Nitrogen exists as a diatomic molecule `(N-=N)`. Due to the presence of a triple bond between the two N-atoms, the bond dissociation energy is larger `(941.4kJ mol^(-1))`. As a result nitrogen is inert and unreactive in its elemental state.
In contrast, white or yellow phosphorus exists as a tetra-atomic molecule `(P_(4))`. Since the P-P single bond is much weaker `(213 kJ mol^(-1))` than `N-=N` triple bond bond `(941.4 kJ mol^(-1))`, therefore, phosphorus is much more reactive than nitrogen.
