Q.Why does the reactivity of nitrogen differ from phosphorus?

Step 1: Bonding in elemental nitrogen.

Nitrogen's small atomic size allows effective sideways ($p\pi$–$p\pi$) overlap between two N atoms, giving the very strong triple bond in $N \equiv N$ (bond enthalpy $\approx 941\ kJ\,mol^{-1}$). This exceptionally strong bond makes $N_2$ kinetically and thermodynamically very stable/unreactive at ordinary temperatures.

Step 2: Bonding in elemental phosphorus.

Phosphorus is a larger atom, so effective $p\pi$–$p\pi$ overlap between two P atoms is not possible (the atomic orbitals are too diffuse/far apart for good sideways overlap). Phosphorus therefore cannot form a stable $P \equiv P$ triple bond, and instead catenates through single P–P $\sigma$ bonds, existing as tetrahedral $P_4$ molecules.

Step 3: Comparing bond strengths.

A single P–P bond is considerably weaker than the N≡N triple bond, so $P_4$ is far more reactive than $N_2$ — white phosphorus even ignites spontaneously in air, while $N_2$ requires very forcing conditions (e.g. lightning, or the Haber process catalyst/high pressure/temperature) to react.

Step 4: Role of d-orbitals.

Additionally, phosphorus has accessible 3d orbitals allowing it to expand its octet and form a wider variety of compounds/oxidation states more readily than nitrogen (2p, no d-orbitals), which also contributes to phosphorus's greater chemical versatility/reactivity.

Contrast the bonding capability of N (small, strong pπ–pπ triple bond, inert N2) against P (larger, no effective pπ–pπ overlap, weaker single-bonded P4), then connect bond strength to reactivity.

• Do not attribute the reactivity difference merely to "nitrogen is unreactive, phosphorus is reactive" without the underlying pπ–pπ overlap/bond-strength reasoning.
• Do not confuse this question with the catenation-tendency question (Q14) — this one is about elemental N₂ vs P₄ reactivity, not the general catenation ranking across the whole group.
• Remember P₄'s reactivity is due to weak P–P bonds, not because phosphorus atoms are "unstable" in some vague sense.