Question

aluminium shows +3 oxidation state in its compounds , but thallium shows +1 oxidation state

Answer 1

In boron family B and Al have the tendency to show +1 ,+3 oxidation state out of these two +3 is more stable and this tendency of higher oxidation state reduces down the group due to the inert pair effect e.g Tl can also show +1 ,+3 but +1 is more stable.
INERT PAIR EFFECT:- The inertness of s subshell electrons towards the bond formation is called inert pair effect or it can be said as the inactiveness of electrons present in outermost shell (i.e. ns²) to get unpaired and involve in bond formation is called inert pair effect.