(2)

As we move down the group, the stability of the +3 oxidation state decreases while that of +1 oxidation state increases. Thus, stability order for +1 oxidation state is $\mathrm{Al}<\mathrm{Ga}<\mathrm{In}<\mathrm{Tl}$ and +3 oxidation state is $\mathrm{Al}>\mathrm{Ga}>\mathrm{In}>\mathrm{Tl}$.

Hence, the stability order is, ${\mathrm{AlCl}}_3>\mathrm{AlCl}, \mathrm{TlI}>{\mathrm{TlI}}_3, \mathrm{TlCl}>{\mathrm{TlCl}}_3$ and $\mathrm{InI}>{\mathrm{InI}}_3$.

(4)

${\mathrm{BF}}_3$ is a $s{p}^2$-hybridised planar molecule. It forms $3\mathrm{\sigma }$-bonds with 3F-atoms, hence has six electrons around it.

(4)

(1)

In group 13 elements, stability of +3 oxidation state decreases down the group while that of +1 oxidation state increases due to inert pair effect. Hence, stability of +1 oxidation state increases in the sequence: $\mathrm{Al}<\mathrm{Ga}<\mathrm{In}<\mathrm{Tl}$.