Group 7 — The HalogensAQA GCSE Chemistry: Revision notes
Section 1
Why are the halogens grouped together?
The periodic table is arranged in order of atomic (proton) number, placing elements with similar properties in the same column, or group. Group 7, the halogens, includes fluorine, chlorine, bromine and iodine.
Every halogen atom has seven electrons in its outer shell, which is why they share similar chemical behaviour.
Section 2
What compounds do halogens form?
Halogens react with metals and non-metals to form a range of compounds:
- With metals, halogens form ionic compounds called metal halides (e.g. sodium chloride, NaCl)
- With non-metals (including hydrogen), halogens form covalent compounds (e.g. hydrogen chloride, HCl)
Halogen molecules themselves exist as diatomic molecules — two atoms sharing a covalent bond, e.g. Cl2, Br2, I2.
Section 3
How do melting point, boiling point and reactivity change down the group?
| Property | Trend down Group 7 |
|---|---|
| Melting point | Increases |
| Boiling point | Increases |
| Relative molecular mass | Increases |
| Reactivity | Decreases |
This is why fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid.
Reactivity decreases down the group because the outer shell is further from the nucleus and more shielded, so it is harder for the atom to attract an extra electron to complete its outer shell.
Halogens get less reactive down the group — the opposite trend to Group 1 metals, which get more reactive down the group.
Section 4
What is a halogen displacement reaction?
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
For example, chlorine (more reactive) displaces bromine from potassium bromide solution:
Cl2 + 2KBr → 2KCl + Br2
The colour change (colourless to orange, as bromine is released) provides evidence for the relative reactivity of the halogens, and can be used to predict and confirm the trend down the group.
Adding chlorine water to potassium iodide solution turns it brown/orange as iodine is displaced, because chlorine is more reactive than iodine.
Must Know
- All Group 7 elements (halogens) have seven electrons in their outer shell
- Halogens form ionic metal halides with metals, and covalent compounds with non-metals
- Melting point, boiling point and relative molecular mass increase down the group
- Reactivity decreases going down the group
- A more reactive halogen displaces a less reactive halogen from a solution of its salt
That's the notes covered.
Carry on to the next subtopic.