8.3 Group VII properties
- Syllabus
- 0620–2026–2027
- Topic
- 8.3
- Level
- —
Learning objectives
8.3.1Group VII halogens, chlorine, bromine• Describe Group VII halogens, chlorine, bromine and iodine, as diatomic non-metals with general trends down the group, (a) increasing density (b) decreasing reactivity8.3.2Appearance of the halogens at r.t.p• State the appearance of the halogens at r.t.p. as: (a) chlorine, a pale yellow-green gas (b) bromine, a red-brown liquid (c) iodine, a grey-black solid8.3.3Displacement reactions of halogens• Describe/explain the displacement reactions of halogens with other halide ions8.3.4Predict the properties of other• Predict the properties of other elements in Group VII, given information about the elements
Describe the Group VII trends
Chlorine, bromine and iodine are Group VII halogens. They are non-metals that exist as diatomic molecules: Cl₂, Br₂ and I₂.
| Going down Group VII | Trend | Chlorine → bromine → iodine |
|---|---|---|
| density | increases | lowest → intermediate → highest |
| reactivity | decreases | most reactive → less reactive → least reactive |
A more reactive halogen gains electrons more readily. Therefore chlorine reacts in situations where bromine or iodine may not, while iodine is less able to displace other halogens from their halides.
Group VII shows the opposite reactivity direction to Group I: halogen reactivity decreases down the group. Density, however, increases down Group VII.
State halogen appearances at r.t.p.
| Halogen | Colour at r.t.p. | State at r.t.p. | Complete appearance |
|---|---|---|---|
| chlorine | pale yellow-green | gas | pale yellow-green gas |
| bromine | red-brown | liquid | red-brown liquid |
| iodine | grey-black | solid | grey-black solid |
From chlorine to bromine to iodine, the colour becomes darker and the state changes from gas to liquid to solid.
When asked for appearance, give both colour and physical state, using room temperature and pressure as the condition.
Do not describe iodine crystals as purple at r.t.p.: solid iodine is grey-black. Purple describes iodine vapour, not the required room-condition solid.
Predict halogen displacement reactions
A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide ions. Reactivity decreases F₂ > Cl₂ > Br₂ > I₂.
| Added halogen | chloride, Cl⁻ | bromide, Br⁻ | iodide, I⁻ |
|---|---|---|---|
| chlorine, Cl₂ | no reaction | reaction: Br₂ forms | reaction: I₂ forms |
| bromine, Br₂ | no reaction | no reaction | reaction: I₂ forms |
| iodine, I₂ | no reaction | no reaction | no reaction |
For chlorine and bromide ions: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). Chlorine gains electrons and is reduced; bromide ions lose electrons and are oxidised.
Compare the elemental halogen with the halogen in the halide ion. Bromine displaces iodide but not chloride, because bromine is more reactive than iodine but less reactive than chlorine.
Predict properties of another Group VII element
Use the position of the unfamiliar halogen and the trends supplied by known Group VII elements to make bounded predictions.
| Position | Density | Reactivity | Likely appearance/state |
|---|---|---|---|
| above chlorine, such as fluorine | lower | higher | paler and gaseous |
| below iodine, such as astatine | higher | lower | darker and solid |
First place the element above, between or below known halogens. Extend each trend separately, and if numerical data are supplied, choose or estimate a value that continues the pattern without claiming unsupported precision.
Do not use only one trend to predict every property. Density increases and reactivity decreases down the group; state and colour predictions must be inferred from the specific information provided.