What do you understand by (a) inert pair effect    (b) allotropy and    (c) catenation?

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 15 elements, the electronic configuration is ns2 np3 .Group 15 elements also show positive oxidation states of +3 & +5 by forming covalent bonds. Due to the inert pair effect, the stability of +5 oxidation state decreases down the group, while that of +3 oxidation state increases. This happens because of the poor shielding of ns2 electrons by the d- and f- electrons.

allotropy: 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 exist in three allotropic forms such as diamond. graphite and fullerenes.

Catenation: carbon atoms can link with one another by means of covalent bonds to form a long chain or rings of carbon atoms. This property of carbon element due to which its atoms can join with one another to form a long chain is called catenation. It is most common in carbon and quite significant in Si and S.