Q.The stability of $+1$ oxidation state increases in the sequence ________.

The tendency of the heavier group-13 elements to show the lower $+1$ oxidation state (instead of the group's characteristic $+3$) grows steadily down the group due to the inert pair effect, becoming most pronounced for thallium.

Group 13 elements (B, Al, Ga, In, Tl) have the outer electronic configuration $ns^2np^1$, so their characteristic oxidation state is $+3$ (using all three valence electrons). However, going down the group, the inert pair effect becomes increasingly significant: the pair of $ns^2$ electrons becomes progressively more reluctant to participate in bonding (due to poor shielding by the intervening $d$- and $f$-electrons in the heavier elements, and relativistic effects, which increase the effective nuclear attraction on the $ns$ electrons), so only the single $np^1$ electron is readily used, favouring the $+1$ oxidation state over $+3$.

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