Properties of Group 7 (Halogens) — AQA GCSE Chemistry
Group 7 elements — the halogens — include fluorine, chlorine, bromine and iodine.
Properties
- They have seven electrons in their outer shell.
- They exist as diatomic molecules (e.g. Cl₂, Br₂, I₂).
- Their appearance changes down the group: chlorine is a green gas, bromine an orange liquid, iodine a grey solid.
- Melting and boiling points increase down the group (molecules get larger, with stronger intermolecular forces).
Forming ions and compounds
Halogens gain one electron to form a −1 ion (e.g. Cl⁻), or share electrons in covalent bonds. With metals they form ionic salts; with non-metals they form covalent compounds.
Reactivity trend
Reactivity decreases down the group (F > Cl > Br > I). This is because:
- the outer shell is further from the nucleus with more shielding,
- so it is harder to attract and gain an electron.
Displacement reactions
A more reactive halogen displaces a less reactive halogen from a solution of its salt. For example: chlorine + potassium bromide → potassium chloride + bromine (Chlorine, being more reactive, displaces bromine.)
Exam tips
- Halogens have seven outer electrons, are diatomic, and form −1 ions.
- Reactivity decreases down the group — explain using distance/shielding.
- Melting/boiling points increase down the group.
- A more reactive halogen displaces a less reactive one from its salt.