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.