Question

`Al` and `Ga` are trivalent in their compounds but monovalent compounds are the most stable down the `13th` group. Why?

Answer 1

Down the group `(13th)`, the stability of `+3` state decreases and that of `+1` state increases due to the prominent "inert pair" effect.
`Al^(3+)gtGa^(3+)gtIn^(3+)gtTI^(3+)`
Most stable `to` least stable
`Tl^(+) gt In^(+) gt Ga^(+) gt Al^(+)`