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 $TlCl _{3}$ and $Tl _{2} O _{3}$. It resembles alkali metals in compounds $Tl _{2} O$ and $TlCl$.