# Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in +1 and +3 oxidation states. This is due to:A. lanthanide contractionB. lattice effectC. diagonal relationshipD. inert pair effect

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Hint: The term inert pair effect is commonly employed in reference to the increasing stability of oxidation states that are two but the group valency for the heavier elements of groups 13, 14, 15 and 16. The name suggests that the outermost s electrons are more tightly sure to the nucleus in these atoms, and thus harder to ionize or share.

Note: Aluminum has three oxidation states. The foremost common one is +3. The opposite two are +1 and +2. One +3 number for Aluminum is often found within the compound aluminium oxide, $A{{l}_{2}}{{O}_{3}}$.