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Group 17 Elements: The Halogens

Learn Group 17 halogen colours and states, the reactivity trend, displacement predictions, balanced ionic equations and precise observations.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • predict the properties of elements in Group 1 and Group 17 using the Periodic Table.
  • describe chlorine, bromine and iodine in Group 17 (the halogens) as a collection of diatomic non- metals showing a trend in colour, state and their displacement reactions with solutions of other halide ions

The Group 17 halogens are diatomic non-metals. O-Level questions connect their colours and physical states with a decreasing reactivity trend and use displacement results as experimental evidence.

Group 17 reactivity and halogen displacement outcomesChlorine is more reactive than bromine, which is more reactive than iodine. Chlorine displaces bromide to form orange bromine and iodide to form brown iodine. Bromine displaces iodide to form brown iodine. Bromine cannot displace chloride, and iodine cannot displace chloride or bromide.Group 17 displacement: compare reactivity firstMore reactiveCl₂Br₂I₂Less reactivereactivity decreases down the groupCl₂(aq) + Br⁻(aq)Yes → Br₂(aq) formsorange solutionCl₂(aq) + I⁻(aq)Yes → I₂(aq) formsbrown solutionBr₂(aq) + I⁻(aq)Yes → I₂(aq) formsbrown solutionBr₂ cannot displace Cl⁻No reaction: bromine is less reactive than chlorine.I₂ cannot displace Cl⁻ or Br⁻No reaction: iodine is the least reactive of the three.Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)Symbolic check: atoms and total charge are balanced; chlorine gains electrons and bromide loses electrons.
A displacement occurs only when the added halogen is more reactive than the halogen in the halide ion. Name the halogen formed before stating its aqueous colour.

1. Definition

A. Halogen (Group 17 element)

Group 17 elements are called halogens. They are reactive non-metals that exist as diatomic molecules, such as Cl₂.

B. Halide ion

A halide ion is a -1 ion formed when a halogen gains one electron (e.g. Cl⁻, Br⁻, I⁻).

C. Displacement reaction (halogens)

A displacement reaction happens when a more reactive halogen replaces a less reactive halogen from a halide solution.

2. Key Ideas

  • The required halogens are diatomic: Cl₂, Br₂ and I₂.
  • Reactivity decreases down Group 17: Cl₂ > Br₂ > I₂.
  • A more reactive halogen displaces a less reactive halogen from its halide solution:
    • Cl₂ displaces Br⁻ and I⁻.
    • Br₂ displaces I⁻.
    • I₂ displaces none.
  • Observations are colour-based: state the new halogen formed and its aqueous colour.
  • Halogen displacement is redox: halogen is reduced; halide ion is oxidised.

3. Detailed Explanations

Quick Recall (displacement + tests)
  • Halogens are diatomic molecules: Cl₂, Br₂, I₂.
  • Halogens form -1 halide ions, X⁻ (e.g. Cl⁻).
  • Reactivity decreases down Group 17: Cl₂ > Br₂ > I₂ (when fluorine is excluded).
  • Displacement rule: a more reactive halogen displaces a less reactive halogen from its halide solution.

A. Physical properties (states and colours)

HalogenFormulaState at room temperatureColour
chlorineCl₂gasyellow-green
bromineBr₂liquidred-brown
iodineI₂solidgrey-black (purple vapour when heated)
Describe the tested trends

State the colour and physical state for a named halogen, or describe the change down the group. Use supplied data if a question asks about another physical property.

B. Reactivity trend

Halogens gain one electron to form -1 halide ions. Reactivity decreases from chlorine to bromine to iodine. Displacement results provide the evidence for this order and allow predictions for unfamiliar comparisons.

C. Displacement reactions (how to predict them)

Rule: a more reactive halogen displaces a less reactive halogen from its halide solution.

Example (chlorine displaces bromide):

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Ionic equation (cleaner):

Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)

What you must say in words: “Bromine is produced” (not just “colour change”).

This is redox

In the ionic equation, chlorine is reduced (gains electrons) and bromide is oxidised (loses electrons): Redox Reactions.

Displacement summary (the table examiners love)

Added halogenHalide solutionDisplacement?Halogen formedTypical colour change (aq)
chlorinepotassium bromideyesbrominepale green → orange
chlorinepotassium iodideyesiodinepale green → brown
brominepotassium iodideyesiodineorange → brown
brominepotassium chloridenononeremains orange
iodinepotassium chloride or potassium bromidenononeno change

Halogen displacement and the silver nitrate test answer different questions. Displacement compares halogen reactivity; silver nitrate identifies a halide ion. Revise the required test procedure and precipitate observations on the Qualitative Analysis page.

E. Safety when handling halogens

Chlorine and bromine vapour hazard

Chlorine and bromine vapour are toxic and irritating. Handle in a fume cupboard and avoid inhalation.

4. Common Mistakes

  • Saying “reactivity increases down the group” (wrong direction).
  • Writing “displacement happens because chlorine is more reactive” with no evidence (you must state the halogen formed).
  • Mixing up Br₂(aq) (orange) and I₂(aq) (brown).
  • Forgetting to write the ionic equation correctly (2 halide ions are needed).
  • Reporting only a colour without naming the halogen responsible for it.
  • Memorising organic-solvent colours or boiling-point explanations as though they were required Group 17 content.

5. Exam Tips

The mark-scheme sentence for displacement

“A more reactive halogen displaces a less reactive halogen from its halide solution.”
Then name the product halogen and its colour.

  • Always state the reactivity order: Cl₂ > Br₂ > I₂ (if fluorine is not in the question).
  • If the question is about halide identification rather than displacement, follow the separate qualitative-analysis procedure.

6. Worked Examples

Modelled example 1

Predict displacement and products

Core

Problem

Chlorine water is added to potassium iodide solution. Predict whether a reaction occurs and write the ionic equation.
Study the worked solution

  1. Compare reactivity

    Method

    Place chlorine above iodine in the Group 17 reactivity order.

    Reason

    A more reactive halogen can displace a less reactive halogen from its halide ions.

    Working

    Cl₂ > I₂ in reactivity.

  2. Predict the species change

    Method

    Convert iodide ions into iodine and chlorine into chloride ions.

    Reason

    Chlorine gains electrons while iodide loses them.

    Working

    Cl₂ → Cl⁻ and I⁻ → I₂.

  3. Balance charge and atoms

    Method

    Use two halide ions for each diatomic halogen molecule.

    Reason

    This balances both atoms and electron transfer.

    Working

    Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)

Guided practice 2

Observation (don’t be vague)

About 5 min

Problem

State a precise observation when chlorine water is added to potassium bromide solution, and name the product responsible.

Pair the colour change with a named halogen

Halogen formed
Final aqueous colour

Hints

Hint 1: apply the displacement order
Chlorine is more reactive than bromine.
Hint 2: name before colouring
The displaced halide becomes its halogen; then recall that halogen’s aqueous colour.
View solution step by step

  1. Identify the reaction

    Method

    Displace bromide ions with chlorine.

    Reason

    Chlorine is more reactive than bromine.

    Working

    Bromine, Br₂(aq), is formed.

  2. Report the observation

    Method

    State the initial and final colours.

    Reason

    The product bromine gives the resulting aqueous colour.

    Working

    Pale-green chlorine water turns orange as bromine forms.

Common misconception 3

No displacement case

Find and correct the mistake

Learner response

Bromine water is added to potassium chloride solution. A student predicts chlorine will form because “bromine displaces chloride.” Correct the prediction and state the observation.

Check the direction before naming a product

Reaction outcome
Observation

View solution step by step

  1. Locate the reversed rule

    Method

    Compare bromine with chlorine.

    Reason

    Reactivity decreases down Group 17, so bromine is less reactive than chlorine.

    Working

    Cl₂ > Br₂.

  2. Apply the displacement condition

    Method

    Require the added halogen to be more reactive than the halogen in the halide.

    Reason

    Bromine does not meet that condition for chloride ions.

    Working

    No displacement occurs.

  3. State the observation

    Working

    The solution remains orange because bromine remains present and no chlorine forms.

Examiner practice 4

Redox language (short)

2 marks

Examination question

In Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq), identify the species oxidised and justify your answer using electrons. [2 marks]

Track one reactant into its product

View solution step by step

  1. Track bromide

    1 mark

    Method

    Follow Br⁻ from reactant to Br₂ product.

    Reason

    Bromide ions are the species whose oxidation state changes from -1 to 0.

    Working

    2Br⁻ → Br₂.

  2. Use electron loss

    1 mark

    Method

    Write the electron change.

    Reason

    Oxidation is loss of electrons.

    Working

    2Br⁻ → Br₂ + 2e⁻, so Br⁻ is oxidised.

Challenge 5

Deduce an order from evidence

Minimal support

Reverse evidence transfer

Halogen X displaces Y⁻ but does not displace Z⁻. Deduce the reactivity order supported by both results.

Turn each result into one inequality

X displaces Y- means
Combined order

Hints

Hint 1: translate success
If X displaces Y-, X must be more reactive than Y.
Hint 2: translate failure
If X cannot displace Z-, Z must be more reactive than X.
View solution step by step

  1. Use the successful displacement

    Method

    Place X above Y.

    Reason

    Only a more reactive halogen displaces a less reactive halogen’s ions.

    Working

    X > Y.

  2. Use the failed displacement

    Method

    Place Z above X.

    Reason

    X cannot displace Z⁻, so X is less reactive than Z.

    Working

    Z > X.

  3. Combine both constraints

    Method

    Join the two inequalities without discarding either result.

    Reason

    The only supported chain places X between Z and Y.

    Working

    Z > X > Y.

7. Mind Stretchers

Mind stretcher 1: Two-step reasoningExtension

Question: You add chlorine water to a mixture of KCl(aq) and KI(aq). Which halogen will be produced and why?

Show Answer

Iodine, I₂, is produced because chlorine is more reactive than iodine and displaces iodide ions. Chloride ions are already the halide of chlorine, so they are not displaced by chlorine.

Mind stretcher 2: Use results to predictExtension

Question: Halogen Q displaces bromide ions but does not displace chloride ions. Place Q relative to chlorine and bromine in a reactivity order.

Show Answer

Q is more reactive than bromine because it displaces bromide, but less reactive than chlorine because it cannot displace chloride. The supported order is chlorine > Q > bromine.

8. Quiz

Quiz Time!

Test colours and states, reactivity trends, displacement predictions, ionic equations and evidence-based deductions.

Go to Quiz Page