The first element of each of the groups 1 (lithium) and 2 (beryllium) and groups 13-17 (boron to fluorine) differs in many respects from the other members of their respective group. The difference in the behaviour of the first member of a group in the s- and p-block compared to the other members in the same group can be due to the following factors:
(i) Small atomic size of the first element.
(ii) Large charge/radius ratio
(iii) High electronegativity
(iv) The absence of d-orbitals in the valence shell of the first element.
The first member of s- and p-block has only four valence orbitals (2s and 2p) available for bonding. As a result, the maximum covalency of the first member of each group is 4. On the other hand, the second member of the groups have nine valence orbitals (3s, 3p, 3d). Hence, these members can expand their covalency beyond 4, through the participation of d-orbital in bond formation.
(v) Ability to form pπ-pπ multiple bonds. The first member of each group of p-block elements has great tendency to form pπ-pπ multiple bonds to itself (C = C. C ≡ N, N = N, N ≡ N and to the other second period elements (C = O, C = N, C ≡ N, N = O).