Nitrogen exists as diatomic molecule and phosphorus as `P_(4)`. Why?

Explanation

In the form of elemental nitrogen it exists as a diatomic molecule `(N_(2))`. This is due to the fact that nitrogen can form `p pi - p pi` multiple bond `(N -=N)` because of small of nitrogen atom. Heavier elements of this group do not able to form `p pi - p pi` bonds as their atomic orbitals are so large and diffuse that they cannot have effective overlapping. Further `P -P` single bond is stronger than `N - N` single bond. Hence phosphorus as tendency to under go catenation