Tendency for  is highest for

For 13Th group elements the highest oxidation state is due to the involvement of valency ns electrons and np electrons  The higher oxidation state stability from top to bottom in a group decreases.Stability of +III oxidation state : Al+3 > Ga+3 > In+3 > Tl+3 Al is more stable in +3 oxidation stateWhere as +1 oxidation state stability from top to bottom increases,due to inert pair effect.Stability of +1 oxidation state : Ga+1 < In+1 < Tl+1