The halogens are a family of non-metals that become less reactive as you move down Group 7. Fluorine is a pale-yellow gas so reactive it attacks glass; iodine is a dark-grey solid that sublimates to a purple vapour. Understanding their trends and displacement reactions is essential for GCSE chemistry.
What are the physical properties of the halogens?
The halogens are diatomic — they exist as pairs of atoms bonded together (F₂, Cl₂, Br₂, I₂). Their physical properties at room temperature and pressure follow clear trends:
| Halogen | Symbol | State at RTP | Colour | Melting point (°C) | Boiling point (°C) |
|---|---|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow | −220 | −188 |
| Chlorine | Cl₂ | Gas | Yellow-green | −101 | −34 |
| Bromine | Br₂ | Liquid | Orange-brown | −7 | +59 |
| Iodine | I₂ | Solid | Grey-black (purple vapour) | +114 | +184 |
| Astatine | At₂ | Solid | Black | +302 | +337 |
Trend: As you go down Group 7, melting and boiling points increase, and the colour deepens. This is because the molecules become larger (more electrons), increasing the strength of the van der Waals (intermolecular) forces between molecules. More energy is required to overcome these forces, hence higher melting and boiling points.
Why do reactivity and electronegativity decrease down Group 7?
Halogens react by gaining one electron to complete their outer shell and achieve a full outer electron configuration (like a noble gas). The ease with which a halogen can attract and gain an electron determines its reactivity.
Moving down Group 7:
- Each element has one more electron shell.
- The outer shell is therefore further from the nucleus.
- The nuclear attraction experienced by an incoming electron is increasingly shielded by inner electron shells.
- It becomes progressively harder to attract an electron to the outer shell.
Therefore reactivity decreases down the group: fluorine is the most reactive non-metal in the periodic table; iodine is considerably less reactive.
This also means that electronegativity (the ability to attract electrons in a bond) decreases down the group.
What happens when halogens react with metals?
Halogens react vigorously with metals to form metal halides, which are ionic salts. For example:
- Iron reacts with chlorine: 2Fe + 3Cl₂ → 2FeCl₃ (iron(III) chloride)
- Sodium reacts with chlorine: 2Na + Cl₂ → 2NaCl (sodium chloride — table salt)
- Aluminium reacts with bromine: 2Al + 3Br₂ → 2AlBr₃ (aluminium bromide)
The more reactive the halogen, the more vigorously the reaction proceeds. Fluorine reacts explosively with most metals; iodine reacts much more slowly and in some cases requires heating.
What are halogen displacement reactions and how do you test for them?
A more reactive halogen can displace a less reactive halogen from a solution of its salt. This follows the same principle as the reactivity series for metals.
Displacement rule: A halogen will displace a halide ion only if the halogen is more reactive (higher up Group 7).
| Reagents added | Reaction? | Observation |
|---|---|---|
| Cl₂(aq) + KBr(aq) | ✅ Yes | Solution turns orange-brown (Br₂ produced) |
| Cl₂(aq) + KI(aq) | ✅ Yes | Solution turns brown/black (I₂ produced) |
| Br₂(aq) + KI(aq) | ✅ Yes | Solution turns brown (I₂ produced) |
| Br₂(aq) + KCl(aq) | ❌ No | No change (Br less reactive than Cl) |
| I₂(aq) + KBr(aq) | ❌ No | No change (I less reactive than Br) |
| I₂(aq) + KCl(aq) | ❌ No | No change (I less reactive than Cl) |
Worked example — writing the ionic equation:
Chlorine displacing bromide: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
The ionic equation (removing spectator K⁺ ions): Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
This is also a redox reaction: chlorine is reduced (gains electrons: Cl₂ → 2Cl⁻) and bromide is oxidised (loses electrons: 2Br⁻ → Br₂).
To confirm which halogen has been produced, add a few drops of cyclohexane (an organic solvent) and shake. The halogen preferentially dissolves in the less polar cyclohexane layer:
- Cl₂: pale yellow cyclohexane layer
- Br₂: orange cyclohexane layer
- I₂: violet/purple cyclohexane layer
What are the uses of halogens?
The halogens' reactivity and specific properties make them useful in many industries:
| Halogen | Key uses | Reason |
|---|---|---|
| Fluorine (as fluoride) | Toothpaste (fluoride ions harden tooth enamel); PTFE (non-stick coatings); HFCs (refrigerants) | Extremely electronegative; forms very stable C–F bonds |
| Chlorine | Water treatment; bleach (NaOCl); PVC plastics; disinfectants | Kills microorganisms; reactive enough to form useful compounds |
| Bromine | Flame retardants in furniture and electronics | Disrupts combustion chain reactions |
| Iodine | Antiseptic (iodine solution); starch test (turns blue-black with starch) | Kills bacteria; specific colour reaction with starch |
How are halogens tested for in the laboratory?
At GCSE you need to know two identification tests:
Testing for chloride, bromide, or iodide ions: Add dilute nitric acid, then silver nitrate solution (AgNO₃):
- Chloride ions (Cl⁻): white precipitate of silver chloride (AgCl), soluble in dilute ammonia
- Bromide ions (Br⁻): cream precipitate of silver bromide (AgBr), soluble in concentrated ammonia only
- Iodide ions (I⁻): yellow precipitate of silver iodide (AgI), insoluble in ammonia
Testing for iodine: Add starch solution → turns blue-black in the presence of iodine. This test is used in food tests and to detect iodine produced in displacement reactions.
How should you answer Group 7 trend questions?
For any question asking you to explain a trend in Group 7, structure your answer around three steps:
- State the trend clearly (e.g. "reactivity decreases down Group 7").
- Explain the electron structure (e.g. "each element has one more electron shell").
- Link to attraction of electrons (e.g. "the outer shell is further from the nucleus and more shielded, so the atom attracts electrons less strongly").
Avoid simply saying "the atom gets bigger" — examiners want the full explanation involving shielding and distance from the nucleus.
Frequently asked questions
Why is fluorine so much more reactive than the other halogens?
Fluorine has only two electron shells — its outer electrons are extremely close to the nucleus and experience minimal shielding. This makes fluorine exceptionally good at attracting electrons, giving it the highest electronegativity of any element. It reacts violently or explosively with most substances and cannot be stored in glass (it reacts with silicon dioxide). For safety, fluorine chemistry at school and university is done with specially designed apparatus and strict protocols.
What colour does iodine turn in cyclohexane?
When iodine (I₂) dissolves in cyclohexane (a non-polar organic solvent), it produces a distinctive violet or purple colour. This is different from its appearance in water, where iodine produces a yellow-brown colour. The colour difference arises because the solvent affects the energy levels of iodine's electrons, changing the wavelength of light absorbed. The violet colour in cyclohexane is the test used to confirm that iodine has been produced in a halogen displacement reaction.
Do halogens react with water?
Yes. Chlorine dissolves in water and partially disproportionates: Cl₂(aq) + H₂O(l) ⇌ HCl(aq) + HClO(aq)
The solution formed (containing hydrochloric acid and hypochlorous acid, HClO) is called chlorine water. Hypochlorous acid is the active bleaching and disinfecting agent. This equilibrium is the basis of both swimming pool treatment (chlorine kills bacteria via HClO) and the manufacture of bleach (sodium hypochlorite, NaOCl). Bromine and iodine undergo similar but less vigorous reactions with water.
Why does the state of halogens change from gas to solid down the group?
Halogens are non-polar diatomic molecules and are held together in bulk only by weak van der Waals (London dispersion) forces between molecules. The strength of these forces depends on the number of electrons in the molecule: more electrons means a larger, more easily distorted electron cloud, which creates larger temporary dipoles and stronger intermolecular forces. Going from F₂ (18 electrons total) to I₂ (106 electrons total), the forces increase significantly — enough to raise the melting and boiling points above room temperature, making iodine a solid.
Let Professor Curie at aitutors.me build your mental model of Group 7 — particle by particle, trend by trend — until the halogen displacement reactions feel completely logical.