Editing Actinide (section)

=== Chemical properties ===
Like the lanthanides, all actinides are highly reactive with [[halogen]]s and [[chalcogen]]s; however, the actinides react more easily. Actinides, especially those with a small number of 5''f''-electrons, are prone to [[Hybridization (chemistry)|hybridization]]. This is explained by the similarity of the electron energies at the 5''f'', 7''s'' and 6''d'' shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.<ref name=g222>Golub, pp. 222–227</ref>

Actinium is chemically similar to lanthanum, which is explained by their similar ionic radii and electronic structures. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced [[Base (chemistry)|basic]] properties. Among other trivalent actinides Ac<sup>3+</sup> is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.<ref name="Himiya aktiniya" /><ref name="Yu. D. Tretyakov" />

Thorium is rather active chemically. Owing to lack of [[electron]]s on 6''d'' and 5''f'' orbitals, tetravalent thorium compounds are colorless. At pH < 3, solutions of thorium salts are dominated by the cations [Th(H<sub>2</sub>O)<sub>8</sub>]<sup>4+</sup>. The Th<sup>4+</sup> ion is relatively large, and depending on the [[coordination number]] can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to hydrolyse. The distinctive ability of thorium salts is their high solubility both in water and polar organic solvents.<ref name="Yu. D. Tretyakov" />

Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V). Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere. Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). [[Fluoride]]s, [[phosphate]]s, [[hypophosphate]]s, [[iodate]]s and [[phenylarsonate]]s of protactinium(IV) are insoluble in water and dilute acids. Protactinium forms soluble [[carbonate]]s. The hydrolytic properties of pentavalent protactinium are close to those of [[tantalum]](V) and [[niobium]](V). The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5''f'' shell in this element.<ref name="Himiya protaktiniya" />

Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the [[group 6 element]]s. Many compounds of uranium(IV) and uranium(VI) are [[non-stoichiometric compound|non-stoichiometric]], i.e. have variable composition. For example, the actual chemical formula of uranium dioxide is UO<sub>2+x</sub>, where ''x'' varies between −0.4 and 0.32. Uranium(VI) compounds are weak [[Oxidizing agent|oxidants]]. Most of them contain the linear "[[uranyl]]" group, {{chem|UO|2|2+}}. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a [[hard acid]] and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. {{chem|NpO|2|2+}} and {{chem|PuO|2|2+}} are also the common form of Np and Pu in the +6 oxidation state. Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen. Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium (as well as many other actinides) forms [[organometallic compound]]s, such as U<sup>III</sup>(C<sub>5</sub>H<sub>5</sub>)<sub>3</sub> and U<sup>IV</sup>(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>.<ref name="Yu. D. Tretyakov" /><ref name=g1278>Greenwood, p. 1278</ref>

Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of [[coordination compound]]s.<ref name="Himiya neptuniya" />

Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium. It is highly reactive, and quickly forms an oxide film in air. Plutonium reacts with [[hydrogen]] even at temperatures as low as 25–50&nbsp;°C; it also easily forms [[halide]]s and [[intermetallic compound]]s. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter [[polymerization]] reactions.<ref name="Plutoniy" /><ref name="Himiya plutoniya">{{cite book|author=M. S. Milyukova|title=Analytical chemistry of plutonium|place=Moscow|publisher=Nauka|year=1965|isbn=978-0-250-39918-5|url=https://archive.org/details/analyticalchemis00inst}}</ref>

The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained only in dry compounds and non-aqueous solutions ([[acetonitrile]]). Oxidation states +3, +5 and +6 are typical for aqueous solutions, but also in the solid state. Tetravalent americium forms stable solid compounds ([[Americium dioxide|dioxide]], [[Americium(IV) fluoride|fluoride]] and [[Americium(IV) hydroxide|hydroxide]]) as well as complexes in aqueous solutions. It was reported that in alkaline solution americium can be oxidized to the heptavalent state, but these data proved erroneous. The most stable valence of americium is 3 in aqueous solution and 3 or 4 in solid compounds.<ref name=m25>Myasoedov, pp. 25–29</ref>

Valence 3 is dominant in all subsequent elements up to lawrencium (with the exception of nobelium). Curium can be tetravalent in solids ([[Curium(IV) fluoride|fluoride]], [[Curium(IV) oxide|dioxide]]). Berkelium, along with a valence of +3, also shows the valence of +4, more stable than that of curium; the valence 4 is observed in solid [[Berkelium tetrafluoride|fluoride]] and [[Berkelium(IV) oxide|dioxide]]. The stability of Bk<sup>4+</sup> in aqueous solution is close to that of [[Cerium|Ce]]<sup>4+</sup>.<ref>{{Cite journal|last1=Deblonde|first1=Gauthier J.-P.|last2=Sturzbecher-Hoehne|first2=Manuel|last3=Jong|first3=Wibe A. de|last4=Brabec|first4=Jiri|last5=Corie Y. Ralston|last6=Illy|first6=Marie-Claire|last7=An|first7=Dahlia D.|last8=Rupert|first8=Peter B.|last9=Strong|first9=Roland K.|date=September 2017|title=Chelation and stabilization of berkelium in oxidation state +IV|journal=Nature Chemistry|volume=9|issue=9|pages=843–849|doi=10.1038/nchem.2759|pmid=28837177|issn=1755-4349|url=http://www.escholarship.org/uc/item/9zn3q96n|bibcode=2017NatCh...9..843D|osti=1436161 }}</ref> Only valence 3 was observed for californium, einsteinium and fermium. The divalent state is proven for mendelevium and nobelium, and in nobelium it is more stable than the trivalent state. Lawrencium shows valence 3 both in solutions and solids.<ref name=m25 />

The redox potential <chem>\mathit E_\frac{M^4+}{AnO2^2+}</chem> increases from −0.32 V in uranium, through 0.34 V (Np) and 1.04 V (Pu) to 1.34 V in americium revealing the increasing reduction ability of the An<sup>4+</sup> ion from americium to uranium. All actinides form AnH<sub>3</sub> hydrides of black color with salt-like properties. Actinides also produce [[carbide]]s with the general formula of AnC or AnC<sub>2</sub> ([[Uranium carbide|U<sub>2</sub>C<sub>3</sub>]] for uranium) as well as sulfides An<sub>2</sub>S<sub>3</sub> and AnS<sub>2</sub>.<ref name=g222 />

<gallery mode="packed" widths="160px" heights="120px">
File:Uranylnitrate_crystals.jpg|[[Uranyl nitrate]] (UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>)
File:U Oxstufen.jpg|Aqueous solutions of uranium III, IV, V, VI salts
File:Np ox st .jpg|Aqueous solutions of neptunium III, IV, V, VI, VII salts
File:Plutonium in solution.jpg|Aqueous solutions of plutonium III, IV, V, VI, VII salts
File:UCl4.jpg|[[Uranium tetrachloride]]
File:Uranium hexafluoride crystals sealed in an ampoule.jpg|[[Uranium hexafluoride]]
File:Yellowcake.jpg|[[Triuranium octoxide|U<sub>3</sub>O<sub>8</sub>]] ([[yellowcake]])
</gallery>