Solution

Thallium is more stable in the +1 state because of the inert pair effect.

$Tl^{+3}$ is less stable than $Tl^{+1}$ (inert pair effect). Going down group 13, stability of lower oxidation state increases. In case of B, Al, Ga and In, higher O.S. +3 remains more stable than lower O.S. +1. But in the last element, thallium (Tl), lower O.S. +1 becomes more stable than higher O.S. +3.

Concept stability of +1 vs +3 oxidation state

The p-Block Elements (Group 13 and 14) in past papers

9 questions from this chapter have appeared across 4 exam years.