Group 17 Halogens: JEE Main Complete Guide

The halogens (F, Cl, Br, I, At) contribute two to four questions per JEE Main session — one of the highest yields in all of p-block chemistry. Fluorine's anomalous behaviour, the interhalogen compounds, the oxyacids of chlorine, and the comparison of hydrogen halides are the hot topics. This guide covers all of them with the specific details JEE tests.

Fluorine: The Great Anomaly

Fluorine is the most electronegative element and shows several anomalous properties compared to other halogens: (1) Highest oxidation state is only −1 (fluorine never shows positive oxidation states because it cannot expand its octet — no d-orbitals, and it is too electronegative to give electrons away). All other halogens can show +1, +3, +5, +7 in their compounds. (2) Fluorine forms HF with very strong H-bonding (bp 293 K — anomalously high compared to other HX). (3) F₂ is the strongest oxidising agent of all halogens (highest reduction potential +2.87 V). Oxidising power of halogens: F₂ > Cl₂ > Br₂ > I₂ (decreases down the group). (4) CaF₂ (fluorspar) is only slightly soluble; other alkaline earth metal halides are soluble. (5) HF is a weak acid (concentrated) due to strong H-bonding forming [F(HF)n]⁻ species; other HX are strong acids in water. (6) Fluorine reacts with glass (SiO₂ + 4HF → SiF₄ + 2H₂O) and even noble gases.

JEE tests: why does F not show positive oxidation states? Why is HF a weak acid? Why is F₂ the strongest oxidant? All answers reduce to the same reasons: high electronegativity, small size, absence of d-orbitals. Take a free p-block mock. See our p-block elements guide.

Interhalogen Compounds

Interhalogens (XY, XY₃, XY₅, XY₇ where X is heavier, less electronegative halogen) exist because halogens combine with each other. General formula: XY_n (n = 1, 3, 5, 7 — always odd). All are polar, reactive, and more reactive than elemental halogens. Structure: ClF (linear), ClF₃ (T-shaped, 2 lone pairs on Cl), BrF₅ (square pyramidal, 1 lone pair on Br), IF₇ (pentagonal bipyramidal, 0 lone pairs — all 7 bonds used). JEE asks: structure and shape of ClF₃ (VSEPR — 5 electron pairs, 2 lone pairs → T-shape), BrF₃ (same analysis), IF₅ (octahedral parent, one lone pair → square pyramidal). Hybridisation: ClF₃ → sp³d; BrF₅ → sp³d²; IF₇ → sp³d³. Hydrolysis: ClF₃ + 3H₂O → HCl + 3HF + Cl₂ (complex). As oxidising agents: IF₇ > BrF₅ > ClF₃ (more F atoms, more powerful oxidant).

Oxyacids of Chlorine

Four oxyacids of chlorine, in increasing oxidation state: HOCl (hypochlorous acid, +1, weakest acid, strongest oxidising agent among oxyacids), HClO₂ (chlorous acid, +3), HClO₃ (chloric acid, +5), HClO₄ (perchloric acid, +7, strongest oxyacid of chlorine, one of the strongest acids known). Trend: acid strength increases with oxidation state (more O atoms pull electron density away from O-H bond, making it easier to lose H⁺). Oxidising power: decreases with increasing oxidation state (HOCl > HClO₂ > HClO₃ > HClO₄ — the lower oxidation state species is more eager to be oxidised and hence is a stronger oxidant). This inverse relationship between acid strength and oxidising power in the chlorine oxyacid series is a favourite JEE conceptual question.

Chlorine and Bleaching Powder

Chlorine preparation: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O (laboratory); Deacon's process (4HCl + O₂ → 2Cl₂ + 2H₂O, CuCl₂ catalyst at 450°C); electrolysis of brine (industrial). Properties: pale yellow-green gas, pungent smell, dense than air, soluble in water (Cl₂ + H₂O ⇌ HCl + HOCl). Bleaching powder: Ca(OCl)Cl or Ca(ClO)Cl (not CaCl₂ + Ca(ClO)₂ — it is a mixed salt). Made by passing Cl₂ over slaked lime Ca(OH)₂ at ~40°C. The effective bleaching agent is HOCl, released when bleaching powder is treated with acid or CO₂. JEE asks: is bleaching powder a mixture or a compound? (compound — double salt, one Ca²⁺ for both Cl⁻ and OCl⁻). For the full halogens context see our halogen compounds guide and our s-block elements guide.