The **inert pair effect** is most pronounced in:

Inert pair effect: the reluctance of the $ns^2$ pair to participate in bonding, found in **heavier** p-block elements (Tl, Pb, Bi, Po). Reason: relativistic effects and poor shielding by inner $d$ and $f$ orbitals make the $ns^2$ pair more tightly held in heavier elements. The 'inert pair' stays as a lone pair, so the heaviest member preferentially adopts an oxidation state 2 less than the group oxidation state. Examples: Pb prefers +2 over +4 (PbO₂ is a strong oxidiser); Bi prefers +3 over +5 (NaBiO₃ is a strong oxidiser); Tl prefers +1 over +3 (Tl(III) compounds are scarce). The lighter members (C, N, O, etc.) show no inert pair effect.