p-Block Elements

Concept overview

The p-block spans groups 13 to 18 — the elements whose valence electrons occupy p-orbitals. It is the longest and most diverse block, covering the semi-metallic boron and aluminium, the biologically vital carbon and silicon, nitrogen and its oxoacids, oxygen family, halogens, and the largely unreactive noble gases.

JEE covers each group with the same recipe: general trends going down the group, key compounds (oxides, halides, hydrides), oxoacids where relevant, and some named reactions. The single most-tested subset is the group 15–17 oxoacids — H₃PO₃ vs H₃PO₄, HNO₂ vs HNO₃, H₂SO₃ vs H₂SO₄, HClO vs HClO₂ vs HClO₃ vs HClO₄. Learn the structure, oxidation state, basicity (number of ionisable H) and acidity trend of each and you cover a large fraction of paper Inorganic marks.

Group 13 highlights: aluminium's amphoteric hydroxide, boron's unusual electron-deficient BF₃ / B₂H₆ (banana bonds), and the diagonal similarity with silicon. Group 14 highlights: allotropes of carbon (graphite conducts, diamond doesn't), silicon's role in silicates and silicones, and the inert-pair effect (Sn²⁺ vs Sn⁴⁺, Pb²⁺ vs Pb⁴⁺ stability).

Group 15 (nitrogen family): NH₃ + HNO₃ synthesis, oxides of nitrogen (colourless N₂O vs brown NO₂, and dimer N₂O₄), P₄ tetrahedral structure, and Bayer's process are workhorse JEE facts. Group 16 (oxygen family): ozone (Δ₀ ≈ 105° bent structure), sulfuric acid manufacture (contact process), and the increasing acidity of H₂S → H₂Te.

Group 17 (halogens): decreasing bond-dissociation energy from Cl₂ to I₂ (F₂ is an anomaly — its bond is weaker than expected due to inter-electronic repulsion), oxoacids of chlorine, and the interhalogen family (ICl, BrF₃, IF₇). Group 18 (noble gases): XeF₂, XeF₄, XeF₆ and their reactions (Bartlett's original 1962 synthesis of XePtF₆).

Key formulas & concepts

• Aluminium amphoteric: Al(OH)₃ + acid → salt; Al(OH)₃ + NaOH → aluminate
• Nitric acid oxidising: Concentrated HNO₃ passivates Fe, Al; dilute reacts
• H₃PO₃ vs H₃PO₄: H₃PO₃ is dibasic (one non-ionisable H on P); H₃PO₄ is tribasic
• Inert pair effect: Sn²⁺ and Pb²⁺ increasingly stable relative to Sn⁴⁺/Pb⁴⁺ down group 14
• Bond dissociation halogens: F–F unusually weak (< Cl–Cl > Br–Br > I–I)
• Ozone structure: Bent, sp², bond angle 116.8° (larger than water)
• Contact process: SO₂ + V₂O₅ / air → SO₃ → oleum → dilute → H₂SO₄

Common mistakes to avoid

• Predicting F–F bond as strongest halogen bond — it's the weakest due to lone-pair repulsion in the compact 2p orbitals.
• Assigning basicity of H₃PO₃ as three — it's two; one H is directly bonded to P and doesn't ionise.
• Confusing amphoteric with weak acid — Al(OH)₃ reacts with both acids and bases, distinct from a merely weak acid.
• Writing NO₂ as a stable monomeric gas at all temperatures — it dimerises to N₂O₄ at low temperatures.

JEE Main relevance

JEE Main asks 3–4 p-block questions every paper — often the highest question count of any Inorganic chapter. Nearly every question is direct-recall from NCERT.

JEE Advanced relevance

Advanced problems combine p-block reactions with mechanisms — for instance, asking why H₃PO₃ is dibasic despite three H atoms, or predicting the structure of a specific interhalogen. Advanced also tests structural depth (S₈ vs S₆ rings, red vs white phosphorus).

Previous-year questions (PYQ)

p-Block PYQs concentrate around structure of oxoacids, allotropes, and specific manufacturing processes. Read NCERT closely — the direct-quote frequency of p-block questions is famous.