Group VII Properties
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
The Halogens: Chlorine, Bromine, Iodine
Group VII contains the halogens. The syllabus focuses on chlorine (Cl), bromine (Br) and iodine (I) - reactive diatomic non-metals (Cl₂, Br₂, I₂), each with 7 outer-shell electrons.
Appearance at Room Temperature and Pressure
| Halogen | State | Colour |
|---|---|---|
| Chlorine, Cl₂ | gas | pale yellow-green |
| Bromine, Br₂ | liquid | red-brown |
| Iodine, I₂ | solid | grey-black (gives a purple vapour when heated) |
Worked Example: Identifying a Halogen's Properties
- Question: which statement about iodine is correct? (A) it is a monatomic non-metallic element (B) it is a solid at room temperature and pressure (C) it is less dense than bromine (D) it is more reactive than chlorine [1]
- Answer: B. Iodine is a grey-black solid at r.t.p. It is diatomic (I₂), not monatomic (ruling out A); it is denser than bromine, not less dense (ruling out C); and it is less reactive than chlorine, since reactivity decreases down the group (ruling out D).
Trends Down Group VII
| Property | Trend down the group (Cl → Br → I) |
|---|---|
| Density | increases |
| Reactivity | decreases |
Reactivity decreases down the group because the outer shell (where an incoming electron must be gained) is further from the nucleus and more shielded, so it's harder to attract an extra electron.
This is the opposite trend to Group I: metals get more reactive down a group (easier to lose an electron), while these non-metals get less reactive down a group (harder to gain one).
Halogen Displacement Reactions
A more reactive halogen will displace a less reactive halogen from a solution of its halide. Since reactivity decreases down the group (Cl > Br > I), these reactions provide direct evidence for the reactivity order.
more reactive halogen + less reactive halide → less reactive halogen + more reactive halide
| Halogen added | Halide solution | Reaction? | Observation |
|---|---|---|---|
| Chlorine | potassium bromide | yes | colourless → orange (Br₂ forms) |
| Chlorine | potassium iodide | yes | colourless → brown (I₂ forms) |
| Bromine | potassium iodide | yes | colourless → brown (I₂ forms) |
| Bromine | potassium chloride | no | no colour change - bromine is less reactive than chlorine |
Equation: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
The more reactive halogen has a greater tendency to gain electrons than the halide ion already in solution, so it takes the halide ion's place, pushing out the less reactive halogen as the free element.
Worked Example: Confirming the Halogen Produced
- Question: chlorine water is added to potassium iodide solution and the mixture turns brown. Describe how you could confirm iodine has been produced. [2]
- Answer: shake the mixture with an organic solvent such as cyclohexane and allow the layers to separate [1]; a purple layer confirms iodine is present (bromine would instead give an orange layer) [1]
Only a more reactive halogen displaces a less reactive one; the colour of the organic layer identifies which halogen was produced
Common Mistakes
- ❌ Assuming a displacement reaction always occurs when two halogens mix
- ✓ check the reactivity order first (Cl > Br > I) - a less reactive halogen added to a more reactive halide gives NO reaction
Predicting Properties of Other Group VII Elements
Fluorine (above chlorine) and astatine (below iodine) are outside the syllabus's named halogens, but their approximate properties can be predicted by extending the trends above.
Worked Example
- Question: predict whether fluorine is more or less reactive than chlorine, and whether it is denser or less dense. [2]
- Answer: fluorine is above chlorine, so it should be more reactive (reactivity decreases down the group) and less dense (density increases down the group)
Common Mistakes
- ❌ Calling iodine "purple-black"
- ✓ the solid itself is grey-black; the purple colour is the vapour given off when iodine is gently heated (it sublimes)
- ❌ Applying the Group I reactivity trend (increases down the group) to Group VII
- ✓ Group VII reactivity decreases down the group - the opposite direction, because non-metals react by gaining electrons, not losing them
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