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Question:

The stability of +1 oxidation state among Al, Ga, In, and Tl increases in the sequence:

Tl<In<Ga<Al

In<Tl<Ga<Al

Al<Ga<In<Tl

Ga<In<Al<Tl

Solution:

The stability of +1 oxidation state among Al, Ga, In, and Tl increases in the sequence: Al < Ga < In < Tl. This is due to the inert pair effect. Due to the inert pair effect, on moving down the group, the stability of +3 oxidation state decreases while the stability of +1 oxidation state increases.