`=>` 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.
`text(Example :)` ● Lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds.
`text(Diagonal Relationship :)` The behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.
`=>` The anomalous behaviour is attributed to :
(i) small size
(ii) large charge/ radius ratio
(iii) high electronegativity of the elements.
(iv) The first member of group has only four valence orbitals (`2s` and `2p`) available for bonding, whereas the second member of the groups have nine valence orbitals (`3s`, `3p`, `3d`).
● As a consequence of this, the maximum covalency of the first member of each group is `4` (e.g., boron can only form `[BF_4]^-`, whereas the other members of the groups can expand their valence shell to accommodate more than four pairs of electrons e.g., aluminium forms `[AlF_6]^(3-)`.
(v) Furthermore, the first member of `p`-block elements displays greater ability to form `pπ – pπ` multiple bonds to itself (e.g., `C = C`, `C ≡ C`, `N = N`, `N ≡ Ν`) and to other second period elements (e.g., `C = O`, `C = N`, `C ≡ N`, `N = O`) compared to subsequent members of the same group.