2.2.2Atoms, Elements and Compounds
Group 7- Reactivity and Electronic Configurations — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 26 Unit quiz
Part 1 of 5Electronic Structure
A halogen atom gains one electron to fill its outer shell, so its ability to attract that electron determines its reactivity.
Outer electrons
- Each halogen atom has seven outer electrons and gains one to form a 1− halide ion with a full outer shell.
Fluorine
- Fluorine has the electron arrangement 2,7, so its outer shell is closest to the nucleus of all the halogens.
Chlorine
- Chlorine has the electron arrangement 2,8,7, with an extra filled shell that puts its outer shell further from the nucleus.
Attraction
- The nucleus attracts an incoming electron more strongly when the outer shell is closer, so smaller halogens are more reactive.
Shielding
- Extra inner shells shield the nucleus as you go down the group, which weakens the attraction for an incoming electron.
Part 2 of 5Explaining the Trend
1
Going down Group 7, each halogen atom has one more filled electron shell than the halogen above it.
2
The outer shell is therefore further from the nucleus, so the nucleus attracts an incoming electron less strongly.
3
Shielding from the inner shells also reduces the pull of the nucleus on incoming electrons.
4
A halogen lower in the group is therefore less able to gain an electron, so it reacts less readily.
5
Reactivity of the halogens therefore decreases as you go down Group 7.
Part 3 of 5Displacement Reactions
Displacement Reactions
Chlorine and bromide
- Cl₂ + 2KBr → 2KCl + Br₂; chlorine displaces bromine from potassium bromide solution.
Chlorine and iodide
- Cl₂ + 2KI → 2KCl + I₂; chlorine displaces iodine, giving a brown solution.
Bromine and iodide
- Br₂ + 2KI → 2KBr + I₂; bromine is more reactive than iodine, so it displaces iodine.
Ionic form
- Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ shows the halide ions swapping places in the reaction.
| Halogen solution added | Potassium chloride | Potassium bromide | Potassium iodide |
|---|---|---|---|
| Chlorine water | No reaction | Orange colour, bromine formed | Brown colour, iodine formed |
| Bromine water | No reaction | No reaction | Brown colour, iodine formed |
Part 4 of 5Worked Example
Question Predict whether bromine water reacts with potassium chloride solution.
- Bromine is below chlorine in Group 7, so bromine is less reactive than chlorine.
- A less reactive halogen cannot displace a more reactive halogen from its halide.
- So bromine cannot displace chlorine from potassium chloride.
Answer
- No reaction;
- the solution does not change colour.
Part 5 of 5Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Displacement reaction A reaction in which a more reactive halogen removes a less reactive halogen from its halide solution.
- Halide ion A negative ion of a halogen, such as Cl⁻, Br⁻ or I⁻.
- Electron shell A layer of electrons at a fixed distance from the nucleus.
- Shielding The reduction in nuclear attraction on outer electrons caused by inner shells.
- Ionic equation An equation showing only the ions that change during a reaction.
Common Mistakes
- Predicting that bromine reacts with potassium chloride;
- bromine is less reactive than chlorine, so there is no reaction.
- Saying halogen reactivity increases down Group 7 when it decreases.
- Explaining reactivity by the number of outer electrons alone;
- the distance of the outer shell and shielding matter.
- Forgetting that a no-reaction result should be recorded clearly in a results table.
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