Lanthanoids and Actinoids: Oxidation States and Properties

The overwhelmingly dominant, most stable oxidation state across the entire lanthanoid series is +3 (Ln₃⁺), not +2 — this is the one general rule, and any statement claiming +2 is the most common lanthanoid state is false. A handful of specific lanthanoids reach +4 (most notably cerium, Ce⁴⁺, but also praseodymium (Pr), terbium (Tb), and neodymium (Nd)/dysprosium (Dy) under specific conditions), and these +4 states show up specifically as OXIDES (like CeO₂, PrO₂, TbO₂) or complex oxide/halide-type species, tied to how close the resulting f-electron configuration sits to an empty (f₀) or half-filled (f₇) shell, which offers extra stability. Separately, europium (Eu) is the standout example of an element for which BOTH +2 and +3 are common, stable oxidation states (Eu²⁺ has a half-filled 4f₇ configuration, which is unusually stable, almost rivalling +3 in accessibility) — and because Eu²⁺ is easily oxidized back to the more standard Eu³⁺, Eu²⁺ behaves as a REDUCING agent in aqueous solution, not an oxidizing agent.

Lanthanoid electron configurations follow the pattern [Xe] 4f^n 6s₂ for most of the series (filling the 4f subshell while the 6s₂ outer shell stays fixed), with occasional promotions of one electron into 5d for elements where an empty, half-filled or filled f-subshell configuration is extra stable (like lanthanum itself, [Xe]5d₁ 6s₂, with zero f-electrons). Praseodymium (Z=59) is [Xe]4f₃ 6s₂, neodymium (Z=60) is [Xe]4f₄ 6s₂, and promethium (Z=61, the synthetic radioactive lanthanoid with no stable isotopes found in nature) is [Xe]4f₅ 6s₂ — each one f-electron ahead of the last as atomic number increases by one, with no unusual promotion needed at this point in the series.

Lanthanoid metals react with hydrogen to form Ln(OH)₃-type hydroxides is INCORRECT — direct reaction with hydrogen gas actually gives a hydride, and it is reaction with WATER (not H₂ gas) that gives the hydroxide Ln(OH)₃ plus H₂ gas; burning in oxygen gives the sesquioxide, Ln₂O₃; and heating with sulphur gives the sulfide, Ln₂S₃. The general, reliable pattern across nearly all lanthanoids for these reactions is: Ln₂O₃ from burning in O₂, and Ln₂S₃ from heating with sulfur, mirroring their dominant +3 oxidation state in both cases (each Ln³⁺ paired with the appropriately charged O²⁻ or S²⁻ counterparts).

Unlike lanthanoids (where +3 dominates almost throughout), several actinoids reach oxidation states as high as +7: specifically neptunium (Np) and plutonium (Pu) are the classic examples cited for reaching +7 under strongly oxidizing conditions (higher than uranium U, which more commonly tops out at +6, as in UO2²+, the uranyl ion). This makes +7 the generally quoted MAXIMUM oxidation state reachable by actinoid elements as a group — a genuinely wider range than lanthanoids ever show, reflecting the greater involvement of 5f, 6d and 7s electrons (energetically closer together than the corresponding 4f, 5d, 6s levels in lanthanoids) in actinoid bonding.

Across a series of trihalides (MF₃, MCl₃, MBr₃, MI₃) of the same lanthanoid, colour and solubility can each follow their own pattern rather than moving in lockstep. For neodymium, the fluoride NdF₃ is characteristically insoluble in both water and alcohol (fluoride salts of lanthanoids are frequently far less soluble than the corresponding chloride/bromide/iodide, a solubility pattern that shows up repeatedly across the whole lanthanoid series, not just for neodymium), while NdCl₃, NdBr₃ and NdI₃ are all soluble in both solvents. Colour tells a different story: NdF₃, NdCl₃ and NdBr₃ are all a similar pale lilac (a typical f-f transition colour for Nd³⁺, largely independent of which halide is attached, since f-orbitals are shielded from the surrounding ligands), but NdI₃ is distinctly dark green — the much larger, more polarisable iodide ion introduces a significant charge-transfer contribution to the colour that the smaller, less polarisable halides do not, overriding the usual halide-independent f-f colour. Reading a question like this means treating solubility (driven mainly by lattice energy, and hence by the fluoride/non-fluoride divide) and colour (driven mainly by f-f transitions, except when a strongly polarisable ligand like iodide adds charge-transfer character) as two separate properties with two separate underlying causes, not one single trend.