Blog · Inorganic
The Inert Pair Effect, Explained Without Memorising
Why Tl⁺ is more stable than Tl³⁺, and Pb²⁺ more than Pb⁴⁺ — from orbital energies.
Going down groups 13–15, the lower oxidation state becomes more stable: Tl⁺ over Tl³⁺, Pb²⁺ over Pb⁴⁺, Bi³⁺ over Bi⁵⁺. Why?
Poor shielding by d and f electrons
Heavy p-block elements have filled 4d, 4f and 5d subshells. These shield the nucleus poorly, so the ns² electrons feel a higher effective nuclear charge and are held more tightly.
Bond energy doesn't compensate
Using the ns² pair costs extra ionisation energy. For light elements, forming two extra bonds pays this back. For heavy elements, bonds are longer and weaker, so the payback is too small — the pair stays 'inert'.
Memory trick: the heavier the element, the lazier the s-pair.
This one idea explains why PbO₂ is a strong oxidising agent and why TlI₃ is really Tl⁺(I₃)⁻.