The lanthanoids

The lanthanoids are the 14 elements after lanthanum (Ce to Lu); La is usually discussed with them, and Ln stands for any of them. They resemble one another more closely than the members of a transition series do, and have essentially one stable oxidation state, +3.

Atoms share 6s² with a variable number of 4f electrons (La 5d¹ 6s², Ce 4f¹ 5d¹ 6s², Eu 4f⁷ 6s², Gd 4f⁷ 5d¹ 6s², Yb 4f¹⁴ 6s², Lu 4f¹⁴ 5d¹ 6s²). Every Ln³⁺ ion is simply 4fⁿ, with n = 1 to 14 across the series.

Lanthanoid contraction: atomic and ionic radii fall steadily from La to Lu because one 4f electron shields another even more poorly than one d electron shields another. The M³⁺ radii fall regularly, from 106 pm for La³⁺ to 86 pm for Yb³⁺; atomic radii fall less regularly (Eu's is 199 pm).

Its big consequence is in the third transition series: 5d elements end up nearly the same size as their 4d partners. Zr (160 pm) and Hf (159 pm) occur together in nature and are hard to separate.

+3 dominates, but +2 and +4 appear where they give an empty, half-filled or filled 4f set. Ce⁴⁺ (noble gas configuration) is a strong oxidant returning to +3; E°(Ce⁴⁺/Ce³⁺) = +1.74 V, which could oxidise water, but the reaction is slow, so Ce(IV) is a good analytical reagent. Tb⁴⁺ (f⁷) is also an oxidant; Pr, Nd, Tb and Dy show +4 only in oxides MO₂.

Eu²⁺ (f⁷) and Yb²⁺ (f¹⁴) are reducing agents that go back to +3; samarium, like europium, shows both +2 and +3.

The metals are silvery-white, soft and tarnish quickly in air; hardness rises with atomic number (samarium is steel-hard). Melting points are 1000-1200 K, with samarium at 1623 K. Properties change smoothly except for Eu and Yb (and sometimes Sm and Tm).

Many Ln³⁺ ions are coloured, from f-electron transitions that give narrow absorption bands; La³⁺ and Lu³⁺ are colourless. All the ions are paramagnetic except f⁰ (La³⁺, Ce⁴⁺) and f¹⁴ (Yb²⁺, Lu³⁺).

First ionisation enthalpies are about 600 kJ mol⁻¹ and second about 1200 kJ mol⁻¹, similar to calcium. Third ionisation enthalpies are unusually low for La, Gd and Lu. Early lanthanoids react like calcium and later ones more like aluminium; E°(Ln³⁺/Ln) is −2.2 to −2.4 V (Eu −2.0 V).

Reactions: they combine with hydrogen on gentle heating, give carbides (Ln₃C, Ln₂C₃, LnC₂) with carbon, release H₂ from dilute acids and from water (giving Ln(OH)₃), burn in halogens to LnX₃, form Ln₂O₃ in oxygen, Ln₂S₃ with sulphur and LnN with nitrogen. Ln(OH)₃ are true compounds and basic, like alkaline earth hydroxides.

Uses: alloy steels for plates and pipes; mischmetall (about 95% lanthanoid metal, 5% iron, traces of S, C, Ca, Al) in magnesium-based alloys for bullets, shells and lighter flints; mixed oxides as petroleum-cracking catalysts; some oxides as phosphors in television screens.