Group 15 Elements (N, P, As): JEE Main Guide

Group 15 (the nitrogen family) contributes two to three questions per JEE Main session and is among the most detail-heavy inorganic chapters. The volume of reactions, oxyacids, and anomalous properties can feel overwhelming, but the facts follow logical patterns. This guide organises Group 15 chemistry by the patterns that make it memorable, covering every topic the exam targets.

General Trends and Anomalous Nitrogen

Group 15: N, P, As, Sb, Bi. Configuration: ns²np³ (half-filled p subshell — extra stability). Common oxidation states: −3, +3, +5 (nitrogen also has −1, +1, +2, +4). Electronegativity and size: N is the most electronegative in the group; size increases down the group. Nitrogen is anomalous: (1) it forms multiple bonds (N₂ has a triple bond, very stable) while heavier members prefer single bonds; (2) N has no d-orbitals (maximum covalency 4, forms NH₄⁺ only), while P, As, Sb can use d-orbitals and form expanded octets (PCl₅, PF₅); (3) N₂ is very stable (bond enthalpy 946 kJ/mol — highest of any diatomic) — hence inert at room temperature.

Allotropes of phosphorus: white P (P₄ tetrahedral, most reactive, waxy solid, stored under water, glows in dark — phosphorescence), red P (polymeric chains, less reactive, no phosphorescence), black P (most stable, least reactive, graphite-like layers). JEE asks about reactivity order (white > red > black) and structure (white P has P-P-P angle 60° — strained). Take a free inorganic chemistry mock. See our p-block elements overview.

Hydrides: NH₃ vs PH₃ and Trend

NH₃: pyramidal, lone pair on N, strong base (accepts H⁺), forms hydrogen bonds (high bp 238 K for its molecular weight), acts as Lewis base (donates lone pair to BF₃, Cu²⁺, etc.). PH₃: similar pyramidal structure but P lone pair less effective (larger atom, orbital overlap with H is poorer) — weaker base than NH₃, no significant H-bonding (bp 185 K), acts as a poor Lewis base. Thermal stability of hydrides: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃ (decreasing bond strength with increasing atomic size). Reducing power increases down the group (reverse order): BiH₃ > SbH₃ > … > NH₃. JEE tests both trends and their explanation (bond dissociation energy vs atomic size).

Oxides of Nitrogen

The oxides of nitrogen are crucial for JEE: N₂O (laughing gas, +1 oxidation state, N₂O analogue of CO₂ structure, neither acidic nor basic), NO (+2, colourless, paramagnetic because of odd electron, becomes brown NO₂ in air), N₂O₃ (+3, anhydride of HNO₂), NO₂ (+4, brown, paramagnetic, dimerises to N₂O₄), N₂O₄ (+4, colourless diamagnetic dimer), N₂O₅ (+5, anhydride of HNO₃). JEE asks: which oxide is neutral (N₂O, NO), which is acidic (N₂O₃, N₂O₄, N₂O₅), and what happens when each dissolves in water.

Oxyacids of Phosphorus

Oxyacids of P are heavily tested: H₃PO₂ (hypophosphorous acid, monobasic — one OH, two direct P-H bonds), H₃PO₃ (phosphorous acid, dibasic — two OH, one P-H bond), H₃PO₄ (phosphoric acid, tribasic — three OH, no P-H). Rule: only OH groups can ionise (P-H bonds do not ionise). Basicity = number of OH groups. JEE loves: why is H₃PO₂ monobasic despite having 3 H? Because two H are directly bonded to P (not ionisable). H₃PO₃ is dibasic (one P-H). Reducing character: H₃PO₂ (strong reducer — has P-H bonds which donate H) > H₃PO₃ > H₃PO₄ (no P-H, not a reducer). For the full inorganic strategy see our inorganic strategy guide and for p-block overview see our periodic table trends guide.