Knowledge Organiser: Group 7 Halogens
This topic summary covers Knowledge Organiser: Group 7 Halogens within Group 7: Halogens for GCSE Chemistry. Revise Group 7: Halogens in Atomic Structure for GCSE Chemistry with 20 exam-style questions and 20 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 12 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 12 of 12
Practice
20 questions
Recall
20 flashcards
Knowledge Organiser: Group 7 Halogens
Key Terms
- Halogen: Group 7, 7 outer electrons, form −1 ions
- Diatomic: Exist as pairs (F₂, Cl₂, Br₂, I₂)
- Halide ion: Cl⁻, Br⁻, I⁻
- Displacement: More reactive halogen replaces less reactive
- Reactivity trend: Decreases down Group 7
Must-Know Facts
- F₂: pale yellow gas; Cl₂: green gas; Br₂: orange-brown liquid; I₂: grey-purple solid
- Reactivity decreases going down Group 7
- Smaller atom = outer shell closer to nucleus = stronger pull on electrons
- More reactive halogen displaces less reactive halide
- Cl₂ + 2KBr → 2KCl + Br₂ (solution turns orange-brown)
- Opposite trend to Group 1!
Key Equations
- Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromide)
- Cl₂ + 2KI → 2KCl + I₂ (chlorine displaces iodide)
- Br₂ + 2KI → 2KBr + I₂ (bromine displaces iodide)
- H₂ + Cl₂ → 2HCl (with hydrogen)
Common Mistakes
- Saying halogen reactivity increases down Group 7: It decreases — larger atoms gain electrons less easily due to greater shielding
- Forgetting halogens are diatomic: Always write Cl₂, Br₂, I₂ — never Cl, Br, I alone in equations
- Getting displacement direction wrong: A more reactive halogen displaces a LESS reactive halide — chlorine displaces bromide, not the other way
- Confusing colour observations: Orange-brown = bromine; brown/black precipitate = iodine with starch; learn specific colours for each halogen
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Read this section alongside the surrounding pages in Group 7: Halogens. That gives you the full topic sequence instead of a single isolated revision point.
Practice Questions for Group 7: Halogens
How many electrons do halogens have in their outer shell?
Explain why chlorine is more reactive than bromine.
Quick Recall Flashcards
20 questions on Group 7: Halogens — practise free
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