Why does the reactivity of nitrogen differ from phosphorus?

Explanation

Reactivity of nitrogen differs from phosphorous due to the fact that nitrogen `(N_2)` exists as a diatomic molecule. Due to presence of triple bond between the two N-atoms `(N-=N)` the bond dissociation energy is very high, consequently nitrogen is inert and unreactive in its elemental state. On the other hand, phosphorous exists as a tetraatomic molecule `(P_4)` since the `P-P` bond strength is very weak and bond angle `PPP=60^@` which also results in high angular strain `P_4` molecule is highly unstable or reactive.