Inert Pair Effect

Just as the alkali and alkaline earth metals show the group oxidation states +1 and +2 (corresponding to the total number of valence electrons they carry), the p-block elements too generally show their group oxidation state as their highest state, alongside other, lower states.

For the heavier post-transition elements of groups 13 to 16, however, the most stable oxidation state turns out to be two less than the group oxidation state, with a growing reluctance to actually reach the full group oxidation state as the group is descended. In group 13, for example, the tendency to favour +1 over the group state of +3 grows from aluminium to thallium: Al³⁺ is more stable than Al⁺, while for thallium the position is reversed and Tl⁺ is more stable than Tl³⁺. This is why aluminium(III) chloride is a perfectly stable compound, while thallium(III) chloride is highly unstable and spontaneously disproportionates to thallium(I) chloride plus chlorine gas. …