The stability of +1 oxidation state among Al, Ga, In and TI increases in the sequence (AIPMT 2015)

Inert pair effect is defined as the tendency of two outermost s-orbital to remain unshared in the compounds, this is due to the poor shielding effect of d- orbitals. Therefore, the effect of the inert pair effect increases down the group. Hence, the stability of the +1 oxidation state increases down the group, and the order is as follows: Al < Ga < In < Tl .