Lanthanoids and actinoids

Why this is asked: Lanthanoid contraction — steady radius shrinkage across the series from poor 4f shielding — is why second- and third-row transition metals end up almost the same size element for element (Zr≈Hf, Nb≈Ta), which is exactly why those pairs are so hard to separate chemically. +3 is the dominant lanthanoid oxidation state, but watch for Ce⁴⁺ and Eu²⁺, both reaching the extra stability of an empty or half-filled f-subshell — the same logic as Cr and Cu in the d-block.

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