ChemistryChapter 12: Halogens
Halogen Physical Trends
Group 17 (the halogens) exist as diatomic molecules X₂.
Learning objectives
- Explain Halogen Physical Trends using Group 17 language.
- Link trends to size, shielding, forces and bond strength.
- Apply equations and observations in FBISE-style answers.
Definition
Group 17 (the halogens) exist as diatomic molecules X₂. At room temperature: F₂ is a pale yellow gas, Cl₂ a greenish-yellow gas, Br₂ a red-brown liquid, and I₂ a dark grey solid that gives violet vapour. Down the group, atomic radius and shielding rise, London forces between X₂ molecules strengthen, melting and boiling points rise, and volatility falls.
Core Concept
Down Group 17: radius ↑ · shielding ↑ · polarizability ↑ · London forces ↑ · m.p. ↑ · b.p. ↑ · volatility ↓ States/colours (rtp): F₂ pale yellow gas · Cl₂ greenish-yellow gas · Br₂ red-brown liquid · I₂ dark grey solid (violet vapour) These are intermolecular (X₂…X₂) trends — not the covalent X–X bond-energy story.
Beginner
Halogens are F₂, Cl₂, Br₂ and I₂ — each is a two-atom molecule. Down the group they get larger, stick together more (London forces), so boiling point rises and they become less volatile. Remember colours: pale yellow gas, greenish-yellow gas, red-brown liquid, dark grey solid (violet vapour).
Intermediate
Link size → polarizability → London forces → higher m.p./b.p. → lower volatility. Keep X₂ intermolecular forces separate from the covalent X–X bond inside the molecule. Quote rtp states and colours accurately; I₂ vapour is violet, not the same as aqueous iodine brown.
Advanced
Polarizability rises with electron count and size, strengthening London dispersion. Volatility is about escaping into the gas phase against intermolecular attractions. Bond enthalpy of X–X is a different trend (F–F is anomalously weak) and must not be used to “explain” boiling points of the elements.
Remember
Down 17: size↑ London↑ bp↑ volatility↓ · F₂ pale yellow gas · Cl₂ greenish-yellow gas · Br₂ red-brown liquid · I₂ dark grey solid / violet vapour · X₂ not X⁻.
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Halogen Physical Trends
Sticky Notes · Quick Revision
Down Group 17
- radius↑ shielding↑
- London↑ mp/bp↑
- volatility↓
Colours at rtp
- F₂ pale yellow gas
- Cl₂ greenish-yellow gas
- Br₂ red-brown liquid
- I₂ dark grey solid / violet vapour
Force story
- Diatomic X₂
- Intermolecular London
- Not X–X bond energy
Exam cue
- Trend + reason
- Name each state/colour
- X₂ ≠ halide X⁻
60-second revision
- Down Group 17: larger X₂ → stronger London → higher bp → lower volatility.
- F₂ pale yellow gas; Cl₂ greenish-yellow gas; Br₂ red-brown liquid; I₂ dark grey solid (violet vapour).
- Physical trends are intermolecular; do not confuse with AgNO₃ test colours.
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