Group VII properties
The halogens: colours and states, the trend in reactivity, and displacement reactions.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 8.3.1Describe the colours and physical states of chlorine, bromine and iodine and the trend down the group.
- 8.3.2Describe and explain displacement reactions of halogens with halide solutions.Supplement
6 minute read
Group VII: the halogens
Group VII contains the halogens: fluorine, chlorine, bromine, iodine and others. All have seven electrons in their outer shell, needing just one more to achieve a full shell — which is why they react readily by gaining an electron.
Colours and states at room temperature
- Chlorine: a pale green/yellow gas.
- Bromine: a red-brown liquid.
- Iodine: a grey-black solid (which sublimes to a purple vapour when heated).
Notice the pattern: melting and boiling points increase down the group, which is why the state changes from gas to liquid to solid as you go down.
Trend in reactivity
Reactivity decreases down the group — the opposite trend to Group I. Going down the group, the outer shell needing one more electron is further from the nucleus and more shielded by inner shells, so it is harder for the nucleus to attract an extra electron in from outside.
Displacement reactions
A more reactive halogen will displace a less reactive halogen from a solution of its salt. For example, chlorine displaces bromine from potassium bromide solution:
Cl₂ + 2KBr → 2KCl + Br₂
The colour change (colourless solution turning orange-brown as bromine is released) is used as visible evidence that a displacement has occurred, and this pattern can be used to rank the halogens' reactivity experimentally.
Think of it like this
Group VII reactivity works like Group I in reverse: instead of an atom trying to let go of one electron more easily as shielding increases, it is trying to grab one more electron, and the same extra shielding down the group makes that pull weaker — so fluorine (least shielded) reacts eagerly, and iodine (most shielded) is comparatively reluctant.
Common misconceptions
- Applying the Group I reactivity trend (increases down the group) to Group VII by mistake. Group VII reactivity decreases down the group — the opposite direction.
- Assuming a less reactive halogen can displace a more reactive one. Displacement only happens from more reactive to less reactive — chlorine can displace bromine and iodine, but iodine cannot displace chlorine.
- Forgetting that both trends (Group I and Group VII) are caused by the same underlying idea — increasing shielding down a group — just with opposite consequences depending on whether an electron is being lost or gained.
In the exam
- For a displacement reaction, describe both the equation AND the visible colour change — questions often ask specifically "what would you observe".
- When explaining the Group VII reactivity trend, use the phrase "harder to attract an extra electron" rather than "harder to react", to show you understand the electron-level mechanism.