Q.Which of the following lanthanoid ions is diamagnetic?

Step 1. A species is diamagnetic only if it has ZERO unpaired electrons -- for an f-block ion, this means either an empty (f⁰) or completely filled (f¹⁴) f sub-shell.

Step 2. Work out each option's f-electron count: Eu²⁺ has the configuration [Xe]4f⁷ (Eu is [Xe]4f⁷6s², and forming Eu²⁺ removes the two 6s electrons) -- 7 unpaired electrons, strongly paramagnetic. Yb²⁺ has the configuration [Xe]4f¹⁴ (Yb is [Xe]4f¹⁴6s², and Yb²⁺ likewise loses only the two 6s electrons) -- a completely filled, fully-paired 4f¹⁴ sub-shell, hence diamagnetic. Ce²⁺ (from Ce, [Xe]4f¹5d¹6s², losing the two 6s electrons) retains 4f¹5d¹, i.e. 2 unpaired elec …

Derive each ion's 4f-electron count from the parent atom's configuration (removing the outer 6s electrons first), and identify the one reaching either …

• Assuming any +2 lanthanoid ion is automatically diamagnetic just because +2 is an 'unusual' oxidation state, without actually counting the resulting f-electrons. …