CBSE Class 11-science Answered

As you go down the group the differentiating electrn enters the f orbital which very less sheilding effect thus increasing the effective nuclear charge. The hold of the nucleas on the outter s orbit electrons increases thus making it inert. Thse pair of electrons in the outtermost s orbital are inert and do not take part in reaction, is the inert pair effect.

The effect is in p block the last elements in group show lower  stable oxidation state than the other members of the group.