2.2Unit 2 · Inorganic Chemistry
Group 7 (halogens) — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 2 of 4 1 quiz
Lesson summary
Recall colours and physical states of chlorine, bromine and iodine.All halogens have 7 electrons in their outer shell.Reactivity decreases down Group 7F > Cl > Br > I.Halogens exist as diatomic moleculesF2, Cl2, Br2, I2.Cl2 + 2KBr → 2KCl + Br2.IonicCl2 + 2Br− → 2Cl− + Br2 (chlori…
- Group 7 (halogens)
- Physical properties
- Why melting points rise
- Electronic configuration
- Reactivity trend
- Displacement reactions
Section 1 of 7Group 7 (halogens)
Essential knowledge
Group 7 (halogens)
Group 7 (halogens) the elements in Group 7 are known as the halogens. These are fluorine, chlorine, bromine, iodine and astatine. They are non-metals that are poisonous and exist as diatomic molecules held together by a single covalent bond.
Physical properties
Physical properties at room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Colour darkens from pale yellow (F2) to yellow-green (Cl2), red-brown (Br2) and grey-black (I2). Iodine sublimes to a purple vapour on gentle heating.
Why melting points rise
Why melting points rise molecules get larger down the group, so the intermolecular forces between them get stronger. More energy is needed to overcome them, so melting and boiling points increase.
Electronic configuration
Electronic configuration every halogen atom has 7 electrons in its outer shell, e.g. Cl is 2.8.7. Halogens react by gaining one electron to form a halide ion with a 1− charge and a full outer shell (Cl− is 2.8.8).
Reactivity trend
Reactivity trend down the group the atoms have more shells, so the outer shell is further from the nucleus and more shielded by inner electrons. The attraction for an incoming electron is weaker, so the atom gains an electron less easily. Therefore reactivity decreases down the group.
Displacement reactions
Displacement reactions a more reactive halogen displaces a less reactive halogen from a solution of its halide. Chlorine displaces bromine and iodine; bromine displaces iodine only; iodine displaces neither. Colour changes show that the less reactive halogen has been released.
Redox in displacement
Redox in displacement in Cl2 + 2I− → 2Cl− + I2, chlorine gains electrons (is reduced; it is the oxidising agent) and iodide ions lose electrons (are oxidised).
Hydrogen halides
Hydrogen halides halogens react with hydrogen to form hydrogen halides, e.g. H2 + Cl2 → 2HCl. HCl is a gas that dissolves in water to give hydrochloric acid, because it ionises to H+ ions. In methylbenzene it stays as molecules, so the solution is not acidic.
Test for chlorine
Test for chlorine chlorine gas bleaches damp litmus paper. Predictions: fluorine is the most reactive halogen; astatine would be a dark solid and the least reactive.
Colours in solution
Colours in solution in water, chlorine is pale green, bromine is orange (yellow-orange when dilute) and iodine is brown. These colours identify the halogen released in displacement reactions.
Metal halides
Metal halides halogens react with metals to form ionic halide salts containing X− ions, e.g. 2K + Br2 → 2KBr and 2Fe + 3Cl2 → 2FeCl3. The reactions get slower down the group.
Reactivity with hydrogen
Reactivity with hydrogen fluorine reacts with hydrogen explosively even in the cold and dark; chlorine needs light or heating; bromine and iodine react more slowly still. This matches reactivity decreasing down the group.
Non-metal halides
Non-metal halides halogens react with non-metals to form simple covalent molecules, e.g. hydrogen halides such as HCl and HBr.
Section 2 of 7Process & sequence
1
Place solutions of potassium chloride, potassium bromide and potassium iodide in separate test tubes.
2
Add a few drops of chlorine water to each and observe any colour change.
3
Repeat with bromine water and then iodine solution.
4
Record whether a colour change (displacement) occurs.
5
Use the results to put the halogens in order of reactivity.
6
Write ionic equations and identify which species is oxidised and reduced.
Why reactivity falls down Group 7write these steps in this order; each step is a marking point.
1
More electron shells down group
2
Outer shell further from nucleus
3
More shielding by inner electrons
4
Weaker attraction for extra electron
5
Electron gained less easily
6
Reactivity decreases
Section 3 of 7Compare
| Halogen | Formula | State | Colour |
|---|---|---|---|
| Fluorine | F2 | Gas | Pale yellow |
| Chlorine | Cl2 | Gas | Yellow-green |
| Bromine | Br2 | Liquid | Red-brown (orange vapour) |
| Iodine | I2 | Solid | Grey-black (purple vapour) |
Melting and boiling points increase and colours darken down the group.
| Halogen added | KCl(aq) | KBr(aq) | KI(aq) |
|---|---|---|---|
| Chlorine water | No reaction | Turns orange (Br2) | Turns brown (I2) |
| Bromine water | No reaction | No reaction | Turns brown (I2) |
| Iodine solution | No reaction | No reaction | No reaction |
A halogen displaces any halogen below it in the group.
Section 4 of 7Worked examples
1
Write the balanced and ionic equations for bromine reacting with potassium iodide solution and state what is oxidised.
- Br2 + 2KI → 2KBr + I2.
- Ionic: Br2 + 2I− → 2Br− + I2.
- Iodide ions lose electrons (2I− → I2 + 2e−), so iodide is oxidised;
- bromine is reduced.
2Explain why chlorine is more reactive than bromine.
- A chlorine atom has fewer electron shells than bromine, so its outer shell is closer to the nucleus with less shielding.
- The nucleus attracts an extra electron more strongly, so chlorine gains an electron more easily and is more reactive.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Halogen · Diatomic molecule · Halide ion · Displacement reaction · Reactivity · Oxidation · Reduction · Hydrogen halide · Intermolecular forces
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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