Q.Discuss the pattern of variation in the oxidation states of

Step 1 - Group 13 (B to Tl)

All group 13 elements have valence configuration $ns^2np^1$, so the group oxidation state is $+3$. B and Al show almost exclusively $+3$. From Ga onward the $+1$ state starts appearing and becomes progressively more stable down the group, because the $ns^2$ electron pair becomes increasingly reluctant to take part in bonding (the inert pair effect, from poor shielding by intervening d/f electrons). For Tl, $+1$ is in fact the MORE stable oxidation state; $\text{Tl}^{3+}$ compounds are comparatively rare and strongly oxidising.

Step 2 - Group 14 (C to Pb)

Valence configuration $ns^2np^2$; the group oxidation state is $+4$. C, Si, Ge predominantly show $+4$ (carbon also shows extensive catenation and a $-4$ state with electropositive elements). Down the group, $+2$ becomes increasingly stable (again the inert pair effect on the $ns^2$ electrons) - Ge shows both $+2$ and $+4$ (with $+4$ more stable), Sn shows both with comparable stability, and for Pb the $+2$ state is the most stable, with $\text{Pb}^{4+}$ compounds being strongly oxidising (e.g. $\text{PbO}_2$ is a good oxidising agent).

Step 3 - Common thread

In both groups, the trend is governed by the increasing inert pair effect down the group - the $ns^2$ pair becomes progressively harder to involve in bonding, so the oxidation state two less than the group oxidation state becomes progressively more stable, culminating in it being the dominant state for the heaviest member ($\text{Tl}^{+1}$, $\text{Pb}^{2+}$).

Write the ns2np1 / ns2np2 valence configurations for the two groups; explain the inert pair effect (ns2 pair increasingly reluctant to bond down the group); trace how the lower oxidation state (+1 for group 13, +2 for group 14) becomes progressively more stable, dominating for Tl and Pb respectively.

• Stating the inert pair effect makes the HIGHER oxidation state more stable - it is the reverse; it stabilises the LOWER state (group oxidation state minus 2).
• Forgetting carbon's distinguishing feature of catenation and its -4 state, treating group 14 purely in terms of +2/+4.
• Saying Tl and Pb "cannot" show +3/+4 at all - they can, but those states are less stable/strongly oxidising, not nonexistent.