The actinoids and applications

The actinoids are the 14 elements from Th to Lr, discussed together with actinium. All are radioactive. The early members have long half-lives; the later ones last from a day down to 3 minutes (lawrencium, Z = 103) and have been made only in nanogram amounts, which makes them hard to study.

Configurations share 7s² with variable 5f and 6d occupancy. Thorium has no 5f electron; from Pa onwards 5f fills, completing at element 103. f⁰, f⁷ and f¹⁴ are favoured: Am is [Rn] 5f⁷ 7s² and Cm is [Rn] 5f⁷ 6d¹ 7s².

5f orbitals are less buried than 4f orbitals, so 5f electrons take part in bonding far more.

Actinoid contraction: atoms and M³⁺ ions shrink across the series, and the shrinkage from one element to the next is larger than in the lanthanoids, because 5f electrons shield poorly.

Because 5f, 6d and 7s are close in energy, actinoids show a wider range of oxidation states. +3 is general, but the first half reaches higher: the maximum rises from +4 (Th) to +5 (Pa), +6 (U) and +7 (Np), then falls. +3 and +4 ions tend to hydrolyse.

The metals are silvery but have varied structures, since their metallic radii are much more irregular than the lanthanoids'. Finely divided, they are highly reactive: boiling water gives a mix of oxide and hydride, HCl attacks all of them, nitric acid barely affects most (a protective oxide forms), and alkalis have no action.

Early actinoids have lower ionisation enthalpies than early lanthanoids, because 5f electrons are better shielded from the nucleus and so more available for bonding. Their magnetic behaviour is more complex; susceptibility varies with the number of unpaired 5f electrons roughly as in the lanthanoids, but lanthanoid values are higher.

Lanthanoid-like behaviour appears only in the second half of the actinoid series. The lanthanoid contraction matters more than the actinoid contraction, because little is known of the elements after the actinoids.

Applications: iron and steel are the chief construction materials (carbon plus Cr, Mn, Ni); MnO₂ goes into dry cells, and batteries also use Zn and Ni/Cd. Group 11 are the coinage metals.

Industrial catalysts: V₂O₅ oxidises SO₂ in making sulphuric acid; TiCl₄ with Al(CH₃)₃ is the basis of Ziegler catalysts for polythene; iron in the Haber process; nickel for hydrogenating fats; PdCl₂ in the Wacker process that makes ethanal; nickel complexes for polymerising alkynes. AgBr is prized in photography for its sensitivity to light.