The anomalous behavior of the first elements in the groups of p block elements is due to their small size, high ionization enthalpies, large charge to radius ratio, unavailability of d orbitals in the valence shell and high electronegativity.
The first element in the p block elements have the four valence orbitals, one orbital is 2s and the other three are 2p so, the maximum covalency of the first element in each group is limited to the four. While in the valance shells of the other p block elements there are vacant d orbitals so they show the covalency of greater than four. Nitrogen can form the pentoxides but it cannot form the pentachloride. Due to their valency, the coordination number is limited to four, however, the higher coordination number is attainable for the compounds of the heavier members.
By moving across the period, the atomic radii are decreased. Due to this reason, the elements of the p block are smaller than the elements of s block and d block. The atom of fluorine has the smallest radius. The electronegativities of fluorine, nitrogen, and oxygen are much higher as their size is small. It is also reflected from the phenomenon that they form relatively stronger bonds with the hydrogen.
The first member in the groups of the p block elements have great ability to make the pi-pi bonds with itself and with other elements of the second period as compare to other members of the groups and it accounts for the greater differences. This is due to the reason that p orbitals of the heavier are large and diffused so effective sideway overlapping is not possible. Nitrogen, carbon, and oxygen are greatly different from the other elements in their respective groups as they have the unique ability to make the multiple bonds which are specifically pπ–pπ and this happens due to the reason that their atomic orbitals are too much large for achieving the effective overlapping.
Due to high electronegativity and small size, and due to the presence of the ion pairs of the electrons, when the first elements are bonded to the hydrogen atoms, they make the intermolecular hydrogen bonds which are stronger than any other intermolecular forces. It results in the high melting points and boiling points of the compounds of the first element of each group in the p block elements.
Oxygen is a diatomic gas while the other members of the group are solids. It is highly non-metallic In nature the molecule of oxygen is paramagnetic while the molecules of the other elements are the diamagnetic. Nitrogen is gas and other members in its group are solids. The molecule of nitrogen is diatomic while the molecules of other members are tetratomic and it can make the trinegative ions. As compare to the other hydrides of the group the hydrides of nitrogen are stable. Among all of the halogens, the fluorine is the most reactive gas. It only shows the negative oxidation state while the oxidation states of other elements of this group are positive.