Group 7Edexcel GCSE Chemistry: Revision notes
Section 1
What are the halogens, and what do they look like?
Group 7 elements are called the halogens. At room temperature:
| Halogen | Colour | State |
|---|---|---|
| Chlorine | Pale green/yellow | Gas |
| Bromine | Red-brown | Liquid |
| Iodine | Grey/purple-black | Solid |
Going down the group, boiling point increases — this is why the state changes from gas (chlorine) to liquid (bromine) to solid (iodine) at room temperature. This trend can be used to predict the physical properties of other halogens, such as astatine (a solid, likely dark in colour).
Section 2
How do halogens react with metals and hydrogen?
Halogens react with metals to form metal halides (ionic salts), e.g.:
sodium + chlorine → sodium chloride
Halogens also form hydrogen halides (e.g. hydrogen chloride, HCl) by reacting with hydrogen. These hydrogen halides dissolve in water to form acidic solutions (e.g. hydrochloric acid).
Chemical test for chlorine: chlorine gas bleaches damp litmus paper, turning it white.
For the chlorine test, be precise: chlorine bleaches damp litmus paper, turning it white (not just 'changes colour').
Section 3
What are displacement reactions, and what pattern do they show?
A more reactive halogen can displace a less reactive halogen from a solution of its metal halide (halide ion). For example:
chlorine + potassium bromide → potassium chloride + bromine
This works because chlorine is more reactive than bromine. If a less reactive halogen is added to a solution of a more reactive halide, no reaction occurs.
These displacement reactions demonstrate the relative reactivity of the halogens: chlorine > bromine > iodine, and can be used to predict that astatine (below iodine) would be less reactive still, and would be displaced by all three named halogens.
Bromine water added to potassium iodide solution turns from orange to brown/black as iodine is displaced and bromide ions form — a visible colour-change test for reactivity.
Section 4
Why are these displacement reactions redox, and why does reactivity fall down the group?
Halogen displacement reactions are redox reactions, explained in terms of electron transfer:
- The more reactive halogen gains electrons to form a halide ion — it is reduced
- The less reactive halide ion loses its electron back to form a halogen molecule — it is oxidised
e.g. Cl2 + 2Br- → 2Cl- + Br2: chlorine is reduced (gains electrons), bromide ions are oxidised (lose electrons).
Reactivity decreases down Group 7 — the opposite trend to Group 1. This is because halogen atoms react by gaining one electron into their outer shell. Going down the group, atoms have more electron shells, so the outer shell is further from the nucleus and more shielded — this makes it harder to attract and gain an extra electron, so reactivity falls.
Group 7 reactivity DECREASES down the group (opposite to Group 1) — a very common exam mix-up, because Group 7 atoms gain electrons rather than lose them.
Must Know
- Group 7 = halogens: chlorine (pale green/yellow gas), bromine (red-brown liquid), iodine (grey/purple-black solid); boiling point increases down the group
- Halogens + metals → metal halides; halogens + hydrogen → hydrogen halides, which dissolve in water to form acidic solutions
- Chemical test for chlorine: bleaches damp litmus paper white
- A more reactive halogen displaces a less reactive one from its halide solution
- Reactivity decreases down Group 7 (Cl > Br > I) — the opposite trend to Group 1
- Displacement reactions are redox: the halogen is reduced (gains electrons), the halide ion is oxidised (loses an electron); harder to gain the extra electron down the group due to more shielding/distance from the nucleus
That's the notes covered.
Carry on to the next subtopic.