What is inert pair effect ?

Explanation

(i) In the elements of `4^("th"),5^("th")` and `6^("th")` period of the p-block the electrons present in the intervening d and f - orbitals do not shield the s-electrons of the valence shell effectively.
(ii) As a result, `ns^(2)`-electons remains more tightly held by the nucleus and hene do not participate in bonding. This is called inert pair effect.