Relative reluctance of a heavier main-group ns electron pair to participate in the higher oxidation state. Retained pairs can nevertheless influence structure through mixing and stereochemical activity.
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The inert-pair effect is a phenomenon observed in the chemistry of certain heavy elements, particularly within the p-block of the periodic table. It refers to the tendency of the outermost s electrons of the heavier elements in groups 13 to 16 (especially the thallium, lead, bismuth, and polonium elements) to remain non-bonding or "inert" when these elements form compounds.