p-Block Elements

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

Groups 15–18 span the nitrogen, oxygen, halogen and noble-gas families — the richest oxidation-state chemistry in the periodic table. NEET focuses on the sharp anomalies at the top of each group (N2's inertness, O's preference for pi bonding over catenation, F's weak bond enthalpy, Xe's surprising reactivity among noble gases), the major industrial processes (Haber, Contact, Ostwald), and the trend of the inert pair effect stabilising lower oxidation states down each group.

Chemistry Syllabus Inorganic Chemistry ~25% weightage

Live Animation1 / 4

1MixN2 and H2 combined in 1:3 ratio

2CompressCompressed to ~200 atm

3CatalysePassed over Fe catalyst at 450°C

4RecycleNH3 liquefied; unreacted gases recycled

1Group 15 Elements (Nitrogen Family)

N, P, As, Sb, Bi share the ns2np3 configuration, but nitrogen's small size and lack of d-orbitals make it the anomalous, chemically inert head of an otherwise reactive family.

Learning Objectives

State the ns2np3 configuration and relate the group's common oxidation states (−3, +3, +5) to it.
Explain why N2 is chemically inert at room temperature while P4 is highly reactive.
Describe the Haber process for NH3 synthesis and its optimum conditions.
Explain the inert pair effect and why Bi(III) is more stable than Bi(V).

Key Terms

CatenationSelf-linking of like atoms into chains/rings; phosphorus catenates more than nitrogen because P–P single bonds are stronger than the lone-pair-repelled N–N single bond.

Haber ProcessIndustrial synthesis of ammonia: N2 + 3H2 ⇌ 2NH3, at ~450°C, 200 atm, using an iron catalyst promoted with Mo/K2O.

Inert Pair EffectThe tendency of the ns2 electron pair to resist bonding for heavier p-block elements, stabilising oxidation states two below the group's maximum (e.g. Bi3+ over Bi5+).

AllotropyExistence of an element in more than one structural form; phosphorus shows white (P4, reactive) and red (polymeric, stable) allotropes.

Group 15 (N, P, As, Sb, Bi) shares the valence configuration ns2np3, giving three common oxidation states: −3 (with H or metals), +3, and +5. Metallic character increases down the group — N and P are non-metals, As and Sb are metalloids, Bi is essentially metallic.

Nitrogen's anomalous inertness

N2's N≡N triple bond (941 kJ/mol) is exceptionally strong because nitrogen's small atoms allow effective side-on p-orbital overlap for two pi bonds on top of the sigma bond. Combined with nitrogen having no valence d-orbitals (capping covalency at 4), this makes N2 kinetically inert despite reacting exothermically once activated (e.g. in the Haber process).

Phosphorus: allotropy and catenation

White phosphorus (P4): discrete tetrahedral molecules, waxy, glows in the dark (chemiluminescence), highly reactive and toxic, strained 60° P–P–P angles.
Red phosphorus: polymeric chains of P4 units, far less reactive, non-toxic, used in matchbox striking surfaces.
P catenates more than N because P–P single bonds are stronger (weaker lone-pair repulsion, larger atoms) than the short, lone-pair-crowded N–N bond.

Key PointInert pair effect down the group: +5 is the most stable state for N and P, but +3 becomes progressively more stable for Sb and especially Bi — Bi(V) compounds like NaBiO3 are in fact strong oxidising agents precisely because Bi(V) readily reverts to the more stable Bi(III).

Haber process: N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92.4 kJ/mol, at ~450°C, 200 atm, Fe catalyst with Mo promoter.