Aluminium shows +3 oxidation staet in its compounds but in case of last element thallium , +1 oxidation state is more common. How will you justify this trend?

Explanation

The above trend is explained with the help of the phenomenon of inert pair effect, it represets the reluctance or inertness of the valence s-electrons of heavier elemetns of p-block to take part in the bond formation because of ineffective shielding of these electrons from the attraction of nucleus by the intervening d and f electrons. The inert pair effect becomes more or more pedominant as we go down the group because of effect of increased nuclar charge outweights the effect of increased atomic ize. inother words, the valence s-electrons become more and more reluctnat to be availbel the bond formation. As a result, the valence p -electron will be more availbel accounting for+1 oxidation state and +3 oxidation state will not be shown by these elemetns to easily. the u=inert parir effect is maximum in the last element thallium (Ti) in boron faimly since it is the heaviest with maximum atomic number (Z=81). therefore, it shows mainly +1 oxidation state or we can say that TiCl exists while `TiCl_(3)` is rather unstable keeping this in mind, the relative stablities of `M^(+)` (monovalent) and `M^(3+)` cations follows the order:
`B^(3+)gtAl^(3)gtGa^(3+)gtIn^(3+)gtTi^(3+)`
`B^(+)gtAl^(+)ltAl^(+)ltGa^(+)ltIn^(+)ltTi^(+)`