Atomic structure and the periodic table: properties of Group 7 (halogens)

What you'll learn

This revision guide covers everything you need to know about Group 7 elements (the halogens) for your AQA GCSE Chemistry exam. You'll learn about the physical and chemical properties of halogens, how their reactivity changes down the group, and how to predict and explain displacement reactions. These concepts are essential for understanding periodic trends and appear regularly in both foundation and higher tier papers.

Key terms and definitions

Halogen — a Group 7 element with seven electrons in its outer shell; includes fluorine, chlorine, bromine, iodine and astatine

Diatomic molecule — a molecule consisting of two atoms bonded together; all halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂)

Reactivity — a measure of how readily an element undergoes chemical reactions; for halogens, this relates to how easily they gain an electron

Displacement reaction — a reaction where a more reactive halogen replaces a less reactive halogen in a compound

Halide ion — a negatively charged ion formed when a halogen atom gains one electron (e.g. Cl⁻, Br⁻, I⁻)

Oxidising agent — a substance that accepts electrons from another substance during a chemical reaction; halogens are strong oxidising agents

Electronegativity — the ability of an atom to attract electrons in a chemical bond; decreases down Group 7

Boiling point — the temperature at which a liquid changes to a gas; increases down Group 7 for the halogens

Core concepts

Physical properties of Group 7 elements

The halogens show clear trends in their physical properties as you go down the group:

Colour and state at room temperature:

• Fluorine (F₂) — pale yellow gas
• Chlorine (Cl₂) — pale green gas
• Bromine (Br₂) — red-brown liquid (volatile, produces orange-brown vapour)
• Iodine (I₂) — dark grey solid (produces purple vapour when heated)
• Astatine (At₂) — black solid (radioactive, rarely encountered)

Melting and boiling points:

The melting and boiling points increase as you go down Group 7. This occurs because:

• Larger atoms have more electrons
• Stronger intermolecular forces (van der Waals forces) exist between larger molecules
• More energy is required to overcome these forces

This explains why fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid at room temperature.

All halogens:

• Exist as diatomic molecules (two atoms joined by a covalent bond)
• Are non-metals
• Have coloured vapours
• Are toxic and must be handled with care

Electronic structure and reactivity

Electronic configuration:

All halogens have seven electrons in their outer shell. This is why they are in Group 7 of the periodic table:

• Fluorine: 2,7
• Chlorine: 2,8,7
• Bromine: 2,8,18,7
• Iodine: 2,8,18,18,7

How halogens react:

Halogens gain one electron to achieve a full outer shell of eight electrons (noble gas configuration). When they react:

• They form halide ions with a 1- charge
• Chlorine atom (Cl) becomes chloride ion (Cl⁻)
• Bromine atom (Br) becomes bromide ion (Br⁻)
• Iodine atom (I) becomes iodide ion (I⁻)

Reactivity trend:

Reactivity decreases as you go down Group 7. This means:

• Fluorine is the most reactive halogen
• Astatine is the least reactive halogen
• The common order: fluorine > chlorine > bromine > iodine

Why does reactivity decrease down the group?

The ability to attract an electron decreases down the group because:

1. Atomic radius increases — atoms get larger down the group as they have more electron shells
2. Shielding increases — inner electron shells shield the outer shell from the nuclear charge
3. Nuclear attraction decreases — the incoming electron is further from the nucleus and experiences more shielding
4. Electronegativity decreases — the ability to attract electrons in a bond decreases

Even though the nuclear charge increases down the group, the effect of increased distance and shielding is greater, so overall attraction for an incoming electron decreases.

Displacement reactions of halogens

A displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from a solution of its salt (halide compound).

The rule: A more reactive halogen will displace a less reactive halogen from its compound.

Observable changes:

When displacement occurs, you observe a colour change in the solution:

Example displacement reactions:

Chlorine + potassium bromide → potassium chloride + bromine Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Chlorine + potassium iodide → potassium chloride + iodine Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)

Bromine + potassium iodide → potassium bromide + iodine Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)

Ionic equations:

You may be asked to write ionic equations showing only the species that change:

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

Cl₂(aq) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)

Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq)

Practical context:

In laboratory tests, halogen water (aqueous halogen solution) is added to colourless halide solutions. If a displacement reaction occurs, the solution changes colour due to the formation of the displaced halogen. Adding an organic solvent like cyclohexane can make the colour changes clearer, as halogens are more soluble in organic solvents and form distinct coloured layers.

Reactions with metals

Halogens react with metals to form ionic compounds called metal halides.

General pattern:

Metal + halogen → metal halide

For example:

• Sodium + chlorine → sodium chloride 2Na(s) + Cl₂(g) → 2NaCl(s)

• Iron + bromine → iron(III) bromide 2Fe(s) + 3Br₂(l) → 2FeBr₃(s)

Key observations:

• Reactions are vigorous and exothermic (release heat energy)
• More reactive halogens react more vigorously
• A metal halide salt is formed as a white or coloured solid
• The metal is oxidised (loses electrons)
• The halogen is reduced (gains electrons)

Reactivity comparison:

If pieces of iron wool are heated and placed in jars containing chlorine gas and bromine vapour:

• With chlorine: vigorous reaction, bright glow, brown fumes of iron(III) chloride
• With bromine: less vigorous reaction, duller glow, iron(III) bromide forms

This demonstrates that chlorine is more reactive than bromine.

Reactions with hydrogen

Halogens react with hydrogen gas to form hydrogen halides, which dissolve in water to form acidic solutions.

General equation:

Hydrogen + halogen → hydrogen halide

H₂(g) + X₂ → 2HX(g) (where X represents any halogen)

Specific examples:

• H₂(g) + Cl₂(g) → 2HCl(g)
• H₂(g) + Br₂(g) → 2HBr(g)
• H₂(g) + I₂(g) → 2HI(g)

Reactivity differences:

• Fluorine: explosive reaction even in the dark and cold
• Chlorine: explosive reaction in sunlight; slower in darkness
• Bromine: reaction occurs on heating with a platinum catalyst
• Iodine: incomplete reaction even with heating; requires a catalyst and forms an equilibrium

This again demonstrates that reactivity decreases down Group 7.

Hydrogen halides in water:

When hydrogen halides dissolve in water, they form acidic solutions:

• HCl(g) → H⁺(aq) + Cl⁻(aq) (hydrochloric acid)
• HBr(g) → H⁺(aq) + Br⁻(aq) (hydrobromic acid)
• HI(g) → H⁺(aq) + I⁻(aq) (hydroiodic acid)

Uses of halogens and their compounds

Understanding practical applications helps contextualise halogen chemistry:

Chlorine:

• Water purification (kills bacteria in drinking water and swimming pools)
• Manufacturing bleach and disinfectants
• Production of plastics like PVC (polyvinyl chloride)
• Manufacturing hydrochloric acid

Bromine:

• Flame retardants in furniture and electrical equipment
• Pesticides and fumigants
• Photography chemicals (historically)

Iodine:

• Antiseptic (iodine solution/tincture for wound treatment)
• Iodised table salt (prevents iodine deficiency)
• Contrast media in medical imaging

Students in Caribbean contexts may encounter chlorine treatment in desalination plants, which is particularly relevant in island nations with limited freshwater resources.

Worked examples

Example 1: Explaining reactivity trend

Question: Explain why chlorine is more reactive than iodine. (4 marks)

Model answer:

• Chlorine has fewer electron shells than iodine / chlorine atoms are smaller (1 mark)
• There is less shielding in chlorine atoms / fewer inner electrons to shield outer electrons (1 mark)
• The outer shell in chlorine is closer to the nucleus (1 mark)
• So the incoming electron is more strongly attracted / chlorine gains an electron more easily (1 mark)

Examiner tip: This is a classic "explain" question requiring multiple linked points. Make sure you refer to both elements, explain the reason (distance/shielding), and link to the consequence (electron attraction).

Example 2: Displacement reaction prediction

Question: A student adds bromine water to solutions of potassium chloride and potassium iodide. (a) Predict what the student would observe in each case. (2 marks) (b) Write a balanced symbol equation for any reaction that occurs. (2 marks)

Model answer: (a)

• With potassium chloride: no change / solution remains orange-brown (1 mark)
• With potassium iodide: solution turns brown / darker (1 mark)

(b) Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq) (2 marks for correct equation; 1 mark if minor error like missing state symbols)

Alternative ionic equation: Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq)

Examiner tip: Remember that a displacement reaction only occurs if the added halogen is more reactive than the halide in solution. Bromine is more reactive than iodine but less reactive than chlorine.

Example 3: Data interpretation

Question: The table shows some properties of the halogens.

(a) Predict the state of bromine at 25°C. Explain your answer. (2 marks) (b) Describe and explain the trend in boiling points. (3 marks)

Model answer: (a)

• Liquid (1 mark)
• Because 25°C is below its boiling point of 59°C (1 mark)

(b)

• Boiling point increases down the group (1 mark)
• The molecules get larger / have more electrons (1 mark)
• Intermolecular forces / van der Waals forces between molecules are stronger / require more energy to overcome (1 mark)

Examiner tip: When explaining trends, always describe the trend first, then explain in terms of structure (size/electrons), then link to the property being measured (forces/energy).

Common mistakes and how to avoid them

• Confusing atoms and molecules: Remember that halogens exist as diatomic molecules (Cl₂, Br₂, I₂), not single atoms in their elemental form. When writing equations, always write Cl₂, not Cl.

• Getting the reactivity trend backwards: Reactivity decreases down Group 7 (opposite to Group 1 metals). Fluorine is the most reactive; iodine is less reactive. Always think about electron gain becoming harder as atoms get larger.

• Incorrect predictions for displacement reactions: A halogen can only displace a halide if it is more reactive. Iodine cannot displace chloride or bromide ions; chlorine can displace both bromide and iodide ions.

• Muddling physical and chemical properties: State at room temperature, colour, and boiling point are physical properties. Reactivity and displacement ability are chemical properties. Make sure you're clear which you're discussing.

• Not balancing symbol equations correctly: In displacement reactions, remember that halogens are diatomic (X₂), so you need 2 halide ions to react with 1 halogen molecule. Check that all atoms balance on both sides.

• Confusing ionic and symbol equations: Ionic equations show only the species that change (spectator ions removed). If asked for a "balanced equation," give the full symbol equation unless specifically asked for ionic equation.

Exam technique for "Atomic structure and the periodic table: properties of Group 7 (halogens)"

• Command word awareness: "Describe" requires observable changes (colour, state); "Explain" requires scientific reasoning (atomic structure, forces, electron shells). For 3-4 mark questions, link multiple points together.

• Use data effectively: In questions providing data tables or graphs, quote values from the data and use them to identify trends before explaining them. This demonstrates analytical skills and often secures marks even if your explanation is incomplete.

• Structure 4-6 mark answers: Start with the trend/observation, then explain using atomic structure concepts (shells, shielding, nuclear charge, distance), and finish with the consequence for reactivity or properties. Use connective words like "therefore," "because," and "so."

• Learn the standard displacement combinations: Know which displacement reactions occur (chlorine displaces both bromide and iodide; bromine only displaces iodide; iodine displaces nothing). This appears frequently and is easy marks if learned.

Quick revision summary

Group 7 halogens have seven outer electrons and exist as diatomic molecules. Physical properties (melting point, boiling point) increase down the group due to stronger intermolecular forces in larger molecules. Reactivity decreases down the group because larger atoms attract incoming electrons less strongly due to increased distance and shielding. More reactive halogens displace less reactive halogens from halide solutions in displacement reactions. Chlorine is more reactive than bromine, which is more reactive than iodine. Halogens react with metals to form ionic halides and with hydrogen to form acidic hydrogen halides.