Q.In some of the reactions thallium resembles aluminium, whereas in others it resembles with group I metals. Support this statement by giving some evidences.

1. Resemblance to aluminium (Group 13, +3 state). As a genuine member of Group 13, thallium can exist in the +3 oxidation state, paralleling aluminium's chemistry:

   • Tl$_2$O$_3$ shows a broadly similar pattern to Al$_2$O$_3$'s Group 13 oxide behaviour.
   • TlCl$_3$, like AlCl$_3$, exists and shows covalent character.
   • Tl$^{3+}$ forms complex ions in solution in a manner comparable to Al$^{3+}$.

2. Resemblance to Group 1 (alkali) metals (+1 state). Down Group 13, the inert pair effect (the reluctance of the $ns^2$ electron pair to participate in bonding) becomes progressively stronger, and for thallium it is strong enough that the +1 oxidation state is actually more stable than +3 — the reverse of aluminium, for which +3 is essentially the only important state. This makes Tl$^+$ chemistry dominant, and Tl$^+$ behaves very much like an alkali metal cation:

   • TlOH is a strong base, freely soluble in water — like NaOH or KOH — quite unlike the weak, essentially insoluble amphoteric Al(OH)$_3$.
   • Tl$_2$CO$_3$ is soluble in water, again paralleling alkali metal carbonates (e.g. Na$_2$CO$_3$), whereas a stable aluminium carbonate does not even exist under normal conditions (Al$^{3+}$ is too small/highly charged to stabilise the carbonate ion).
   • Tl$^+$ salts in general show ionic, alkali-metal-like solubility and lattice behaviour rather than the more covalent character typical of Al$^{3+}$ compounds. …

Recall that thallium, at the foot of Group 13, is strongly affected by the inert pair effect, making Tl(I) more stable than Tl(III) — unlike lighter Group 13 members. Compare specific compounds: Tl(III) compounds resembling Al (TlCl$_3$, Tl$_2$O$_3$) vs Tl(I) compounds resembling alkali metals …

• Stating thallium "is like aluminium" or "is like an alkali metal" without distinguishing which oxidation state (+3 vs +1) each resemblance applies to.
• Forgetting to invoke the inert pair effect as the underlying reason +1 becomes the more stable, dominant state for Tl. …