Three pairs of compounds are given below. Identify that compound in each of the pairs which has group 13 element in more stable oxidation state. Give reason for your choice. State the nature of bonding also.

1.  TlCl3, TlCl

2.  AlCl3, AlCl

3.  InCl3, InCl

(a) TlCl more stable than TlCl3 due to inert pair effect. TlCl3 is less stable and covalent in nature but TlCl is more stable and ionic in nature.
(b) Due to absence of d-orbitals, Al does not show inert pair effect. Hence, its most stable oxidation state is +3. Thus, AlCl3 is much more stable than AlCl. Further, in the solid or the vapour state, AlCl3 covalent in nature but in aqueous solutions, it ionises to form a Al3+ (aq) and Cl- (aq) ions.
(c) Due to inert pair effect, indium exists in both +1 and +3 oxidation states out of which +3 oxidation state is more stable than +1 oxidation state. In other words, InCl3 is more stable than InCl. Being unstable, InCl undergoes disproportionation reaction.
$3\mathrm{InCl}\left(\mathrm{aq}\right)\to 2\mathrm{In}\left(\mathrm{s}\right)+{\mathrm{In}}^{3+}\left(\mathrm{aq}\right)+3{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)$