalous [Rn]6d<sup>2</sup>7s<sup>2</sup> 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. This is due to [[relativistic effects]], which become 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 results in thorium almost always losing all four valence electrons and occurring in its highest possible oxidation state of +4. This is different from its lanthanide congener cerium, in which +4 is also the highest possible state, but +3 plays an important role and is more stable. Thorium complexes in the trivalent and divalent oxidation states are known, however.<ref>{{cite journal |last1=Parry |first1=Julian |title=Synthesis and Characterization of the First Sandwich Complex of Trivalent Thorium: A Structural Comparison with the Uranium Analogue |journal=J. Am. Chem. Soc. |date=1999 |volume=121 |issue=29 |pages=6867–6871 |doi=10.1021/ja9903633|bibcode=1999JAChS.121.6867P }}</ref><ref>{{cite journal |last1=Evans |first1=Bill |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 |journal=Chem Sci |date=2014 |volume=6 |issue=1 |pages=517–521 |doi=10.1039/c4sc03033h|pmid=29560172 |pmc=5811171 }}</ref> Thorium is much more similar to the [[transition metal]]s zirconium and hafnium than to cerium in its ionization 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, from actinium to americium.<ref name="CottonSA2006" /><ref name="NIST">{{cite journal |last1=Martin |first1=W. C. |last2=Hagan |first2=Lucy |last3=Reader |first3=Joseph |last4=Sugar |first4=Jack |title=Ground Levels and Ionization Potentials for Lanthanide and Actinide Atoms and Ions |journal=Journal of Physical and Chemical Reference Data |date=July 1974 |volume=3 |issue=3 |pages=771–780 |doi=10.1063/1.3253147 |bibcode=1974JPCRD...3..771M }}</ref><ref name=johnson>{{cite book |last=Johnson |first=David |date=1984 |title=The Periodic Law |url=https://www.rsc.org/images/23_The_Periodic_Law_tcm18-30005.pdf |location= |publisher=The Royal Society of Chemistry |page= |isbn=0-85186-428-7 |archive-date=31 March 2022 |access-date=11 January 2024 |archive-url=https://web.archive.org/web/20220331224430/https://www.rsc.org/images/23_The_Periodic_Law_tcm18-30005.pdf |url-status=live }}</ref>

[[File:CaF2 polyhedra.png|thumb|alt=Crystal structure of fluorite|Thorium dioxide has the [[fluorite]] crystal structure. <br/> {{chem2|Th(4+)}}: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;{{chem2|O(2−)}}: <span style="color:#9c0; background:#9c0;">__</span>]]

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement. A hypothetical metallic state of thorium that had the [Rn]6d<sup>2</sup>7s<sup>2</sup> 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-centred cubic as it actually is. The actual crystal structure can only be explained when the 5f states are invoked, proving that thorium is metallurgically a true actinide.<ref name="Johansson" />

Tetravalent thorium compounds are usually colourless or yellow, like those of [[silver]] or lead, as the {{chem2|Th(4+)}} 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=978-0-471-18602-1}}</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. The only other commonly-encountered actinide, uranium, retains some echoes of main-group behaviour. The chemistry of uranium is 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 low enough not to require special handling in the laboratory.{{sfn|Greenwood|Earnshaw|1997|p=1262}}

===Reactivity===
Thorium is a highly [[reactivity (chemistry)|reactive]] and electropositive metal. With a [[standard reduction potential]] of −1.90&nbsp;V for the {{chem2|Th(4+)}}/Th couple, it is somewhat more electropositive than zirconium or aluminium.{{sfn|Stoll|2005|p=6}} Finely divided thorium metal can exhibit [[pyrophoricity]], spontaneously igniting in air.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} When heated in air, thorium [[swarf|turnings]] ignite and burn with a brilliant white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}} Such samples slowly tarnish, becoming grey and finally black at the surface.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}

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 a black insoluble residue of ThO(OH,Cl)H.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="CRC">{{cite book| last= Hammond| first= C. R.| title= The Elements, in Handbook of Chemistry and Physics| edition= 81st| publisher= [[CRC Press]]| isbn= 978-0-8493-0485-9| date= 2004| url-access= registration| url= https://archive.org/details/crchandbookofche81lide}}</ref> It dissolves in concentrated [[nitric acid]] containing a small quantity of catalytic [[fluoride]] or [[fluorosilicate]] ions;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}<ref name="ekhyde">{{cite book|url= http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf|author= Hyde, E. K.|title= The radiochemistry of thorium|publisher= [[National Academy of Sciences]]|date= 1960|access-date= 29 September 2017|archive-date= 5 March 2021|archive-url= https://web.archive.org/web/20210305130415/http://www.radiochemistry.org/periodictable/pdf_books/pdf/rc000034.pdf}}</ref> if these are not present, [[passivation (chemistry)|passivation]] by the nitrate can occur, as with uranium and plutonium.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=61–63}}{{sfn|Greenwood|Earnshaw|1997|p=1264}}<ref>{{cite journal |last1=Moore |first1=Robert Lee |last2=Goodall |first2=C. A. |first3=J. L. |last3=Hepworth |first4=R. A. |last4=Watts |date=May 1957 |title=Nitric Acid Dissolution of Thorium. Kinetics of Fluoride-Catalyzed Reaction |journal=Industrial & Engineering Chemistry |volume=49 |issue=5 |pages=885–887 |doi=10.1021/ie50569a035}}</ref>

[[File:Kristallstruktur Uran(IV)-fluorid.png|thumb|alt=Crystal structure of thorium tetrafluoride|Crystal structure of thorium tetrafluoride<br/>{{chem2|Th(4+)}}: <span style="color:silver; background:silver;">__</span>&nbsp;&nbsp;/&nbsp;&nbsp;{{chem2|F−}}: <span style="color:#9c0; background:#9c0;">__</span>]]

===Inorganic compounds===
Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.{{sfn|Greenwood|Earnshaw|1997|p=1267}} In air, thorium burns to form {{chem2|ThO2}}, which has the [[fluorite]] structure.<ref name="Yamashita">{{cite journal |title= Thermal expansions of NpO<sub>2</sub> and some other actinide dioxides |journal= J. Nucl. Mater. |volume= 245 |issue= 1 |date= 1997 |pages= 72–78 |last1= Yamashita |first1=Toshiyuki |last2= Nitani |first2=Noriko |last3= Tsuji |first3=Toshihide |last4= Inagaki |first4=Hironitsu| doi= 10.1016/S0022-3115(96)00750-7 |bibcode=1997JNuM..245...72Y}}</ref> Thorium dioxide is a [[refractory material]], with the highest melting point (3390&nbsp;°C) of any known oxide.<ref name="Emsley2011" /> It is somewhat [[hygroscopic]] and reacts readily with water and many gases;{{sfn|Wickleder|Fourest|Dorhout|2006|pp=70–77}} it dissolves easily in concentrated nitric acid in the presence of fluoride.{{sfn|Greenwood|Earnshaw|1997|p=1269}}

When heated in air, thorium dioxide emits intense blue light; the light becomes white when {{chem2|ThO2}} is mixed with its lighter homologue [[cerium dioxide]] ({{chem2|CeO2}}, ceria): this is the basis for its previously common application in [[gas mantle]]s.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=70–77}} A flame is not necessary for this effect: in 1901, it was discovered that a hot Welsbach gas mantle (using {{chem2|ThO2}} with 1% {{chem2|CeO2}}) remained at "full glow" when exposed to a cold unignited mixture of flammable gas{{which|date=November 2022}} and air.<ref name="Ivey" /> The light emitted by thorium dioxide is higher in wavelength than the [[blackbody]] emission expected from [[incandescence]] at the same temperature, an effect called [[candoluminescence]]. It occurs because {{chem2|ThO2}} : Ce acts as a catalyst for the recombination of [[free radical]]s that appear in high concentration in a flame, whose deexcitation releases large amounts of energy. The addition of 1% cerium dioxide, as in gas mantles, heightens the effect by increasing emissivity in the visible region of the spectrum; but because cerium, unlike thorium, can occur in multiple oxidation states, its charge and hence visible emissivity will depend on the region on the flame it is found in (as such regions vary in their chemical composition and hence how oxidising or reducing they are).<ref name="Ivey" />

Several binary thorium [[chalcogen]]ides and oxychalcogenides are also known with [[sulfur]], [[selenium]], and [[tellurium]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=95–97}}

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.{{sfn|Greenwood|Earnshaw|1997|p=1271}} Many related polyhalide ions are also known.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} Thorium tetrafluoride has a [[monoclinic crystal system|monoclinic]] crystal structure like those of [[zirconium tetrafluoride]] and [[hafnium tetrafluoride]], where the {{chem2|Th(4+)}} ions are coordinated with {{chem2|F−}} ions in somewhat distorted [[square antiprism]]s.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} The other tetrahalides instead have dodecahedral geometry.{{sfn|Greenwood|Earnshaw|1997|p=1271}} Lower iodides {{chem2|ThI3}} (black) and {{chem2|ThI2}} (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead contain {{chem2|Th(4+)}} and could be more clearly formulated as [[electride]] compounds.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} Many polynary halides with the alkali metals, [[barium]], thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=78–94}} For example, when treated with [[potassium fluoride]] and [[hydrofluoric acid]], {{chem2|Th(4+)}} forms the complex anion {{chem2|[ThF6](2-)}} (hexafluorothorate(IV)), which precipitates as an insoluble salt, {{chem2|K2[ThF6]}} (potassium hexafluorothorate(IV)).<ref name="ekhyde" />

Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible [[nuclear fuel]]s.{{sfn|Greenwood|Earnshaw|1997|p=1267}} All four heavier [[pnictogen]]s ([[phosphorus]], [[arsenic]], [[antimony]], and bismuth) also form binary thorium compounds. Thorium germanides are also known.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=97–101}} Thorium reacts with hydrogen to form the thorium hydrides {{chem2|ThH2}} and {{chem2|Th4H15}}, the latter of which is superconducting below 7.5–8&nbsp;K; at standard temperature and pressure, it conducts electricity like a metal.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=64–66}} The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.{{sfn|Greenwood|Earnshaw|1997|p=127}}

[[File:Uranocene-3D-balls.png|thumb|upright|alt=Structure of thorocene|Sandwich molecule structure of thorocene]]

===Coordination compounds===
In an acidic aqueous solution, thorium occurs as the tetrapositive [[aqua ion]] {{chem2|[Th(H2O)9](4+)}}, which has [[tricapped trigonal prismatic molecular geometry]]:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}<ref>{{cite journal |last=Persson |first=I. |date=2010 |title=Hydrated metal ions in aqueous solution: How regular are their structures? |journal=Pure and Applied Chemistry |volume=82 |issue=10 |pages=1901–1917 |doi=10.1351/PAC-CON-09-10-22|doi-access=free }}</ref> at pH&nbsp;&lt;&nbsp;3, the solutions of thorium salts are dominated by this cation.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} The {{chem2|Th(4+)}} ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95&nbsp;and&nbsp;1.14&nbsp;Å.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} 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 {{chem2|[[iron|Fe]](3+)}}), predominantly to {{chem2|[Th2(OH)2](6+)}} in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide {{chem2|Th(OH)4}} forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} As a [[HSAB theory|hard Lewis acid]], {{chem2|Th(4+)}} 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=S. |year=2006 |title=Lanthanide and Actinide Chemistry|publisher=[[John Wiley & Sons]]}}</ref>

High 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.{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} These thorium salts are known for their high solubility in water and 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, [[phosphate]]s, [[vanadate]]s, [[molybdate]]s, and [[chromates]], and their hydrated forms.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} 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.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} 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.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} Thorium complexes with organic ligands, such as [[oxalate]], [[citrate]], and [[EDTA]], are much more stable. In natural thorium-containing waters, organic thorium complexes usually occur in concentrations orders of magnitude higher than the inorganic complexes, even when the concentrations of inorganic ligands are much greater than those of organic ligands.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}

[[File:Thorium half sandwich.svg|thumb|upright|alt=Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2|Piano-stool molecule structure of ({{chem2|η^{8}\-C8H8)ThCl2(THF)2}}]]

In January 2021, the aromaticity has been observed in a large [[metal cluster]] anion consisting of 12 [[Bismuth|bismuth atoms]] stabilised by a center thorium cation.<ref>{{Cite magazine |last=Krämer |first=Katrina |date=2021-01-04 |title=Heavy-metal cluster sets size record for metal aromaticity |url=https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |access-date=2 July 2022 |magazine=Chemistry World |language=en |archive-date=4 January 2021 |archive-url=https://web.archive.org/web/20210104151533/https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |url-status=live }}</ref> This compound was shown to be surprisingly stable, unlike many previous known [[Metal aromaticity|aromatic metal clusters]].

===Organothorium compounds===
Most of the work on organothorium compounds has focused on the [[cyclopentadienyl complex]]es and [[cyclooctatetraenide anion|cyclooctatetraenyls]]. Like many of the early and middle actinides (up to [[americium]], and also expected for [[curium]]), thorium forms a cyclooctatetraenide complex: the yellow {{chem2|Th(C8H8)2}}, [[thorocene]]. It is [[isotypic]] with the better-known analogous uranium compound [[uranocene]].{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} It can be prepared by reacting [[potassium cyclooctatetraenide|{{chem2|K2C8H8}}]] with thorium tetrachloride in [[tetrahydrofuran]] (THF) at the temperature of [[dry ice]], or by reacting thorium tetrafluoride with {{chem2|MgC8H8}}.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} It is unstable in air and decomposes in water or at 190&nbsp;°C.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} [[Half sandwich compound]]s are also known, such as {{chem2|(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 {{chem2|Th(C5H5)3}} and {{chem2|Th(C5H5)4}}: many derivatives are known. The former (which has two forms, one purple and one green){{sfn|Wickleder|Fourest|Dorhout|2006|pp=116–117}} is a rare example of thorium in the formal +3 oxidation state;{{sfn|Greenwood|Earnshaw|1997|pp=1278–1280}} a formal +2 oxidation state 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 {[C<sub>5</sub>H<sub>3</sub>(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub>Th}<sup>1−</sup> anion containing thorium in the formal +2 oxidation state |journal=[[Chemical Science (journal)|Chemical Science]] |volume=6 |year=2015 |issue=1 |pages=517–521 |doi=10.1039/C4SC03033H|pmid=29560172 |pmc=5811171 }}</ref> The chloride derivative {{chem2|[Th(C5H5)3Cl]}} is prepared by heating thorium tetrachloride with [[limiting reagent|limiting]] {{chem2|KC5H5}} used (other univalent metal cyclopentadienyls can also be used). The [[alkyl]] and [[aryl]] derivatives are prepared from the chloride derivative and have been used to study the nature of the Th–C [[sigma bond]].{{sfn|Greenwood|Earnshaw|1997|pp=1278–1280}}

Other organothorium compounds are not well-studied. Tetraallylthorium, {{chem2|Th(CH2CH\dCH2)4}}, is known, but its structures has not been determined. The molecular structure of tetrabenzylthorium, {{chem2|Th(CH2C6H5)4}}, without ancillary ligands has been reported.<ref>{{cite journal |last1=Bart |first1=Suzanne |title=Isolation and Characterization of Elusive Tetrabenzylthorium Complexes |journal=Organometallics |date=August 2, 2023 |volume=42 |issue=15 |page=2079-2086 |doi=10.1021/acs.organomet.3c00248 }}</ref> They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion {{chem2|[Th(CH3)7](3−)}}, heptamethylthorate(IV), which forms the salt {{chem2|[Li(tmeda)]3[Th(CH3)7]}} (tmeda = {{chem2|(CH3)2NCH2CH2N(CH3)2}}). Although one methyl group is only attached to the thorium atom (Th–C distance 257.1&nbsp;pm) and the other six connect the lithium and thorium atoms (Th–C distances 265.5–276.5&nbsp;pm), they behave equivalently in solution. Tetramethylthorium, {{chem2|Th(CH3)4}}, is not known, but its [[adduct]]s are stabilised by [[phosphine]] ligands.<ref name="CottonSA2006" />