Question

Nitrogen exists as a diatomic molecule, while the other elements of group 15 exist as polyatomic molecules. Explain.

Answer 1

1. Nitrogen has a very small atomic size compared to other group 15 elements.
2. It has high ionisation energy and high electronegativity.
3. Nitrogen lacks vacant d-orbitals in its valence shell, unlike other elements which have d-orbitals.
4. Due to its small size, nitrogen forms strong pπ-pπ multiple bonds (triple bond) with itself, leading to a very stable diatomic N₂ molecule.
5. Other elements like phosphorus have a larger size and possess vacant d-orbitals, allowing them to form polyatomic molecules such as P₄ by forming single bonds.
6. The bond energy of the nitrogen triple bond (N ≡ N) is very high (~945 kJ/mol), making the diatomic form very stable and less reactive.

Therefore, nitrogen exists as N₂ (diatomic), while phosphorus, arsenic, etc., exist as P₄, As₄ (polyatomic) molecules due to their inability to form strong multiple bonds like nitrogen.