Aluminium is usually found in +3 oxidation state. In contarast, thallium exists in +1 and +3 oxidation states. This is due to :
For "Tl" +1 oxidation state is more stable,due to inert pair effect.
NOTE: For IIIA group higher oxidation state is due to involvement of ns and np electrons.
IIIA group ns2 np1 +3 oxidation state
Lower oxidation state is due to involvement of only valency "np" electrons.
IIIA group ns2 np1 +1 oxidation state
Higher oxidation state stability decreases from top to bottom.
Lower oxidation state stability increases from top to bottom.
Stability order of +3 oxidation state: B+3 >Al+3 > Ga+3 > In+3 >Tl+3
Stability order of +1 oxidation state: B+1 > Al+1 >Ga+1 > In+1 >Tl+1
Therefore, Tl+3 is acting as strong oxidising agent.
Due to poor shielding effect by the innershell f- and d-orbital electrons ,the valency s-orbial electrons are strongly held by the nucleus,therefore valency s-orbital electron pair does not involve in bonding [inert pair] and hence only one p-orbital electron involves in bonding and shows mainly +1 oxidation state. Hence +1 oxidation state is stable for Tl is due to inert pair effect.