The inert-pair effect is the tendency of the two electrons in the outermost atomic-s orbital to remain unshared in compounds of post-transition elements.
Inert-pair effect is seen in the elements of 13th,14th, 15th and 16th groups.
Due to the inert-pair effect elements down the group exhibit oxidation state which is 2 units lesser than the elements above them.
Stability of +1 oxidation state due to inert pair effect is in the order Al<Ga<In<Tl