Group 7 (halogens) - chlorine, bromine and iodine

Chemistry· 2.5–2.8, 2017 spec section 2(b)· 12 min read

1. Physical Properties of Chlorine, Bromine and Iodine★★☆☆☆⏱ 3 min

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📘 Definition

Group 7 Halogens

Elements in group 7 of the periodic table, all with 7 electrons in their outer shell, forming diatomic molecules (X₂) in their elemental state.

At room temperature and pressure (RTP), the three common halogens have distinct physical properties you must recall for exams:

Halogen	Colour	State at RTP	Key observation
Chlorine (Cl₂)	Pale green	Gas	Forms pale green aqueous solutions
Bromine (Br₂)	Red-brown	Liquid	Evaporates easily to orange-brown vapour; forms orange aqueous solutions
Iodine (I₂)	Grey-black	Solid	Sublimes on heating to purple vapour; forms brown aqueous solutions, purple in organic solvents

Melting and boiling points of halogens increase down Group 7, as diatomic molecule size increases, leading to stronger intermolecular forces that require more energy to overcome.

📐 Worked Example

State and explain the relative boiling points of chlorine and iodine.

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Step 1: Recall the boiling point trend for Group 7: boiling point increases down the group.
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Step 2: Identify positions: chlorine is above iodine in Group 7.
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Step 3: Compare boiling points: iodine has a higher boiling point than chlorine.
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Step 4: Explain: Iodine molecules are larger than chlorine molecules, so intermolecular forces between iodine molecules are stronger, requiring more energy to break for boiling.

Exam tip:

Always link boiling/melting point trends to intermolecular force strength, not covalent bond strength, for full marks.

2. Predicting Properties of Other Group 7 Halogens★★☆☆☆⏱ 2 min

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You can use trends for Cl₂, Br₂ and I₂ to predict properties of halogens higher (fluorine, F₂) or lower (astatine, At₂) in the group, even if you have never studied these elements explicitly.

Fluorine is above chlorine, so it has a lower boiling point, is a gas at RTP, and is more reactive than chlorine.
Astatine is below iodine, so it has a higher melting/boiling point, is a solid at RTP, and is less reactive than iodine.

📐 Worked Example

Predict the state, colour, and relative reactivity of astatine compared to bromine.

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Step 1: Astatine is below iodine in Group 7.
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Step 2: Predict state: following the trend gas → liquid → solid down the group, astatine is a solid at RTP.
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Step 3: Predict colour: colours darken down Group 7, so astatine will be a dark black solid.
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Step 4: Predict reactivity: reactivity decreases down Group 7, so astatine is less reactive than bromine.

Exam tip:

Always explicitly state the trend you are using to make your prediction, even if it seems obvious, to secure full marks.

3. Displacement Reactions of Halogens★★★☆☆⏱ 4 min

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📘 Definition

Halogen displacement reaction

A reaction where a more reactive halogen (higher in Group 7) displaces a less reactive halide ion from its ionic compound in solution.

Displacement reactions provide clear experimental evidence for the Group 7 reactivity trend: reactivity decreases down the group. Colour changes in solution confirm when a reaction has occurred.

📐 Worked Example

Write balanced molecular and ionic equations for the reaction between chlorine water and potassium bromide solution, and state the observation you would make.

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Step 1: Confirm reactivity: chlorine is more reactive than bromine, so displacement occurs.
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Step 2: Write molecular equation:
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$Cl_2 + 2KBr \rightarrow 2KCl + Br_2$
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Step 3: Write ionic equation, omitting spectator K⁺ ions:
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$Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2$
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Step 4: Observation: Pale green chlorine solution turns orange as bromine is formed.

If you add a less reactive halogen to a more reactive halide solution, no reaction occurs, so there is no colour change. For example, adding iodine solution to sodium chloride solution results in no visible change, as iodine is less reactive than chlorine.

Exam tip:

Always include both starting and final colour in your observation answers, not just the final colour, for full marks.

4. Higher Tier: Reactivity Trend Explanation (Electronic Configuration)★★★★☆Higher only⏱ 3 min

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🔬 Derivation

Goal:

Explain why reactivity decreases down Group 7

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1. All Group 7 elements have 7 electrons in their outer shell, so they gain 1 electron to form a stable 1- ion with a full outer shell.
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1. As you go down Group 7, the number of electron shells increases, so the outer shell is further from the positively charged nucleus.
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1. Extra inner electron shells create more shielding, reducing the pull of the nucleus on the outer shell.
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1. The nucleus is less able to attract an incoming electron to fill the outer shell, so the element is less reactive.

Result:

Reactivity of Group 7 halogens decreases down the group.

📐 Worked Example

Explain why fluorine is more reactive than chlorine, with reference to their electronic configurations.

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Step 1: Both elements have 7 outer electrons, so they gain 1 electron to form stable ions.
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Step 2: Fluorine has 2 electron shells, chlorine has 3: fluorine's outer shell is closer to the nucleus.
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Step 3: Fluorine has less inner shell shielding than chlorine.
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Step 4: Fluorine's nucleus attracts an incoming electron more strongly than chlorine's, so fluorine is more reactive.

Exam tip:

Examiners require you to mention both increasing distance from the nucleus and increasing shielding to get full marks for this explanation.

5. Common Pitfalls

Wrong move:

Stating boiling points decrease down Group 7

Why:

Confusing Group 7 trends with Group 1, where boiling points decrease down the group

Correct move:

Recall halogen molecules get larger down the group, so intermolecular forces are stronger, increasing boiling points

Wrong move:

Predicting astatine is a gas at RTP

Why:

Forgetting the state trend down Group 7: gas (Cl) → liquid (Br) → solid (I)

Correct move:

Apply the state trend sequentially to predict astatine is a dark solid at RTP

Wrong move:

Writing that iodine displaces chlorine from sodium chloride solution

Why:

Confusing the reactivity trend: less reactive halogens cannot displace more reactive halides

Correct move:

Only halogens higher in the group (more reactive) displace lower halides: chlorine displaces iodine, not the reverse

Wrong move:

Only stating the final colour for a displacement reaction observation

Why:

Examiners require evidence of change, so you need to compare starting and final colour

Correct move:

Write e.g. 'pale green chlorine solution turns orange' instead of just 'orange solution forms'

Wrong move:

Explaining reactivity trends by saying electrons are lost more easily down Group 7

Why:

Mixing up Group 7 (gain electrons) and Group 1 (lose electrons) reactivity mechanisms

Correct move:

State that Group 7 elements gain electrons, so weaker attraction for incoming electrons down the group reduces reactivity

6. Quick Reference Cheatsheet

Concept	Core Facts	Higher Tier Only
Physical properties (Cl₂, Br₂, I₂)	Cl₂: pale green gas; Br₂: red-brown liquid; I₂: grey-black solid. Mp/bp increases down group.	N/A
Predicting halogen properties	Fluorine = gas, more reactive than Cl; Astatine = solid, less reactive than I.	N/A
Displacement reactions	More reactive (higher) halogen displaces less reactive (lower) halide. Include full colour change observations.	Write ionic equations for displacement reactions (omit spectator ions)
Reactivity trend	Reactivity decreases down Group 7 (opposite of Group 1).	Explain using: increasing outer shell distance from nucleus + more shielding → weaker attraction for incoming electron → less reactive

7. Frequently Asked

Why does reactivity decrease down Group 7, unlike Group 1?

Group 7 elements gain 1 electron to form stable negative ions. As you go down the group, the outer shell is further from the nucleus with more shielding, so the nucleus attracts an incoming electron less strongly, reducing reactivity. Group 1 elements lose electrons, so the opposite trend applies.

What observation confirms a displacement reaction between chlorine and potassium bromide occurred?

The pale green chlorine solution turns orange, as red-brown bromine is displaced into the solution. You must include both starting and final colour for full marks.

Going deeper

study_guidePeriodic Table Basic Trends
study_guideBalancing Chemical Equations

What's Next

Now that you have mastered Group 7 halogen properties and reactions, you are ready to move on to related inorganic chemistry topics frequently tested alongside Group 7 content in Edexcel IGCSE Chemistry exams. Next, you will learn how to test for halide ions and chlorine gas, as well as study other periodic table groups including Group 1 alkali metals and Group 0 noble gases. Practicing past paper questions on displacement reactions and trend explanations will help you secure full marks on this common topic.