(a) Inert pair effect
As one moves down the group, the tendency of s-block electrons to participate in chemical
bonding decreases. This effect is known as inert pair effect. In case of group 13 elements,
the electronic configuration is ns2 np1 and their group valency is +3. However, on moving
down the group, the +1 oxidation state becomes more stable. This happens because of
the poor shielding of the ns2 electrons by the d- and f- electrons. As a result of the poor
shielding, the ns2 electrons are held tightly by the nucleus and so, they cannot participate
in chemical bonding.
Allotropy is the existence of an element in more than one form, having the same chemical properties but different physical properties. The various forms of an element are called allotropes. For example, carbon exists in three allotropic forms: diamond, graphite, and fullerenes.
The atoms of some elements (such as carbon) can link with one another through strong covalent bonds to form long chains or branches. This property is known as catenation. It is most common in carbon and quite significant in Si and S.