In some of the reactions thallium resembles aluminium, whereas in others it resembles with group I metals.

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