Halogen Family (Group 17): Trends, HCl, Bleaching Powder & Interhalogens Group 17 (Halogen Family): Big picture Group 17 elements — Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At) — are called halogens ( salt-formers ). They are highly electronegative, very reactive non-metals with valence configuration ns 2 np 5 . This gives them a strong tendency to gain one electron to reach noble-gas configuration. In daily life: Cl₂ disinfects water, Br₂ helps in flame retardants, Iodine is essential for thyroid (iodized salt), and fluorine chemistry enables Teflon and many modern drugs. 2026-05-26T17:05:00.257Z Halogen color and state at room temperature: F₂ (pale yellow gas), Cl₂ (greenish-yellow gas), Br₂ (red-brown liquid), I₂ (violet solid that sublimes). Lineup illustration of halogens in labeled containers: left to right F2 gas (pale yellow), Cl2 gas (greenish-yellow), Br2 liquid (red-brown in a vial), I2 solid crystals (violet) with faint purple vapors. White background, clean vector style, each labeled with formula and color. gpt-image-2 Position and electronic configuration Valence shell configuration: ns 2 ,np 5 (seven valence electrons). Common oxidation states: F shows only −1 (most electronegative, no d-orbitals available for expansion); Cl, Br, I show −1, +1, +3, +5, +7 in their compounds (as taught in NCERT, higher oxidation states arise due to availability of vacant d-orbitals and formation of multiple bonds with oxygen/fluorine). Astatine is radioactive with very short-lived isotopes, so its chemistry is limited in scope. Fast periodic trends you must remember Atomic/ionic size: increases down the group (F < Cl < Br < I). Ionization enthalpy: decreases down the group. Electronegativity (Pauling): decreases F (4.0) > Cl > Br > I. Bond dissociation enthalpy (X–X): F–F is unusually low (≈159 kJ mol⁻¹) — lower than Cl–Cl (≈243 kJ mol⁻¹). Oxidizing power: F₂ > Cl₂ > Br₂ > I₂ (F₂ is the most reactive non-metal). Member Relative atomic radius 1st ionization enthalpy Electronegativity (Pauling) X–X bond enthalpy (qualitative) Oxidizing power Color Physical state Group 17 snapshot at room temperature Element F₂ Smallest in group Highest in group 4.0 (highest in periodic table) F–F unusually LOW (≈159 kJ mol⁻¹) Strongest (most reactive) Pale yellow Gas Cl₂ Larger than F Lower than F High Cl–Cl higher than F–F (≈243 kJ mol⁻¹) Strong oxidizer Greenish-yellow Gas Br₂ Larger than Cl Lower than Cl Moderate Lower than Cl–Cl (trend) Moderate oxidizer Red-brown Liquid I₂ Largest (among stable) Lowest (among these) Lower Lower than Br–Br (trend) Weakest oxidizer Violet Solid (sublimes) Why is F–F bond so weak? You might expect the smallest atom to make the strongest bond, but F-F has unusually low bond enthalpy. Reason: tiny F atoms bring three lone pairs each very close, increasing lone pair–lone pair repulsions and lengthening/weakening the bond. In contrast, Cl-Cl has less crowding, so a stronger bond. Trap: Do not rank X–X bond enthalpy simply by atomic size. D( F-F ) < D( Cl-Cl ) — the F–F anomaly is a favorite NEET check. neet-alert Reactivity and oxidizing power Halogens are strong oxidizing agents because they readily gain one electron to form halide ions (X⁻). Oxidizing strength decreases down the group: F 2 > Cl 2 > Br 2 > I 2 . A more powerful halogen displaces a less powerful one from its halide salt in water. Example: Cl 2 liberates Br 2 and I 2 from bromides/iodides, but I 2 cannot displace Br 2 or Cl 2 . FO-CL-BR-I: Follows Oxidizing order — F₂ > Cl₂ > Br₂ > I₂. Halide ions (X⁻): size and basic reactivity Ionic size increases down the group: F - < Cl - < Br - < I - . Reducing power of halide ions increases down the group (I⁻ is the best reducing agent among halides). Silver halide precipitates get less soluble and more photosensitive down the group (AgCl > AgBr > AgI solubility order). Hydrogen halides (HX): bond polarity vs acid strength Bond polarity decreases down the group: HF > HCl > HBr > HI (because electronegativity decreases). But acid strength in water is the reverse: HI > HBr > HCl > HF . Why? The H–X bond becomes longer and weaker down the group — easier to ionize in water. HF is a weak acid in water despite highest bond polarity because its H–F bond is very strong and HF molecules form strong H-bonded networks, limiting ionization. IUPAC: hydrogen chloride; aqueous solution is hydrochloric acid. Preparation (lab): dry HCl gas from NaCl and conc. H 2SO 4 (at room temperature: NaCl + H 2SO 4 NaHSO 4 + HCl ). Properties: turns moist blue litmus red; forms dense white fumes of NH 4Cl with ammonia ( NH 3 + HCl NH 4Cl ). SMILES (HCl): [H]Cl Hydrogen chloride (HCl): quick facts Industrial chlorine: chlor-alkali (electrolysis of brine) The chlor-alkali process electrolyses aqueous sodium chloride (brine) to produce three co-products: chlorine gas (anode), hydrogen gas (cathode), and sodium hydroxide in solution. Membrane cells are standard today (separating products to avoid mixing and back-reaction). Uses: Cl₂ (PVC, water treatment, bleaching), NaOH (soaps, paper), H₂ (refining, ammonia synthesis). Membrane chlor-alkali cell: anode (Cl₂ out), cathode (H₂ out), membrane separates brine feed from NaOH product compartment. Schematic diagram of a membrane chlor-alkali cell: left anode compartment with brine feed; show Cl- oxidized to Cl2 bubbles at anode; right cathode compartment with Na+ migrating through membrane, OH- formed at cathode with H2 bubbles; labeled outlets: Cl2, H2, NaOH. Clean vector, red arrows for electron flow. 2026-05-26T17:05:00.314Z gpt-image-2 Overall balanced reaction for electrolysis of brine (membrane cell). Chlor-alkali overall Chlor-alkali: Cl₂ at anode, H₂ at cathode, NaOH in solution. Keep products straight: Anode (oxidation) → Cl₂; Cathode (reduction) → H₂; Solution accumulates NaOH. remember Deacon’s process: chlorine from HCl + O₂ Where HCl is available (e.g., from chlorination reactions), chlorine can be made by catalytic oxidation of HCl with oxygen in the Deacon process (historically important; now partly replaced by more energy-efficient routes). Catalyst: CuCl 2 ; temperature about 720 K. Deacon’s process Catalytic oxidation of hydrogen chloride to chlorine. HCl + O₂ over CuCl₂ at ~720 K → Cl₂ + H₂O Bleaching powder Ca(OCl)Cl (calcium oxychloride) Bleaching powder is produced by passing chlorine over dry slaked lime. It acts as a disinfectant and bleaching agent due to “available chlorine” — it releases Cl 2 and forms hypochlorous acid (HOCl) in moist conditions, both strong germicides/oxidizers. Formation of calcium oxychloride (bleaching powder). Preparation of bleaching powder Disinfection of drinking water (chlorination). Bleaching of cotton, linen, and wood pulp. Sanitation: deodorizing and disinfecting public places. Uses of bleaching powder gpt-image-2 Bleaching powder in water treatment: dosing into a water channel and microbial kill via HOCl/Cl₂. Illustration of a water-treatment channel with a hopper dosing bleaching powder into flowing water; zoom inset shows Ca(OCl)Cl releasing HOCl/Cl2 that attack microbes. Clean educational vector, labeled arrows for HOCl formation. 2026-05-26T17:05:00.857Z Interhalogen compounds: formula, examples, shapes Interhalogens have general formula AB x where A is the larger, less electronegative halogen (Cl, Br, I) and B is the smaller, more electronegative halogen (F or Cl). They are often more reactive than parent halogens and are used as fluorinating agents (e.g., ClF 3 , BrF 3 , IF 5 , IF 7 ). VSEPR predicts distinctive shapes that are NEET-favorites. Type Example(s) Central-atom e⁻ pairs (AXmEn) Shape (VSEPR) Hybridization NEET tip Interhalogens by type (XY, XY₃, XY₅, XY₇) Series XY ICl, BrCl AX2E0 (diatomic) Linear Just two atoms → linear XY₃ ClF₃, BrF₃ AX3E2 T-shaped sp3d Two lone pairs equatorial XY₅ IF₅ AX5E Square pyramidal sp3d2 One lone pair on octahedral frame XY₇ IF₇ AX7E0 Pentagonal bipyramidal sp3d3 No lone pairs; all bonding gpt-image-2 2026-05-26T17:05:00.599Z Four-panel vector diagram showing 3D VSEPR geometries with central halogen (colored) and surrounding F atoms. Panel labels: ClF3 (T-shaped AX3E2; two equatorial lone pairs), BrF3 (T-shaped), IF5 (square pyramidal AX5E), IF7 (pentagonal bipyramidal AX7). Use bond wedges/dashes, concise labels. VSEPR shapes of key interhalogens: ClF₃ (T-shaped), BrF₃ (T-shaped), IF₅ (square pyramidal), IF₇ (pentagonal bipyramidal). Oxoacids of chlorine: HOCl, HClO₂, HClO₃, HClO₄ Chlorine forms a series of oxoacids with increasing oxidation state on Cl: HOCl (+1), HClO₂ (+3), HClO₃ (+5), HClO₄ (+7). As the number of O atoms increases, acid strength increases (inductive effect of more electronegative O atoms stabilizes the conjugate base). Thermal stability also increases. Oxidizing power in dilute solutions generally decreases from HOCl to HClO₄; perchloric acid is the strongest acid but is a comparatively poor oxidizer in dilute solution (perchlorate is very stable), though hot concentrated HClO₄ can oxidize strongly. Chlorine oxoacids: oxidation state, acidity and oxidizing behavior Acid Cl oxidation state pKa (approx.) Relative stability Oxidizing power (aqueous, qualitative) Oxoacid HOCl (hypochlorous acid) +1 ~7.5 (weak) Lowest Strong oxidizer HClO₂ (chlorous acid) +3 ~2 (stronger) Low–moderate Strong–moderate oxidizer HClO₃ (chloric acid) +5 ~−1 (very strong) High Moderate oxidizer HClO₄ (perchloric acid) +7 << 0 (very strong) Highest Weak in dilute solution; strong only when hot/concentrated High-yield: Acid strength order — HOCl < HClO₂ < HClO₃ < HClO₄. Among Cl-oxoacids, HClO₄ is the strongest acid and thermally most stable. neet-alert Pseudohalogens and pseudohalides Pseudohalogens are neutral molecules that mimic halogens, and their corresponding anions are pseudohalides (behaving like halide ions). Examples: cyanogen ( (CN) 2 ) behaves like X 2 and its pseudohalide ions are CN - (cyanide), OCN - (cyanate), SCN - (thiocyanate). They form salts with metals, precipitate with Ag + (e.g., AgCN ), and show acid analogs like HCN (similar in behavior to HX in some tests). Key terms The 17th group of the periodic table: F, Cl, Br, I, At — highly electronegative, reactive non-metals. halogens Group 17 (halogen family) Halide The anion X⁻ derived from a halogen (e.g., Cl⁻, Br⁻, I⁻). Oxoacid of halogen An acid containing halogen, oxygen, and hydrogen (e.g., HOCl, HClO₄). Hypochlorous acid HOCl; chlorine oxoacid with Cl in +1 state; weak acid, strong oxidizer. Chlorous acid HClO₂; Cl in +3 state; stronger acid than HOCl. Chloric acid HClO₃; Cl in +5 state; very strong acid. HClO₄; Cl in +7 state; very strong acid; thermally very stable conjugate base (perchlorate). Perchloric acid Bleaching powder Calcium oxychloride, Ca(OCl)Cl; releases HOCl/Cl₂ for bleaching and disinfection. Industrial electrolysis of brine to produce Cl₂, H₂, and NaOH. Chlor-alkali process Interhalogen A compound containing two different halogens (e.g., ClF₃, IF₅); often more reactive than parent halogens. Neutral molecule behaving like a halogen (e.g., (CN)₂) and its anion (e.g., CN⁻) behaving like a halide. Pseudohalogen / pseudohalide Catalytic oxidation of HCl with O₂ over CuCl₂ (≈720 K) to make Cl₂ and H₂O. Deacon’s process Public health: Iodized salt prevents goitre. Typical fortification uses KI at about 10–15 mg per kg of salt (or equivalent iodine from KIO₃). clinical Industry and environment: Chlorine enables PVC (via vinyl chloride), paper bleaching (shifting to ECF/TCF methods to reduce chlorinated byproducts), and water sanitation. Fluorine in pharmaceuticals is common (~30% of marketed drugs have at least one F), tuning bioactivity. CFC “freons” have been largely phased out due to ozone depletion. Preview: aromatic halogenation (organic unit). False. F–F bond enthalpy is unusually LOW (≈159 kJ mol⁻¹) because of strong lone pair–lone pair repulsions in tiny F atoms. Cl–Cl is stronger (≈243 kJ mol⁻¹). “Fluorine has the highest X–X bond enthalpy among halogens.” “I₂ is more reactive (stronger oxidizer) than Br₂.” False. Oxidizing strength decreases down the group: F₂ > Cl₂ > Br₂ > I₂. I₂ is the weakest oxidizer among these. “HF is a strong acid in water like HCl/HBr/HI.” False. HF is a weak acid in water due to very strong H–F bond and extensive H-bonding, which limit ionization. Many interhalogens (e.g., ClF₃, BrF₃, IF₅, IF₇) are stable enough to handle industrially and are powerful fluorinating agents when used with proper precautions. “Interhalogen compounds are inherently unstable and unusable.” remember SMILES (simple halogens): Cl₂ = ClCl; Br₂ = BrBr; I₂ = II; HF = [H]F; HCl = [H]Cl. tip Solve trend questions by combining: (1) size effect, (2) bond strength, (3) solvation/hydrogen bonding. When two factors oppose, check which dominates in NCERT context.