Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in +1 and +3 oxidation states. This is due to:

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Strategy:
 The "Inert Pair Effect" refers to the reluctance of the outermost -electrons to participate in bonding in heavy -block elements (Periods 5 and 6). This leads to a stable oxidation state that is 2 units less than the group state.

Step 1: Apply to Group 13
Group configuration: .
- Aluminum: Readily uses all 3 electrons ( state).
- Thallium (Period 6): The  electrons are deeply penetrating and exhibit poor shielding by  and  electrons, effectively becoming "inert".
- Consequently, Thall

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