Chemistry - Complete Pblock- Question with Solution | TestHub

The decreasing stability of the +3 oxidation state in Group 13 with increasing atomic number is due to the "inert pair effect." This effect causes the $n{s}^2$ electrons to become increasingly reluctant to participate in bonding down the group. The energy required to unpair these $n{s}^2$ electrons is not sufficiently compensated by the energy released from forming two additional bonds, leading to the preference for the +1 oxidation state for heavier elements like Tl.