Thallium belongs to group $13$ of the periodic table. The most common oxidation state for this group is $+3$. However, heavier members of this group also display the $+1$ oxidation state.

This happens because of the inert pair effect. Aluminium displays the $+3$ oxidation state and alkali metals display the $+1$ oxidation state.

Thallium displays both the oxidation states. Therefore, it resembles both aluminium and alkali metals.

Thallium, like aluminium, forms compounds such as $\mathrm{TlCl}_{3}$ and $\mathrm{Tl}_{2} \mathrm{O}_{3} .$ It resembles alkali metals in compounds $\mathrm{Tl}_{2} \mathrm{O}$ and $\mathrm{TlCl}$.