The first member of each group of p-block differs in many respects from its succeeding members (called congeners) of their respective groups. For example, boron shows anomalous behaviour as compared to rest of the members of the 13 group elements. The main reasons for the different behaviour of the first member as compared to other members is because of:
i) small size of the atom and its ion
ii) high electronegativity and
iii) absence of d-orbitals in their valence shell
These factors have significant effect on the chemistry of first element as compared to other elements (specially second). For example
a) Covalence upto four:
First member of each group belongs to second period elements and have only four valence orbitals i.e., one 2s and three 2p orbitals available for taking part in chemical combinations. They do not have vacant d-orbitals in their valence shell. Therefore, they may have maximum covalence of four (using one 2s and three 2p orbitals). In contrast, the next members belonging to third or higher periods have vacant d-orbitals. For example, the elements of third period of p-block with the electronic configuration 3s2 3p1-6 has vacant 3d-orbitals lying between 3p and 4s energy sub-shells.
b) Reactivity:
Due to availability of d-orbitlals of elements of third period, they are more reactive than elements of second period which do not have d-orbitals. For example, tetrahalides of carbon are not hydrolysed by water whereas tetrahalides of other elements of group 14 are readily hydrolysed. The hydrolysis involves the nucleophilic attack of water molecule.
c) Tendency to form multiple bonds:
Because of the combined effect of smaller size and nonavailability of d-orbitals, the first member of each group shows tendency to form pπ –pπ multiple bonds either with itself (such as C = C, C ≡ C, N ≡ N, O = O) or with other members of the second period of elements (such C = O, C ≡ N, N = O, etc). The other members of the group do not have strong tendency to form π - bonding. The heavier elements do form π -bonding but they involve d- orbitals and form dπ –pπ or dπ - dπ bonding. For example, the bonds between sulphur and oxygen in oxides of sulphur (SO2 and SO3 ) are much shorter than might be expected for a single bond. In these molecules, in addition to normal π bond, a π bond is also formed by the sidewise overlap of a filled 2p-orbital of oxygen with a vacant 3d-orbital on the sulphur). This is called pπ - dπ bond and results in bringing the two atoms closer and thus accounts for shorter bond length of S-O bond.
Because the d-orbitals are of higher energy than p-orbitals, they contribute less to the overall stability of molecules than does the pπ -pπ bonding of second row elements. However, the coordination number in species of heavier elements may be higher in those of first element in the same oxidation state. For example, both nitrogen and phosphorus form ions in + 5 oxidation state as NO\(^-_3\)(three coordination with bonding using one p-orbital of N) ; PO\(^{3-}_4\) (having four coordination using s, p and d orbitals contributing to the π -bonding).
