Aliminium is usually found in +3 oxidation state ,In contrast, thallium exiists in +1 and +3 oxodtion states .This is due to :
inert pair effect
diagonal relation ship
Inert pair effect -
The phenomenon in which outer shell s electrons (ns2) penetrate to (n-1) d-electrons and thus become closer to the nucleus and are more effectively pulled by the nucleus
This results in less availability of ns2 electrons pair for bonding or ns2 electron pair becomes iner
As we know that
The inert pair effect is defined as the tendancy of electrond is the outermost atomic 'S' orbital to remaon unionized is compounds of post-transition metals.Due to thos ineret pair effect Al usually shows +3 O.S. and Tl shows +I oxidation state.