The +1 oxidation state in group 13 and +2 oxidation state in group 14 becomes more and more stable with increased atomic number. Explain.

Explanation

The term inert pair effect is often used in relation to the increasing stability of oxidation states that are 2 less than the group valency for the heavier elements of groups 13,14,15 and 16. in group 13 all the elements show +3 oxidation state whereas Ga, In and Tl show +1 oxidation state also
Boron, being small in size can lose its valence electrons to form `B^(3+)` ions and shows +3 oxidation state
+1 oxidation state of Tl, Ga is due to inert pair effect. The outer shell s-electrons `(ns^(2))` penetrate to (n-1) d-electrons and thus becomes closer to nucleus and are more effectively pulled towards the nucleus
The results in less availability of `ns^(2)` electrons pair for bonding or `ns^(2)` electron pair becomes inert. This reluctance in the participation of `ns^(2)` electrons in bonding is termed as inert pair effect. The inert pair is more effective after `n ge 4` and increases with incresing value of n. For groups 14, in spite of four valence electrons, they do not form `M^(4+)` or `M^(4-)` ionic compounds. They form covalent compounds with four bonds.
Ge,Sn and Pb also exhibit +2 oxidation state due to inert pair effect `Sn^(2+) and Pb^(2+)` show ionic nature. The tendency to form +2 ionic state increases on moving down the group due to inert pair effect.