Group 7 Halogens – Properties, Reactions, Colours
100% Exam-Focused | O Level Chemistry 5070 | IGCSE Chemistry 0620 | 2025-2027 Syllabus
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Group 7 Halogens (The Halogens) – Quick Definition
Group 7 elements (Fluorine, Chlorine, Bromine, Iodine, Astatine) are highly reactive non-metals with 7 valence electrons. They exist as diatomic molecules (X₂) and form 1− ions (halides). Reactivity decreases down the group.
Group 7 elements (Fluorine, Chlorine, Bromine, Iodine, Astatine) are highly reactive non-metals with 7 valence electrons. They exist as diatomic molecules (X₂) and form 1− ions (halides). Reactivity decreases down the group.
Colours & Physical States – Memorise This Table (12–15 marks every year)
| Halogen | Symbol | Colour | State (25°C) | Boiling Point Trend |
|---|---|---|---|---|
| Fluorine | F₂ | Pale yellow | Gas | Increases down group |
| Chlorine | Cl₂ | Greenish-yellow | Gas | |
| Bromine | Br₂ | Red-brown liquid | Liquid | |
| Iodine | I₂ | Dark grey solid → purple vapour | Solid |
Trends Down Group 7 Halogens (Most Important 8-marker)
| Property | Trend Down Group | Explanation (Exam Answer) |
|---|---|---|
| Melting & Boiling point | Increases | Larger molecules → stronger van der Waals/London forces |
| Reactivity | Decreases (F > Cl > Br > I) | Atomic radius ↑ → outer shell farther from nucleus → harder to gain electron |
| Colour | Darker | Absorbs lower energy light |
Displacement Reactions – Guaranteed Question Every Paper
- Cl₂ + 2KBr → 2KCl + Br₂ (colourless → red-brown)
- Cl₂ + 2KI → 2KCl + I₂ (colourless → brown → purple vapour)
- Br₂ + 2KI → 2KBr + I₂ (red-brown → purple)
- I₂ + KCl → no reaction
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Past Paper Solutions by Prof. Faisal (Group 7 Questions)
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