Group 17 Elements (Halogens)

• Identify periodic trends in halogens from F to I (atomic radius, electronegativity, bond energy, boiling point)
• Explain the anomalous behaviour of HF — why it is a weak acid despite F being the most electronegative element
• Write formulas and structures of interhalogen compounds and predict their geometry
• Describe the preparation, properties, and uses of bleaching powder
• Write the step-by-step preparation of HCl from NaCl and H₂SO₄
• Arrange oxoacids of chlorine in order of oxidising power and acid strength and justify the trend

Level 1 — Foundations

General configuration: ns² np⁵ (one electron short of noble gas)

Periodic Trends (F → I)

Note: Cl has higher bond energy than F because F–F bond is weakened by lone-pair repulsion (small atom, short bond, repulsion between lone pairs on both F atoms).

Anomalous Behaviour of HF

1. Highest boiling point among hydrogen halides (19.5 °C vs HCl −85 °C) due to strong intermolecular H-bonding (F is small and highly electronegative).
2. Weak acid in water (Ka ≈ 6.8 × 10⁻⁴) — counter-intuitive! Reasons:
   • H–F bond is very strong (569 kJ/mol); high bond dissociation energy offsets the high hydration enthalpy of F⁻.
   • F⁻ forms H-bonds with water strongly, lowering effective dissociation.
   • Compare: HCl, HBr, HI are all strong acids because H–X bond energy decreases down the group.
3. HF etches glass: SiO₂ + 4HF → SiF₄ + 2H₂O.
4. HF forms bifluoride ion: HF + F⁻ → [HF₂]⁻ (unique to F due to H-bonding).

Hydrogen Halides — Preparation

HCl preparation detail:

• Step 1 (below 150 °C): NaCl + H₂SO₄ → NaHSO₄ + HCl↑
• Step 2 (above 600 °C): NaHSO₄ + NaCl → Na₂SO₄ + HCl↑
• HCl gas is dried over anhydrous CaCl₂ (not P₂O₅, which reacts with HCl).

Bleaching Powder

• Formula: Ca(OCl)Cl or Ca(ClO)Cl — a mixed salt (calcium hypochlorite + calcium chloride).
• Made by passing Cl₂ over slaked lime at 40 °C: 2Ca(OH)₂ + 2Cl₂ → Ca(OCl)Cl + CaCl₂ + H₂O (simplified: Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O).
• Bleaching action: Ca(OCl)Cl + CO₂ + H₂O → CaCO₃ + 2HClO; HClO → HCl + [O] (nascent oxygen bleaches).
• Uses: bleaching cotton/linen, disinfecting drinking water.

Level 2 — JEE Depth

Interhalogen Compounds

Halogens combine with each other to form interhalogen compounds of types: XX′, XX′₃, XX′₅, XX′₇ (X = larger halogen, X′ = smaller halogen, fluorine).

Key reasoning (VSEPR):

• XX′₃: 3 bond pairs + 2 lone pairs → T-shape
• XX′₅: 5 bond pairs + 1 lone pair → Square pyramidal
• XX′₇: 7 bond pairs + 0 lone pairs → Pentagonal bipyramidal

Why F is always the smaller halogen X′? F cannot be the central atom — it has no d-orbitals and the highest electronegativity, so it never expands its octet.

Reactivity: Interhalogens are generally more reactive than Cl₂ but less reactive than F₂. The X–F bond is polar and weak, making them good fluorinating agents.

Oxoacids of Chlorine

Trend in acid strength: HClO₄ > HClO₃ > HClO₂ > HOCl

• More oxygen atoms → more electron withdrawal from O–H bond → easier H⁺ release → stronger acid.

Trend in oxidising power: HOCl > HClO₂ > HClO₃ > HClO₄

• Lower oxidation state of Cl → higher tendency to gain electrons → stronger oxidising agent.
• HClO₄ is the strongest acid but the weakest oxidising agent (Cl already at +7, reluctant to oxidise further).

JEE Trap: HClO₄ is the strongest acid among all known acids. It is NOT the strongest oxidising agent — that is HOCl/HClO.

Fluorine's Unique Oxidising Power

• F₂ is the strongest oxidising agent of all halogens.
• Can oxidise Cl⁻, Br⁻, I⁻, and even H₂O: 2F₂ + 2H₂O → 4HF + O₂.
• F₂ displaces all other halogens from their salts.
• F₂ shows only −1 oxidation state (no positive oxidation state; no d-orbitals, smallest size, highest electronegativity).

Cl₂ — Preparation from Lab

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂↑

Purification: pass through water (removes HCl), then through conc. H₂SO₄ (dries gas).

Example 1: Predict the shape of ClF₃ and BrF₅ using VSEPR theory.

ClF3:
- Central atom: Cl
- Valence electrons on Cl: 7
- Bonds formed: 3 (with 3 F atoms)
- Lone pairs on Cl: (7 - 3) / 2 = 2 lone pairs
- Electron geometry: Trigonal bipyramidal (3 bp + 2 lp)
- Lone pairs occupy equatorial positions (more space)
- Molecular shape: T-shaped
- Bond angles: approx 87.5° (< 90° due to lone pair repulsion)

BrF5:
- Central atom: Br
- Valence electrons on Br: 7
- Bonds formed: 5 (with 5 F atoms)
- Lone pairs on Br: (7 - 5) / 2 = 1 lone pair
- Electron geometry: Octahedral (5 bp + 1 lp)
- Lone pair occupies one axial position
- Molecular shape: Square pyramidal
- Bond angle: slightly < 90°

Example 2: Arrange HCl, HBr, HI, HF in order of acid strength and boiling point. Justify.

Acid strength order:
HI > HBr > HCl >> HF

Reason (acid strength):
- Acid strength depends on ease of H–X bond breaking.
- Bond energy: H–F (569) > H–Cl (432) > H–Br (366) > H–I (297) kJ/mol
- Weaker H–X bond → easier dissociation → stronger acid
- HF is anomalously weak because H–F bond energy is very high
  AND F⁻ forms strong H-bonds with water, reducing effective dissociation.

Boiling point order:
HF >> HI > HBr > HCl

Reason (boiling point):
- HF: intermolecular H-bonding (F–H···F) → highest BP (19.5 °C)
- HCl, HBr, HI: only van der Waals forces → BP increases with molar mass
  HI (−35 °C) > HBr (−67 °C) > HCl (−85 °C)

• Q1: Why is the bond energy of F₂ (159 kJ/mol) less than that of Cl₂ (243 kJ/mol), even though F is more electronegative?
• Q2: Write the equation for the reaction of Cl₂ with NaOH at room temperature. What products form at 70 °C?
• Q3: Which oxoacid of chlorine has chlorine in the +5 oxidation state? Is it a stronger or weaker acid than HOCl?
• Q4: What is the hybridisation of the central iodine atom in IF₇? What is its shape?
• Stretch: Q5: HF dissolves glass but HCl does not. Explain this in terms of the reaction with SiO₂ and the relative reactivity of F⁻ versus Cl⁻ with silicon.

NCERT Chapter 7 link: Class 12 Chemistry — The p-Block Elements (Group 17 section, pages 171–185)

Exam connections: JEE Main frequently tests: HF anomaly, acid strength order of HX, oxidising power of halogens, bleaching powder reaction, interhalogen shapes (VSEPR). JEE Advanced tests oxidation state reasoning in oxoacids and comparative reactivity.

Study strategy: Draw the oxoacid structures side-by-side, label oxidation states, then derive acid strength and oxidising power trends from first principles. Do NOT memorise trends blindly — derive them each time for 2 minutes to build permanence.

• Use the platform-native live simulation or PhET-style tool for this topic: simulate halogen displacement reactions (F₂ displaces Cl⁻, Cl₂ displaces Br⁻, etc.) in a virtual lab and observe colour changes.
• Mirror / body / home activity: make a simple bleaching experiment with household bleach (dilute NaOCl solution), a strip of coloured fabric, and vinegar (CO₂ source) — photograph the bleaching and annotate the chemistry for your portfolio.
• Voice or text reflection with AI Mentor: explain to a younger sibling why HF can etch glass but HCl cannot, using only everyday language.

• "Explain why HF is a weak acid using one analogy from an Indian home, school, or market — for example, compare the bond strength to a very tight knot."
• "What is one common mistake students make when predicting the shape of ClF₃, and how would you catch yourself making it?"
• Stretch: "How are fluorine compounds used in water fluoridation and dental health? Is there a connection to the chemistry of HF you just studied? How might this relate to a career in materials science or public health?"

• Complete the core practice + one extension activity (photo of bleaching experiment, drawn structures of interhalogens, or a comparison table you created) for base XP + Group 17 Halogens badge.
• 5-7 day streak or family discussion note (e.g., explaining bleaching powder to a parent) = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• Hands-on project: use a Drishti kit or a digital conductivity meter (or homemade lemon-battery probe) to compare HCl vs. HF (vinegar as a safe HF proxy) conductivity — relate to strong vs. weak acid dissociation.
• Future Skill track: Green Tech — research how chlorine and its oxoacids are used in water treatment plants, and propose an improvement to reduce Cl byproducts (trihalomethanes) in drinking water.
• Coding extension: write a simple Python script that takes an oxoacid formula as input, counts the oxygen atoms around the central atom, and outputs the predicted relative acid strength.

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path." (Example: "Understanding bleaching chemistry connects to water safety engineering — a field I could work in.")

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.