Question

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

• A. Al < Ga < In < Tl
• B. Tl < In < Ga < Al
• C. In < Tl < Ga < Al
• D. Ga < In < Al <Tl

Answer 1

The correct option is A Al < Ga < In < Tl

Within the group, as we go down, the stability of the +1 oxidation increases while the stability of the +3 oxidation state decreases.
This is because the outermost (valence) $n{s}^2$ electrons tend to not take part in bond formation. This is popularly referred to as
the inert pair effect.