(i) The tendency of formation of long open or closed atom chains by the combination of same atoms in themselves is known as catenation.

(ii) The catenation is maximum in carbon and decreases down the group.

(iii) This is due to high bond energy of catenation.

(iv) Only carbon atoms also form double or triple bonds involving pπ-pπ multiple bond within itself.

(v) Carbon also possesses the tendency to form closed chain compounds with O, S and N atoms as well as forming pπ-pπ multiple bonds with other elements particularly nitrogen and oxygen e.g. C =O; C=N; C ≡ N; C = S are the functional groups present in numerous molecules due to this reason.

(vi) Carbon can form chain containing any number of carbon atoms Si and Ge cannot extend the chain beyond 6 atoms, while Sn and Pb do not form chains containing more than one or two atoms.

(vii) The reason for greater tendency of carbon for catenation than other elements in the group may further be explained by the fact that the C – C bond energy is approximately of the same magnitude as the energies of the bond between C and other elements. On the other hand, the Si – Si bond is weaker than the bond between silicon and other elements.