Discuss the pattern of variation in the oxidation states of (i) B to Tl and (ii) C to Pb.

Explanation

(i)B to Tl : The electric configuration of group 13 elements is `ns^2np^1`. Therefore , the most common oxidation state exhibited by them should be +3. However, it is only boron and aluminium which practically show the +3 oxidation state.
The remaining elements i.e., Ga,In, Tl , Show both the +1 and +3 oxidation states . On moving down the group , the +1 state becomes more stable . For example , Tl(+1) is more stable than Tl (+3). This is because of the inert pair effect.
The two electrons present in the s-shell are strongly attracted by the nucleus and do not participate in bonding .
This inert pair effect becomes more and more prominent on moving down the group . Hence, Ga(+1) is unstable , In (+1) is fairly stable and Tl (+1) is very stable.

The stability of the +3 oxidation state decreases on moving down the group.
(ii)C to Pb : The electronic configuration of group 14 elements is `ns^2 np^2`. Therefore , the most common oxidation state exhibited by them should be +4. However, the +2 oxidation state becomes more and more common on moving down the group . C and Si mostly show the +4 state.
On moving down the group, the higher oxidation state becomes less stable. This is because of the inert pair effect. Thus, although Ge, Sn and Pb show both the +2 and +4 states, the stability of the lower oxidation state increases and that of the higher oxidation state decreases on moving down the group.