Group 17: halogens and displacement
Key idea: A more reactive halogen displaces a less reactive one from its halide solution. Reactivity decreases down Group 17 because gaining an electron becomes harder.
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Guided lesson
Guided steps
- Compare the pure halogens and their solutions.
- Look for the colour of a newly formed halogen.
- Decide whether the colour shows displacement.
- Choose the more reactive halogen and the product.
- Explain which particles gain and lose electrons.
- Write the ionic equation and check atoms and charge.
- Compare distance, shielding and electron gain.
- Explain why halogen reactivity decreases down the group.
- Use the trends to predict fluorine’s properties.
- Predict astatine’s properties and explain what remains unknown.
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ContinueChlorine, bromine and iodine can displace one another from halide solutions, but only in one direction. Use reaction evidence to order the halogens, explain their electron-gain pattern, and tell apart the added halogen and the halide ions already in solution.
You need Periodic Table position and outer electrons, plus the distinction between an atom and an ion.
Group 17: the halogens
Chlorine, bromine and iodine are the Group 17 elements in this course. Each is a non-metal made of diatomic molecules: Cl₂, Br₂ and I₂.
| Element | Formula | State at room temperature | Colour | Melting point (°C) | Boiling point (°C) |
|---|---|---|---|---|---|
| chlorine | Cl₂ | gas | pale green | −101 | −34 |
| bromine | Br₂ | liquid | red-brown | −7 | 59 |
| iodine | I₂ | solid | grey-black (gives a purple vapour) | 114 | 184 |
Down the group the colour gets darker, and the melting and boiling points increase, so the state changes from gas to liquid to solid.
Displacement reactions
To compare how reactive the halogens are, add a solution of each halogen to colourless solutions of potassium halides. These are the results:
| Solution added | to potassium chloride | to potassium bromide | to potassium iodide |
|---|---|---|---|
| chlorine water (very pale green) | — | turns orange | turns brown |
| bromine water (orange) | stays orange | — | turns brown |
| iodine solution (brown) | stays brown | stays brown | — |
In these comparisons, the formation of orange bromine or brown iodine is evidence of displacement. Chlorine turns bromide ions into orange bromine and iodide ions into brown iodine. Bromine turns iodide ions into iodine. Iodine displaces neither chloride nor bromide. “Stays orange” is not evidence of displacement: the orange is the bromine that was added.
Keep pure elements and aqueous solutions separate. Pure iodine is a grey-black solid; iodine dissolved in water is brown. Adding a colourless halide solution can dilute the added halogen and make its colour paler even when no displacement occurs. Look for the colour of a newly formed halogen, rather than treating every change in colour intensity as a reaction.
So chlorine is the most reactive and iodine the least. A more reactive halogen displaces a less reactive halogen from a solution of its halide:
The potassium ions are unchanged, so the second equation shows only the particles that react.
Why reactivity decreases. A halogen atom has seven outer electrons and reacts by gaining one to form a 1− ion. Going down the group, the incoming electron enters a shell farther from the nucleus and more shielded by inner shells. It is attracted less strongly, so it is gained less readily and the halogen is less reactive.
Check your understanding 1 Does orange mean displacement?
Orange bromine water is added to colourless potassium chloride solution. The mixture remains orange, although dilution makes it paler. Which conclusion follows?
Show answer
No displacement is demonstrated. Bromine is less reactive than chlorine, and the orange colour belongs to the bromine added. A paler mixture can result simply from dilution.
Spot the mistake 1
Which halogen does the displacing?
Learner claim
Compare the halogens, not their places in the list
Show solution step by step
Apply the group trend
Method
Rank chlorine above bromine.Reason
Group 17 reactivity decreases down the group.Working
Answer
Cl₂ is more reactive than Br₂.Predict the displacement
Method
The solution turns orange as bromine forms.Reason
The more reactive chlorine takes the electron from each bromide ion.Working
Answer
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Common mistake
Check your understanding 2 Explain the displacement direction
Chlorine water is added to potassium bromide solution. Correct the claim that bromide displaces chlorine because bromine is lower in Group 17. Explain the direction and electron transfer on paper or in the optional notes.
Show answer
Chlorine is more reactive than bromine; reactivity decreases down Group 17. Chlorine gains electrons from bromide ions to form chloride ions. Bromide ions lose electrons and form bromine, which makes the solution orange. The added halogen is chlorine, not bromide: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). Potassium ions remain unchanged.
Check your understanding 3 Chlorine and iodide ions
Chlorine water is added to potassium iodide solution. Name the halogen formed and write the ionic equation.
Write the equation on paper or in the optional notes, then compare the product, states, coefficients, atoms and charge.
Show answer
Iodine forms: Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq). The solution becomes brown. Each side has two Cl and two I atoms and total charge −2; potassium ions are unchanged spectators.
Spot the mistake 2
Explain the Group 17 trend
Learner claim
A learner says bromine is more reactive than chlorine because bromine atoms are larger. First compare distance, shielding and electron gain. The next activity asks for your complete explanation of the corrected trend.
Connect shells and shielding to attraction
Show solution step by step
Compare the atoms
Method
Chlorine has three occupied shells; bromine has four.
Reason
The electron chlorine gains ends up closer to the nucleus, with less shielding.
Working
Answer
Chlorine: shorter distance, less shielding.
Connect to reactivity
Method
Chlorine gains an electron more readily.Reason
Its nucleus attracts the incoming electron more strongly.
Working
Answer
Chlorine is more reactive than bromine; reactivity decreases down Group 17.
Common mistake
“Atoms get larger” is true for both Group 1 and Group 17, yet the reactivity trends run in opposite directions. Size explains a trend only when you say whether the atom is losing or gaining an electron.
Check your understanding 4 Explain the complete electron-gain chain
Correct the claim that bromine is more reactive than chlorine because its atoms are larger. Explain the Group 17 trend using occupied shells, shielding, attraction of the incoming electron and ease of electron gain. Work on paper or in the optional notes.
Show answer
A halogen atom gains one electron to form a 1− ion. Down Group 17 there are more occupied shells, so the incoming electron is farther from the nucleus and more shielded. It is attracted less strongly and gained less readily; reactivity decreases. Chlorine has three occupied shells compared with bromine’s four, so chlorine gains an electron more readily and is more reactive. Size alone does not explain the direction: electron gain matters here, while Group 1 reacts by electron loss.
Check your understanding 5 Predict fluorine
Fluorine is above chlorine in Group 17. Use the trends to predict its state, colour relative to chlorine, and reactivity.
Show answer
The trends predict a gas, paler than chlorine, and greater reactivity. Fluorine’s incoming electron is closer to the nucleus and less shielded.
Check your understanding 6 Predict astatine
Astatine is below iodine. What state and colour would you predict by extending the trend? Does this tell you what a bulk sample looks like?
Show answer
The simple trend predicts a dark solid, but a prediction is not an observation. Astatine’s bulk appearance is uncertain. The trend also predicts that it would not displace iodide; an exact melting point cannot be read from its position.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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