2.2Unit 2 · Inorganic Chemistry
Group 7 (halogens) — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 2 of 4 1 quiz
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 4Group 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:
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:
- 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:
- 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:
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:
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:
- in Cl2 + 2I− → 2Cl− + I2, chlorine gains electrons (is reduced;
- it is the oxidising agent) and iodide ions lose electrons (are oxidised).
- in Cl2 + 2I− → 2Cl− + I2, chlorine gains electrons (is reduced;
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:
- 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:
- 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:
- 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:
- 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.
- fluorine reacts with hydrogen explosively even in the cold and dark;
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 4Process & sequence
- Place solutions of potassium chloride, potassium bromide and potassium iodide in separate test tubes.
- Add a few drops of chlorine water to each and observe any colour change.
- Repeat with bromine water and then iodine solution.
- Record whether a colour change (displacement) occurs.
- Use the results to put the halogens in order of reactivity.
- 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.
- More electron shells down group
- Outer shell further from nucleus
- More shielding by inner electrons
- Weaker attraction for extra electron
- Electron gained less easily
- Reactivity decreases
Section 3 of 4Compare
| 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 4Worked examples
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.
Explain 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.
- A chlorine atom has fewer electron shells than bromine, so its outer shell is closer to the nucleus with less shielding.
Marked practiceQuizzes for this lesson
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