Separate halogen molecules from halide ions, and explain why boiling point rises while oxidising strength falls down the group.
Know which particle the question names
Halogens are Group 17 elements, also called Group 7 in A-Level teaching. Their atoms have outer configuration ns² np⁵. A halogen molecule is X₂; a halide ion is X⁻. Chlorine, Cl₂, and chloride, Cl⁻, do different jobs in a redox reaction.
A halogen molecule accepts two electrons to make two halide ions. It is an oxidising agent and is itself reduced. Halide ions can donate electrons to form a halogen; the ions then act as reducing agents.
| Halogen | Pure substance near room temperature | Typical aqueous appearance |
|---|---|---|
| F₂ | Very pale yellow gas | Not a school aqueous displacement reagent |
| Cl₂ | Pale green gas | Pale green; very dilute solutions may look almost colourless |
| Br₂ | Red-brown liquid with coloured vapour | Yellow-orange to orange-brown, depending on concentration |
| I₂ | Grey-black solid; purple vapour on heating | Brown; excess iodide increases solubility through I₃⁻ formation |
Boiling point: compare forces between molecules
Down the group the X₂ molecules have more electrons and a larger, more polarisable electron cloud. Instantaneous dipoles more readily induce dipoles in neighbouring molecules, giving stronger London forces. More energy is required to separate the molecules, so boiling points increase.
The covalent X–X bond is inside each molecule and is not the bond broken during boiling. “Bonds get stronger down the group” is therefore an ambiguous and potentially wrong explanation. Melting points also generally rise, but melting depends on the crystal arrangement as well as intermolecular attraction.
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Halogen boiling-point explanation
Labels to include:
- Separate pairs of X₂ molecules, with covalent bonds inside each molecule
- Instantaneous δ+ and δ− ends on one molecule
- Induced opposite dipole on a neighbouring molecule
- Dotted intermolecular attraction, distinct from the solid covalent bond
- Larger electron cloud down the group; no invented numerical scale
Heating overcomes attractions between molecules. The molecules remain X₂ in the vapour.
Electronegativity: specify the bonding pair
Electronegativity is an atom’s relative tendency to attract the shared electrons of a covalent bond. It decreases down the halogens. Additional shells increase distance and shielding, reducing the attraction between the nucleus and the bonding pair despite the greater proton number.
All X₂ molecules are non-polar: their atoms have equal electronegativity, so the bond has no permanent dipole. This does not mean an individual halogen atom has zero electronegativity.
Oxidising strength and predicting displacement
Oxidising ability decreases down the group. In the A-Level explanation, added electrons are accepted into a shell farther from the nucleus with more shielding, so attraction for an incoming electron is weaker. For the aqueous Cl₂/Br₂/I₂ tests, chlorine is the strongest oxidising agent and iodine the weakest.
A halogen can oxidise halide ions below it in the group. Cl₂ reacts with Br⁻ and I⁻; Br₂ reacts with I⁻; I₂ does not displace Cl⁻ or Br⁻. Do not predict a displacement just because the mixture is coloured: the reagent itself may be coloured.
| Added halogen | Cl⁻ solution | Br⁻ solution | I⁻ solution |
|---|---|---|---|
| Cl₂ | No net displacement | Br₂ forms | I₂ forms |
| Br₂ | No displacement | No net displacement | I₂ forms |
| I₂ | No displacement | No displacement | No net displacement |
Observation → product → electron transfer
Example: add a small quantity of chlorine water to excess potassium bromide solution. Formation of an orange-brown solution indicates bromine. Chlorine accepts electrons, so it is reduced; bromide loses electrons, so it is oxidised. Potassium ions are spectators.
Use matching quantities and compare with the starting solutions. In an optional organic-solvent extraction, bromine gives an orange layer and iodine a violet/purple layer. Specify the solvent layer: purple iodine in an organic solvent is not the usual colour of aqueous iodine.
Halogen solutions and organic solvents require the supervised practical method, appropriate ventilation and eye protection. Fluorine is not used for this school experiment.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Explain why iodine has a higher boiling point than bromine.Show answer
I₂ has more electrons and a larger, more polarisable electron cloud. Stronger London forces act between its molecules, so more energy is needed to separate them. Do not attribute this to breaking I–I bonds.
Q2. Why does electronegativity decrease from chlorine to iodine?Show answer
The bonding pair is farther from the nucleus and more shielded by inner shells. Attraction between nucleus and bonding pair becomes weaker despite the increase in proton number.
Q3. Predict what happens when bromine water is added separately to KCl and KI solutions.Show answer
KCl: no displacement. KI: iodide is oxidised to iodine, giving a brown aqueous mixture under suitable conditions. Br₂ + 2I⁻ → 2Br⁻ + I₂.
Q4. Identify both agents in Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.Show answer
Cl₂ is the oxidising agent: it gains electrons. Br⁻ is the reducing agent: it loses electrons. Name the charged ion Br⁻, not Br₂, as the reducing agent.
Q5. 0.00300 mol Br₂ reacts completely with excess iodide. Find the amounts of I⁻ consumed and I₂ formed.Show answer
Br₂: I⁻: I₂ = 1:2:1. Therefore 0.00600 mol I⁻ is consumed and 0.00300 mol I₂ forms.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise — AQA 2.3 Halogens (January 2022) — Primary coverage checklist, pp1–3; original explanations and questions.
- AQA 7405 specification — 3.2.3.1–3.2.3.2 and Required Practical 4.
- AQA 7404/1 June 2023 mark scheme — Q06.1, p18.
- AQA 7404/1 June 2023 examiner report — Q06.1, p4.
- AQA 7404/1 2021 mark scheme (November archive) — Q05.1, p15: instantaneous and induced dipoles.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.