Structure and bonding · GCSE Chemistry
Group 7 halogens
GCSE Chemistry revision on Group 7 halogens: diatomic molecules, displacement reactions, colours, and why reactivity decreases down the group.
Seven outer electrons, diatomic X₂. Reactivity falls down the group because an incoming electron is less strongly attracted. A more reactive halogen displaces a less reactive one from its salt.
The important bits
What you need to know
- 1
Halogens (F, Cl, Br, I, At) have seven outer electrons: F 2,7; Cl 2,8,7. They form 1− ions (F⁻, Cl⁻, Br⁻, I⁻) or a single covalent bond in X₂ and in HX.
- 2
They exist as diatomic molecules: F₂, Cl₂, Br₂, I₂. Forces between molecules are weak, but they get stronger as the molecules get larger, so melting and boiling points increase down the group.
- 3
At room temperature fluorine and chlorine are gases, bromine is a red-brown liquid, iodine is a grey-black solid that sublimes to a purple vapour.
- 4
Reactivity decreases down Group 7. The atom must gain an electron. Extra shells and shielding weaken the attraction for that electron, so it is gained less easily.
- 5
Displacement: a more reactive halogen displaces a less reactive halogen from a solution of its salt. Cl₂ + 2KBr(aq) → 2KCl(aq) + Br₂(aq). The solution turns orange-brown.
- 6
Chlorine displaces bromine and iodine. Bromine displaces iodine but not chlorine. Iodine displaces neither. That order is the reactivity series of the group.
- 7
With metals they form ionic salts: 2Na(s) + Cl₂(g) → 2NaCl(s). With hydrogen they form hydrogen halides, which dissolve to make acids: HCl(aq) is hydrochloric acid.
- 8
Chlorine water is pale green; bromine water is orange; iodine in potassium iodide is brown. Colour change is the practical evidence of displacement.
Quotations worth analysing
Short evidence. Real method.
“Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)”
Chlorine is more reactive, so it takes the metal and bromine is set free. Ionic equation: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine is reduced; bromide is oxidised.
“Cl + e⁻ → Cl⁻”
Gaining one electron fills the outer shell. Down the group this is harder, so fluorine is the most reactive halogen and iodine the least of those you meet.
“F₂ and Cl₂ gases; Br₂ liquid; I₂ solid.”
Same bonding type (simple molecules), stronger intermolecular forces down the group as molecules get larger. That is why iodine is a solid and chlorine is a gas.
Go deeper
Displacement is a redox story on Higher tier
Chlorine atoms gain electrons: Cl₂ + 2e⁻ → 2Cl⁻ (reduction). Bromide ions lose electrons: 2Br⁻ → Br₂ + 2e⁻ (oxidation). OIL RIG still applies. The more reactive halogen is the better oxidising agent because it takes electrons more readily. You can answer a Foundation question with “chlorine is more reactive so it displaces bromine” and the colour change. Higher tier wants the electron transfer as well. If iodine solution is added to potassium chloride, nothing happens: iodine cannot take electrons from chloride. Write “no reaction” and give the trend reason, not a made-up colour.
Go deeper
Opposite trend to Group 1, same physics
Group 1 loses an electron; Group 7 gains one. Extra shells make loss easier and gain harder. That is why potassium is more reactive than lithium, but iodine is less reactive than chlorine. Do not say “halogens get more reactive down the group” — that is the classic mix-up. Melting points do increase down Group 7, which is a different trend about intermolecular forces, not about reactivity. Keep the two trends in separate sentences. Fluorine is rarely handled in school labs; the papers use chlorine, bromine and iodine for displacement.
Go deeper
Colours must be learned as a set
Chlorine water: very pale green, often looking colourless in dilute solution. Bromine water: orange. Iodine: brown in aqueous potassium iodide, purple in organic solvents, grey-black as the solid. After Cl₂ is added to KBr, orange bromine appears. After Cl₂ is added to KI, brown iodine appears. After Br₂ is added to KI, brown iodine appears. After Br₂ is added to KCl, no change. A table of halogen versus halide, with colour or “no reaction”, is the highest-yield revision page in this topic. State symbols: Br₂(aq) in these tests, not Br₂(l), because you are looking at an aqueous mixture.
See the idea in action
Chlorine water is added to potassium iodide solution. The mixture turns brown because chlorine displaces iodine: Cl₂ + 2KI → 2KCl + I₂. Chlorine atoms have fewer shells than iodine, so they attract an extra electron more strongly. Iodine solution added to potassium chloride does not react: iodine is less reactive than chlorine. Bromine is a liquid at room temperature because I₂-sized thinking applies on a smaller scale: Br₂ molecules have stronger intermolecular forces than Cl₂, but weaker than I₂.
Exam technique
Turn knowledge into marks
Learn the displacement table with colours. Explain reactivity with attraction for an extra electron, distance and shielding. Keep the melting-point trend (up the group as molecules get larger) separate from the reactivity trend (down the group, less reactive).
Common mistakes
Do not give these marks away
- 01
Saying halogen reactivity increases down the group, copying Group 1.
- 02
Forgetting bromine is a liquid, or mixing up the displacement colours.
- 03
Writing I₂ + KCl as a reaction that produces chlorine.
What is observed when chlorine is added to potassium bromide solution?
ANo change, because bromine is more reactive
BThe solution turns orange-brown as bromine is displaced
CA purple vapour of iodine forms
DA white precipitate of potassium chloride appears
Show the answer
The solution turns orange-brown as bromine is displaced. Chlorine is more reactive than bromine, so Cl₂ + 2KBr → 2KCl + Br₂. Aqueous bromine is orange-brown.
Quick questions
If this is the bit you searched
Why does reactivity decrease down Group 7?
Halogen atoms must gain an electron. Down the group there are more shells, so the extra electron is further from the nucleus and more shielded, and is gained less easily.
What is a halogen displacement reaction?
A more reactive halogen takes the place of a less reactive halogen in a salt solution, for example chlorine displacing bromine from potassium bromide.
Why are fluorine and chlorine gases but iodine a solid?
All are simple molecules. Intermolecular forces get stronger as the molecules get larger down the group, so boiling point rises and iodine is a solid at room temperature.
What is the colour of bromine water and iodine solution?
Bromine water is orange. Aqueous iodine (in potassium iodide) is brown. These colours are how you spot a displacement product.