What is insert pair effect?

Answer

In the elements of `4^(th),5^(th)` and `6^(th)` period of the `p` block elements which come after `d`-block elements, the electrons presents in the intervening `d-` and `f-` orbitals do not shield the `s-`electrons of the valence shell effectively.As a result, `ns^(2)` electrons remain more tightly held by the nucleus and hence do not participate in bonding This is called inert pair effect.