1. B to Tl- they belong to group 13 elements with electric configuration as ns² np¹. The atoms of these elements have 3 valence electrons, two in s subshell & one in p subshell,therefore all these elements show maximum of + 3 oxidation state. Boron shows only +3 oxidation state in its compound & other elements also show +1 oxidation state.The +1 oxidation state becomes more stable as we move down to the group from boron to thallium.The +1 oxidation state is more stable than +3 oxidation state because of inert pair effect .In the case of last element ,after the removal of one electron from p orbital, the remaining ns² electrons behave like stable noble gases& donot take part in compound formation. This reluctance of the s electron pair to take part in chemical combination is called inert pair effect. The two electrons present in the s-shell are strongly attracted by the nucleus and do not participate in bonding. This inert pair effect becomes more and more prominent on moving down the group. Hence, Ga (+1) is unstable, In (+1) is fairly stable, and Tl (+1) is very stable

The stability of the +3 oxidation state decreases on moving down the group.

(ii) C to Pb- they belongs to group 14 in periodic table & are also called carbon family with electronic configuration of ns² np². Therefore, the most common oxidation state exhibited by them should be +4. However, the +2 oxidation state becomes more and more common on moving down the group. C and Si mostly show the +4 state. On moving down the group, the higher oxidation state becomes less stable. This is because of the inert pair effect. Thus, although Ge, Sn, and Pb show both the +2 and + 4 states, the stability of the lower oxidation state increases and that of the higher oxidation state decreases on moving down the group.