Which is more stable among `Tl^(+3)` and `Tl^(+1)`. Why?

Explanation

`Tl^(1+)` is more stable than `Tl^(3+)` . It is due to inert pair effect.
With increases in size of atom in a group. The s electrons present in the outermost orbit are reluctant in involving bond formation.
Hence, in thallium, only p electron involves in bond formation.