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. Aluminum displays the + 3 oxidation state and alkali metals display the + 1 oxidation state. Thallium displays both the oxidation states. Therefore, it resembles both aluminum and alkali metals. Thallium, like aluminum, forms compounds such as TlCl3 and Tl2O3. It resembles alkali metals in compounds Tl2O and TlCl.
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