1. Nitrogen has a small size, high electronegativity, high ionisation enthalpy as compared to phosphorus.
2. Nitrogen does not contain vacant d-orbitals in its valence shell whereas phosphorus contains vacant d-orbitals in its valence shell.
3. Nitrogen has the ability to form triple bond ( N ≡ N) as a result of which its bond enthalpy (941.4 kJ mol-1) is very high making it less reactive.