Why does the reactivity of nitrogen differ from phosphorus ?

Explanation

Reactivity of nitrogen differs from phosphorus because :
(i) Bond dissociation enthalpy of nitrogen is very high because it forms pn-pn multiple bond due to its small size while P4 molecule is tetrahedral with high angle strain (60°) which makes it more reactive than nitrogen.
(ii) Phosphorus can expand its covalency due to presence of d-orbitals in its valance shell. Also, it forms pn-dn bonds (`R_3P` = O). Whereas maximum covalency of nitrogen is 4 due to absence of d-orbitals.