The inert pair effect is the reluctance of $ns^2$ electrons to participate in bonding as we go down groups 13-15. Manifestation: lower OS becomes more stable. Group 13: $Tl^+$ >> $Tl^{3+}$ (TlCl stable, TlCl3 decomposes). Group 14: $Pb^{2+}$ >> $Pb^{4+}$ (PbCl2 stable ionic, PbCl4 unstable covalent, PbO2 strong oxidiser). $Sn^{2+}$ is a reducing agent (→ $Sn^{4+}$). Group 15: $Bi^{3+}$ >> $Bi^{5+}$ (BiCl3 stable, BiF5 exists but rare, Bi2O5 strong oxidiser). Cause: poor shielding by inner d and f electrons increases effective nuclear charge on ns electrons; relativistic effects (6s orbital contracts in heavy elements). Practical impact: PbO2 in lead-acid batteries, SnCl2 as reducing agent in organic synthesis.