CBSE Class 12-science Answered

Inert pair effect:

The electronic configuration of Group 13 elements is ns²np¹. Hence they are expected to be trivalent. In most of their compounds this is the case, however for the heavy elements lower oxidation states are more stable. This is explained by the s electrons remaining paired and not participating in bond formation. This inertness of s-subshell electrons towards the bond formation is called inert pair effect. This happens because the s orbitals are held closer to the nucleus, therefore the electrons present in s orbitals are held strongly by nucleus because of large electrostatic forces. Hence the energy required to unpair the s-electrons is high because of which they remain paired.

Examples are as follows:

1) In 13^th group, thallium can exhibit +1 and+3 oxidation states but it is stable in +1 oxidation state only due to inert pair effect.

2) In 14^th group , lead shows both +2 and +4 oxidation states but it is stable in +2 oxidation state due to inert pair effect.