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
Memory Aid
- Group 7 order: "Fluorine Chlorine Bromine Iodine Astatine" — "Fast Cars Burn In Accidents" (F, Cl, Br, I, At).
- States at room temperature: "Gas Gas Liquid Solid" for F₂, Cl₂, Br₂, I₂ — the only liquid element in the group is bromine (memorise this as the exception).
- Displacement rule: "More reactive = can displace less reactive" — if you can remember the order F > Cl > Br > I (decreasing reactivity), you can work out any displacement question.
- Group 7 vs Group 1: "Group 1 wants to LOSE (gets easier going down), Group 7 wants to GAIN (gets harder going down)" — opposite trends for opposite processes.