CBSE Class 12-science Answered

The inert pair effect is the reluctance of s-electrons to take part in bond formation. A chemical consequence is the occurrence of oxidation states which are two units less than the group oxidation.

Let us understand this in detail.

The electronic configuration of Group 13 elements is ns²np¹. Hence, they would be expected to be trivalent. 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.

For example:

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.