2.2 Group 7 (Halogens)Edexcel IGCSE Chemistry: Revision notes
Section 1
What are the halogens and what do they look like?
The halogens are the non-metal elements in Group 7 of the Periodic Table: fluorine, chlorine, bromine, iodine and astatine. At room temperature and pressure they show a clear trend in physical state:
| Halogen | Colour | State at room temperature |
|---|---|---|
| Chlorine (Cl2) | Pale green/yellow-green | Gas |
| Bromine (Br2) | Red-brown | Liquid (with an orange-brown vapour) |
| Iodine (I2) | Grey-black solid / purple vapour | Solid (sublimes to a purple vapour on heating) |
Going down the group, the halogens get darker in colour, and their melting and boiling points increase, which is why the state changes from gas to liquid to solid.
Examiners often ask for colour AND state together — always give both, e.g. 'bromine is a red-brown liquid', not just 'red-brown'.
Section 2
How do melting and boiling points change down the group?
Melting and boiling points increase down Group 7 as the relative molecular mass increases. This is because the molecules are held together by weak intermolecular forces, and these forces get stronger as the molecules get bigger (more electrons), so more energy is needed to separate them.
This trend lets you predict properties of other halogens:
- Fluorine (above chlorine) is a pale yellow gas, even more reactive than chlorine.
- Astatine (below iodine) is predicted to be a dark solid, less reactive than iodine.
Section 3
How do displacement reactions show the trend in reactivity?
Reactivity in Group 7 decreases down the group. This can be shown using displacement reactions: a more reactive halogen will displace a less reactive halide ion from a solution of its salt.
For example:
- Chlorine + potassium bromide solution → potassium chloride + bromine (chlorine displaces bromine)
- Chlorine + potassium iodide solution → potassium chloride + iodine (chlorine displaces iodine)
- Bromine + potassium iodide solution → potassium bromide + iodine (bromine displaces iodine)
- Bromine does not displace chlorine from potassium chloride, because bromine is less reactive than chlorine.
You can observe these reactions by colour change, e.g. a colourless potassium bromide solution turns orange as bromine is formed when chlorine is added.
Chlorine gas bubbled through colourless potassium iodide solution turns the solution brown, because chlorine displaces iodine: Cl2 + 2KI → 2KCl + I2
Section 4
Why does reactivity decrease down Group 7?
Halogen atoms react by gaining one electron to form a stable, negatively charged halide ion with a full outer shell (electron configuration like the nearest noble gas).
Going down the group:
- Atoms get larger (more electron shells).
- The outer shell is further from the nucleus and is increasingly shielded by inner shells.
- The nucleus therefore attracts an incoming electron less strongly.
This means it becomes harder to gain an electron as you go down the group, so reactivity decreases down Group 7 — the opposite trend to Group 1, where reactivity increases down the group because it becomes easier to lose an electron.
Do not say halogens 'lose' electrons to react — halogens gain one electron each to become halide ions; it is Group 1 metals that lose electrons.
Must Know
- Chlorine: pale green gas; bromine: red-brown liquid; iodine: grey-black solid (purple vapour).
- Melting/boiling points increase down the group; reactivity decreases down the group.
- A more reactive halogen displaces a less reactive halide from solution (e.g. chlorine displaces bromine and iodine).
- Reactivity decreases down the group because atoms get larger, so the outer shell is further from the nucleus and more shielded, making it harder to gain an electron.
- Halogens react by gaining one electron each to form a 1- halide ion.
- Trends can be used to predict the properties of fluorine and astatine.
That's the notes covered.
Carry on to the next subtopic.