Q.Write the electronic configuration of Ce⁴⁺ and Co²⁺.

Step 1. Cerium (Z=58) has the atomic configuration [Xe]4f¹5d¹6s². Forming Ce⁴⁺ requires removing four electrons: the two 6s electrons, the one 5d electron, and the one 4f electron, leaving [Xe]4f⁰ -- an empty, extra-stable f sub-shell, which is precisely why Ce⁴⁺ is such a favourable, accessible oxidation state for cerium (reverting further to Ce³⁺, [Xe]4f¹, is also thermodynamically easy, underlying Ce⁴⁺'s use as an oxidant). …

Write the atomic configuration of each element, then remove the appropriate number of electrons (outermost s and, where needed, d …

• Writing Ce⁴⁺ as [Xe]4f¹ (only removing three electrons, i.e. stopping at the Ce³⁺ configuration) instead of continuing to remove the fourth (4f) electron required for the +4 charge. …