(i) In pentahalides, the oxidation state is +5 and in trihalides, the oxidation state is +3. Since the metal ion with a high charge has more polarizing power, pentahalides are more covalent than trihalides.

(ii) As we move down a group, the atomic size increase and the stability of the hydrides of group 15 elements decrease. Since the stability of hydrides decrease on moving from NH₃ to BiH₃, the reducing on moving from NH₃ to BiH₃.

(iii) Nitrogen has a small size due to which the lone pair of electrons is concentrated in a small region. This means that the charge density per unit volume is high.

On moving down a group, the size of the central atom increases and the charge gets distributed over a large area decreasing the electron density. Hence, the electron donating capacity decreases on moving down the group.

(iv) Nitrogen owing to its small has a tendency to from pπ− pπ multiple bonds with itself. Nitrogen thus forms a very stable diatomic molecule, N₂. On moving down a group, the tendency to form pπ − pπ bonds decrease (because of the large size of heavier elements). Therefore, phosphorus (like other heavier metals) exists in the P₄ state.

(v) N (unlike P) lacks the d-orbital. This restricts nitrogen to expand its coordination number beyond four. Hence, R₃N = O does not exist.