SPEC 4.1.2.6 • PERIODIC TABLE
Group 7 — The Halogens
Non-metals with seven outer electrons. The mirror image of Group 1: reactivity decreases down the group, displacement reactions, and the trap that catches students who try to apply Group 1 logic.
Group 7 — The Halogens
Group 7 sits in the second-to-last column of the periodic table — just before Group 0. Five elements, all non-metals, all with seven outer-shell electrons.
The Five Halogens
F • Cl • Br • I • At
fluorine, chlorine, bromine, iodine, astatine — in order of increasing atomic number down the group.
Halogen
An element in Group 7 — forms compounds with similar formulae because all halogens have seven outer-shell electrons.
Three Defining Features
Non-metals — all five elements
Diatomic molecules — exist as pairs of atoms (F₂, Cl₂, Br₂, I₂) bonded covalently
React by gaining one electron → form 1− ions (halide ions like Cl⁻, Br⁻)
Physical Properties at Room Temperature
The four examinable halogens (F, Cl, Br, I) cover all three physical states at 20 °C — a unique feature of Group 7.
Fluorine
Pale yellow
Gas
Cl₂
Chlorine
Yellow-green
Gas
Br₂
Bromine
Red-brown / orange
Liquid
Iodine
Dark grey solid; purple vapour
Solid
Iodine Note
Iodine sublimes when heated — it goes directly from solid to a purple vapour without melting first. The colour of the vapour is a useful identification clue.
Astatine
Astatine is intensely radioactive and rare. From the trend (gas → liquid → solid), astatine is predicted to be a dark solid at room temperature with very high mp/bp. Used in spec questions as a "predict the property" target.
Physical Trends Down Group 7
Three physical properties all change in the same direction going down Group 7:
PropertyTrend down the groupReason
Relative molecular massIncreasesLarger atoms / more protons + neutrons
Melting pointIncreasesStronger intermolecular forces between bigger molecules
Boiling pointIncreasesSame as melting point — more energy needed to separate molecules
Why the State Changes Down the Group
The state at room temperature reflects the boiling point. Fluorine and chlorine boil well below 20 °C, so they're gases. Bromine boils at 59 °C — just above room temperature, so it's a liquid (only just). Iodine has a much higher mp/bp so it's a solid.
Past Paper Pattern (P1 Jun24 Q02.7)
Recent MCQ format: "Which describes the trends going down Group 7?" Answer: relative molecular mass increases AND boiling point increases. Both go up — same direction.
Reactivity DECREASES Down Group 7
This is the central trap of the topic — and the opposite direction to Group 1.
Most Reactive (top)
F → Cl → Br → I → At
Least Reactive (bottom)
(reactivity decreases down the group)
The Group 7 Trap
Group 1 reactivity increases down the group. Group 7 reactivity decreases down the group. Same underlying reason — opposite direction — because Group 1 metals lose electrons but halogens gain electrons.
In One Sentence
Going down Group 7, it gets harder to gain an electron — so reactivity decreases.
The HT Reactivity Chain HT only
Higher Tier explanation — mirror image of the Group 1 chain. The mechanism here is electron gain, not loss. Mark allocation varies: typically 3 marks for comparing two halogens; up to 4 marks for explaining the whole-group trend.
1More shells. Going down Group 7, each atom has more electron shells.
2Outer shell further from the nucleus, and more shielded by inner electrons.
3Weaker (electrostatic) attraction between the nucleus and an incoming electron.
4Harder to gain an electron → halogen is less reactive.
Memory Hook
Same shells/distance/shielding/attraction logic as Group 1 — but the OUTCOME is "harder to GAIN" instead of "easier to LOSE". For 3-mark comparison questions, steps 1+2 are typically combined into one mark. For 4-mark whole-group trend questions, all four steps each score separately.
Group 1 vs Group 7 — Mirror Images
The cleanest way to understand both trends: they share a mechanism but differ in what happens to electrons.
Group 1 (Alkali Metals)
Lose 1 outer electron → 1+ ion
Outer shell further down → easier to lose
Reactivity INCREASES down the group
Most reactive: caesium (bottom)
Group 7 (Halogens)
Gain 1 electron → 1− ion (halide)
Outer shell further down → harder to gain
Reactivity DECREASES down the group
Most reactive: fluorine (top)
Common Error
Students apply Group 1 logic and write "reactivity increases down Group 7". Wrong. Halogens GAIN electrons, and gaining gets harder down the group, so reactivity decreases.
Reactions of Halogens
Halogens react with both metals (gaining electrons) and non-metals (sharing electrons). The spec asks specifically about the compounds formed when chlorine, bromine, and iodine react.
With Metals → Ionic Halide Salt
2Na + Cl₂ → 2NaClForms a white crystalline ionic solid (sodium chloride). The halogen gains electrons from the metal to form 1− halide ions. Same pattern with Br and I: 2Na + Br₂ → 2NaBr; 2Na + I₂ → 2NaI.
With Hydrogen → Hydrogen Halide (Covalent)
H₂ + Cl₂ → 2HClForms a covalent molecular gas (hydrogen chloride). Dissolves in water to give a strong acid (hydrochloric acid). Same pattern: H₂ + Br₂ → 2HBr; H₂ + I₂ → 2HI.
Naming the Ion
The element is chlorine; the 1− ion is chloride (Cl⁻). Same pattern: bromine → bromide; iodine → iodide. Calling Cl⁻ a "chlorine ion" loses marks.
Displacement Reactions — The Rule
Displacement Reaction
A reaction in which a more reactive halogen replaces a less reactive halogen in one of its compounds (a halide salt solution).
The Pattern
Add a halogen to a halide salt solution (like KBr or KI). If the added halogen is higher up Group 7 (more reactive), it kicks out the lower halogen and takes its place in the salt. If the added halogen is lower in the group, no reaction occurs.
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Chlorine is more reactive → displaces bromine. Solution turns from colourless to orange-brown as bromine is produced.
Why It Works
Chlorine is more reactive than bromine because it gains an electron more easily (fewer shells, less shielding). When Cl₂ meets Br⁻ ions, the chlorine takes their electrons — forming Cl⁻ ions and freeing Br atoms (which combine to form Br₂).
Displacement Outcomes — The Grid
Use this grid to predict any halogen-displacement question quickly. Reactivity rule: F > Cl > Br > I. A halogen only displaces those below it.
Add this halogen……to potassium chloride (KCl)…to potassium bromide (KBr)…to potassium iodide (KI)
Chlorine (Cl₂)(same halogen)✔ Displaces → orange-brown solution (Br₂)✔ Displaces → brown solution (I₂)
Bromine (Br₂)✗ No reaction(same halogen)✔ Displaces → brown solution (I₂)
Iodine (I₂)✗ No reaction✗ No reaction(same halogen)
Key Colour Observations
Bromine produced: solution turns orange / orange-brown
Iodine produced: solution turns brown / dark yellow
All starting halide solutions (KCl, KBr, KI) are colourless — describe the change clearly
"Colourless" — Not "Clear"
One very small word matters here. AQA mark schemes consistently penalise students who describe halide solutions as "clear".
✗ Loses Mark
"The solution was clear before the reaction."
"Clear" is a transparency property, not a colour. KBr solution is clear AND colourless — but the colour-change question asks about colour.
✔ Wins Mark
"The solution was colourless, then turned orange-brown as bromine was produced."
"Colourless" is a colour. Colour observation = colour answer.
Universal Rule
Anywhere in the spec where a colourless solution is described — Group 1 + water, displacement, neutralisation, electrolysis — the answer is colourless, never "clear". This is one of the easiest marks to lose and the easiest to fix.
Practice Question 1
3 MARKS
Pattern: P1 Jun24 Q02.6 (1-mark MCQ — "chlorine + KBr") expanded to a 3-mark Foundation/Higher question.
P
Observation: The solution changes colour from colourless to orange-brown (or orange / brown). [1 mark]
E
What it shows: Bromine (Br₂) has been produced — that's what gives the orange-brown colour. [1 mark]
E
Why: Chlorine is more reactive than bromine (chlorine is higher in Group 7), so chlorine displaces bromine from potassium bromide: Cl₂ + 2KBr → 2KCl + Br₂. [1 mark]
P
Don't write: "Solution goes clear" (clear is not a colour); "bromide is produced" (bromide is the ion Br⁻ — the orange colour comes from Br₂, the element).
Practice Question 2 HT only
3 MARKS
Pattern: AQA spec 4.1.2.6 / Question Pattern Library — typical 3-mark HT comparison.
1
Chlorine has fewer electron shells than bromine — its outer shell is closer to the nucleus and less shielded by inner electrons. [1 mark]
2
So the (electrostatic) attraction between the nucleus and an incoming electron is stronger in chlorine than in bromine. [1 mark]
3
So chlorine gains an electron more easily → chlorine is more reactive than bromine. [1 mark]
P
Watch out: Don't say "chlorine has fewer electrons" — that's wrong; chlorine has fewer SHELLS (17 electrons total vs bromine's 35). And the force is electrostatic attraction on an incoming electron — not magnetic, not intermolecular.
Practice Question 3
three
3 MARKS
Pattern: AQA spec 4.1.2.6 — "predict properties from given trends down the group".
P
State at room temperature: Astatine is a solid (continuing the gas → liquid → solid trend; iodine is solid, so astatine has even higher mp/bp). [1 mark]
E
Reactivity: Astatine is less reactive than iodine (reactivity decreases down Group 7). [1 mark]
E
Molecular form: Astatine exists as diatomic molecules (At₂), like the other halogens. [1 mark]
P
Other valid answers: "Higher melting/boiling point than iodine"; "darker colour than iodine"; "would not displace iodine from KI"; "has 7 outer electrons"; "would form a 1− ion (At⁻)".
Common Errors — Quick Check
If a sentence on your answer matches the left column, swap it for the right one.
Loses the markWins the mark
"Reactivity increases down Group 7""Reactivity decreases down Group 7"
"Halogens lose electrons""Halogens gain one electron to form 1− ions"
"Chlorine ions" (the ion in NaCl)" Chloride ions" (Cl⁻)
"Solution goes clear""Solution is colourless "
"Halogens are made of single atoms""Halogens are diatomic molecules (F₂, Cl₂, Br₂, I₂)"
"Chlorine has fewer electrons than bromine""Chlorine has fewer shells than bromine"
"Iodine displaces chlorine from KCl"" No reaction — iodine is less reactive than chlorine, so it can't displace it"
Summary
Group 7 — The Halogens (4.1.2.6)
F, Cl, Br, I, At — all non-metals, all diatomic molecules
All have 7 outer electrons → react by gaining 1 e⁻ → form 1− halide ion
Physical states at 20°C: F₂/Cl₂ = gas; Br₂ = liquid; I₂ = solid
Down the group: relative molecular mass ↑, mp ↑, bp ↑
Reactivity Trend (Critical)
Reactivity DECREASES down Group 7 (opposite to Group 1)
Mechanism: harder to gain an electron going down the group
HT chain (typically 3 marks): more shells / outer shell further + more shielded → weaker attraction on incoming electron → harder to gain → less reactive
Reactions & Displacement
With metals: 2Na + Cl₂ → 2NaCl (ionic halide)
With hydrogen: H₂ + Cl₂ → 2HCl (covalent gas)
Displacement: more reactive halogen displaces less reactive from a halide solution
Cl₂ + 2KBr → 2KCl + Br₂ — solution turns colourless → orange-brown
Top Exam Tip
Three traps to memorise: (1) Reactivity goes DOWN down Group 7 (don't apply Group 1 logic). (2) Halogens GAIN electrons (not lose). (3) Use "colourless" not "clear" for halide solutions. The HT mark scheme always wants the full 3-step mechanism: shells/distance/shielding → weaker attraction on incoming electron → harder to gain.
Next: Group 7 — The Halogens