Group 7 — the halogens
Group 7 (or Group 17 in modern numbering) contains fluorine, chlorine, bromine, iodine and astatine — the halogens. They sit just before Group 0 and are the most reactive non-metals.
Electronic structure
Each halogen has 7 electrons in its outer shell:
- F: 2,7
- Cl: 2,8,7
- Br: 2,8,18,7
So each needs to gain just one electron to fill its outer shell, forming a −1 ion (F⁻, Cl⁻, Br⁻, I⁻). They readily react with metals to form ionic halides (e.g. NaCl) and with non-metals to form covalent compounds (e.g. HCl).
Diatomic molecules
Halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) — two atoms share a covalent bond to give each atom a full outer shell.
Physical properties (and trends down the group)
| Halogen | State at room T | Colour |
|---|---|---|
| F₂ | Gas | Pale yellow |
| Cl₂ | Gas | Yellow-green |
| Br₂ | Liquid | Red-brown |
| I₂ | Solid | Dark grey/purple |
Trend:
- Melting / boiling points increase down the group (more electrons → stronger intermolecular forces).
- Colours get darker / more intense down the group.
- Density increases.
- Reactivity decreases down the group (the opposite of Group 1).
Reactivity trend — and why
Reactivity decreases down Group 7 because:
- Atoms get larger down the group.
- The outer shell is further from the nucleus.
- More inner shells shield the incoming electron from the positive nucleus.
- So the atom attracts an electron less strongly → less reactive.
This is the inverse of Group 1, but the same explanation principle.
Displacement reactions — the classic exam question
A more reactive halogen will displace a less reactive halogen from a solution of its salt:
- Cl₂ + 2NaBr → 2NaCl + Br₂ (chlorine displaces bromine; orange-brown colour appears)
- Cl₂ + 2NaI → 2NaCl + I₂ (chlorine displaces iodine; brown/black colour appears)
- Br₂ + 2NaI → 2NaBr + I₂ (bromine displaces iodine)
But: iodine cannot displace chlorine or bromine — it's less reactive.
A typical experiment uses a 3 × 3 grid of test tubes adding each halogen to each halide solution and recording colour changes.
Reaction with metals
2Na(s) + Cl₂(g) → 2NaCl(s) — exothermic, white ionic salt.
This works for Mg, Fe and others, with reactivity matching the halogen's place in the group.
Hydrogen halides (extension)
Halogens react with hydrogen to form gases that dissolve in water as strong acids (HCl, HBr, HI). Reactivity decreases down the group: F₂ + H₂ is explosive in dark; I₂ + H₂ requires heating and is partially reversible.
⚠Common mistakes
- Saying halogens form +1 ions. They form −1 ions.
- Saying reactivity increases down Group 7. It decreases — the opposite of Group 1.
- Forgetting halogens are diatomic. Always Cl₂, not Cl.
- Mixing displacement direction. A more reactive halogen displaces a less reactive one from solution; the other way doesn't happen.
Links
Builds on C1.4 (electronic structure) and contrasts with C1.7 (Group 1 reactivity). Halogen ions appear again in C4 (acids), C8.5 (anion tests).
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