Group 17 Elements Revision Notes, Mnemonics and Trends

Group 17 Elements (Halogens): F, Cl, Br, I, At

This quick revision Notes, Trends and Mnemonics are very important for CSIR-NET, GATE, SLET, NEET, JEE Main/Advanced, IIT-JAM.

Mnemonics: F: First | Cl: Class | Br: Biriyani | I: In | At: Atlanta

1. General Electronic Configuration

ns² np⁵ (one electron short of stable octet → high electron affinity)

2. Key Periodic Trends (Down the Group: F → At)

Property	Trend	Exception/Important Note
Atomic & Ionic Radius	Increases	F < Cl < Br < I < At
Ionisation Energy	Decreases	F > Cl > Br > I > At
Electronegativity	Decreases	F (4.0) > Cl (3.0) > Br (2.8) > I (2.5) > At (2.2) F is most electronegative element
Electron Affinity	Decreases (generally)	Cl > F > Br > I Exception: Cl > F (due to small size of F → high e⁻-e⁻ repulsion)
Bond Dissociation Energy (X-X)	Decreases (F to I)	F-F (159 kJ/mol) < Cl-Cl > Br-Br > I-I Exception: F-F weakest (small size → high lone pair repulsion)
Oxidising Power	Decreases	F₂ > Cl₂ > Br₂ > I₂ F₂ strongest oxidising agent
Colour	Deepens	F₂ (pale yellow) → Cl₂ (greenish yellow) → Br₂ (red-brown) → I₂ (violet)
Physical State	—	F₂, Cl₂ (gas) → Br₂ (liquid) → I₂ (solid)
Oxidation States	-1 most common; +1, +3, +5, +7 (except F)	F shows only -1 (no d-orbitals) +7 stability: I > Br > Cl >> F (absent)
Acidic Nature of HX	Increases	HF < HCl < HBr < HI
Thermal Stability of HX	Decreases	HF > HCl > HBr > HI

3. Important Compounds & Reactions

• Hydrides (HX):
  • HF: weakest acid, extensive H-bonding → high BP, polymeric (HF)ₙ
  • HI: strongest acid, strongest reducing agent
  • Bond strength: HF > HCl > HBr > HI
• Interhalogen Compounds: AX, AX₃, AX₅, AX₇ (A = larger halogen)
• Oxides: Mostly unstable; I₂O₅ most stable (used in estimation of CO)
• Oxyacids:
  • HOCl < HOBr < HOI (acid strength)
  • HOCl > HOBr > HOI (oxidising power)
  • HClO₄ > HBrO₄ > HIO₄ (acid strength & oxidising power)
  • HClO₄: strongest acid among oxyacids
• Halides Reactivity: For same element, XF > XCl > XBr > XI (due to bond polarity)
• Reactions:
  • F₂ + H₂O → 2HF + O₂ (cold dilute)
  • Cl₂ + H₂O → HCl + HOCl
  • Br₂/I₂ + H₂O → no reaction (reversible)
  • 2F₂ + 2NaOH (cold dilute) → 2NaF + OF₂ + H₂O

4. Exam Tricks & Mnemonics

Anomalous Behaviour of Fluorine:
Small size, highest electronegativity, low F-F bond energy, absence of d-orbitals → only -1 state, weakest X-X bond, most reactive.

Oxidising Power Order: F₂ > Cl₂ > Br₂ > I₂
Mnemonic: Fear Clean Bright Interior → decreasing order.

Acid Strength of HX: HF << HCl < HBr < HI
Reason: H-X bond strength decreases down the group.

Reducing Power of HX: HF < HCl < HBr < HI
Same as acid strength (HI strongest reducing agent).

Oxyacid Oxidising Power (HOX): HOCl > HOBr > HOI
Opposite to acid strength (Cl has intermediate EN).

Stability of Higher Oxidation States: +7 more stable in I than Cl/Br (inert pair effect opposite here).

Reaction with Water:
F₂: oxidises water to O₂
Cl₂: disproportionation
Br₂/I₂: no reaction or reversible.

5. Frequently Asked Exam Questions

• Why is F anomalous in Group 17?
• Why Cl has higher electron affinity than F?
• Why F-F bond energy is less than Cl-Cl?
• Order of oxidising/reducing/acid strength of halogens/HX?
• Why HF is weakest acid but HClO₄ is strongest?
• Which halogen forms most stable higher oxyacids? (Iodine)
• Why I₂ is solid and violet in colour?
• Which interhalogen has maximum bond energy? (ClF)

6. Quick Revision One-Liners

• Fluorine: most reactive non-metal, strongest oxidising agent.
• Chlorine: greenish yellow gas, used in bleaching.
• Bromine: only non-metal liquid at RTP, red-brown.
• Iodine: sublimes to violet vapour, used in goiter treatment.
• Astatine: radioactive, least reactive halogen.
• HF: liquid due to H-bonding, used in glass etching.
• HClO₄: strongest Brønsted acid.
• Bleaching action: Cl₂/HOCl permanent (oxidising), SO₂ temporary.