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The inert pair effect is the reluctance of $ns^2$ electrons to participate in bonding, becoming more pronounced in heavier elements of groups 13-15. The lower oxidation state becomes progressively more stable going down each group.

Group 13: Al(+3 only) to Tl(+1 dominant). TlCl stable, TlCl3 unstable. Group 14: Sn(+4 slightly preferred, +2 exists) to Pb(+2 strongly preferred). PbO2 oxidises HCl to Cl2. Group 15: As(+5 accessible) to Bi(+3 dominant). Bi2O5 strong oxidiser.

Causes: (1) Poor shielding by intervening d and f electrons increases effective nuclear charge on ns electrons. (2) Relativistic contraction of 6s orbital in 6th period elements. (3) The $ns^2$ pair becomes increasingly stable energetically.

Chemical consequences: SnCl2 is a reducing agent ($Sn^{2+}$ easily gives electrons to become $Sn^{4+}$). PbO2 is an oxidising agent in lead-acid batteries ($Pb^{4+}$ takes electrons to become $Pb^{2+}$). BiCl3 hydrolyses to BiOCl.

The inert pair effect explains why the chemistry of 6th period p-block elements (Tl, Pb, Bi) differs fundamentally from their lighter congeners.