IGCSE Chemistry Group 7 (Halogens) Notes
The halogens — F, Cl, Br, I — are reactive non-metals with seven outer electrons. Reactivity decreases down the group.
Key concepts
- Diatomic molecules
- Halogens exist as X₂ molecules held by a single covalent bond.
- Trend in reactivity
- Down the group, atoms get bigger and outer shell is further from the nucleus, so it is harder to gain an electron.
- Displacement
- A more reactive halogen displaces a less reactive one from a solution of its salt.
- Chlorine: pale green gas. Bromine: red-brown liquid. Iodine: grey-black solid.
- Chlorine displaces bromide and iodide; bromine displaces only iodide.
- Halogens react with hydrogen to form hydrogen halides, e.g. H₂ + Cl₂ → 2HCl.
- Hydrogen halides dissolve in water to form acidic solutions.
Worked example
Chlorine water is added to potassium bromide solution. State what you would see and write the ionic equation.
- 1Chlorine is more reactive than bromine, so it displaces bromine.
- 2Observation: colourless solution turns orange-brown (bromine).
- 3Ionic equation: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Answer: Solution turns orange-brown; Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Common mistakes
✗ Saying halogens 'lose an electron' to react.
✓ Halogens gain one electron to form 1− ions.
✗ Predicting bromine displaces chlorine.
✓ Less reactive halogens cannot displace more reactive ones — check the order.
✗ Forgetting the '2' in ionic displacement equations.
✓ Balance charges — each Br₂ needs 2 electrons, so 2 Br⁻ are produced.
Quick check
Try these, then reveal the answers.
What is the colour of iodine solution?Reveal answer
Brown / dark brown.
Which halogen would displace iodine from potassium iodide?Reveal answer
Chlorine or bromine — either is more reactive than iodine.
Why do halogens form 1− ions?Reveal answer
They have 7 outer electrons and gain 1 to complete the shell.
Exam tips
- Learn the colours of halogens (gases, liquids, solids) and their solutions.
- Explain the reactivity trend using atomic size, shielding and attraction on incoming electron.
- Use ionic equations to score full marks in displacement questions.
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Frequently asked questions
Why does reactivity decrease down Group 7?+
Atoms get larger with more shielding, so the pull on an incoming electron is weaker and it is harder to gain one.
Why are halogens diatomic?+
Two halogen atoms share a pair of electrons in a covalent bond, giving each a full outer shell.
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