Lesson focus: Group 7 non-metals become less reactive down the group, and displacementA reaction where a more reactive element pushes a less reactive element out of its compound. reactions reveal their relative reactivity.
Describe Group 7 trends and use displacement evidence and atomic structure to predict reactions.
Build on earlier learning
The elements of Group 1Group 1 showed how an outer-electron pattern creates a chemical family. Group 7 uses the same periodic reasoning but atoms gain, rather than lose, one electron.
You will be successful when you can…
- Identify halogens as diatomic Group 7 non-metals.
- Describe trends in state, boiling point and reactivity down the group.
- Use a reactivity order to predict displacement reactions.
- Explain decreasing reactivity using distance, shielding and attraction for an incoming electron.
Words that carry the science
- halogen
- A Group 7 non-metal; this KS3 group is also numbered Group 17 in the modern 1–18 system.
- diatomic
- Existing as molecules containing two atoms, such as Cl₂.
- halide
- A negative ion formed when a halogen atom gains one electron, such as Cl⁻ or Br⁻.
- displacement
- A reaction in which a more reactive element replaces a less reactive element in a compound.
- incoming electron
- An electron attracted to and gained by a halogen atom during ion formation.
- reactivity series
- An ordering of elements from more reactive to less reactive based on evidence.
Track physical trends and electron gain in Group 7
01Halogens share seven outer electrons
The halogenOne of the elements in Group 7 of the periodic table, such as chlorine. are non-metals in Group 7. Fluorine, chlorine, bromine and iodine exist as diatomic moleculeA molecule made of two atoms joined together, such as chlorine (Cl₂)., written F₂, Cl₂, Br₂ and I₂.
A halogenOne of the elements in Group 7 of the periodic table, such as chlorine. atom has seven electrons in its outer shell. It can gain one electron to form a 1− halideThe negative ion a halogen forms, such as chloride (Cl⁻). with a full outer shell. Chlorine is a halogen; chloride is the ion Cl⁻, so the names are not interchangeable.
02Physical properties change down the group
At room temperature, fluorine and chlorine are gases, bromine is a liquid and iodine is a solid. Their boiling points increase down the group, so the physical state changes from gas towards liquid and solid.
Colour also becomes generally darker down the group, but colour descriptions depend on state, concentration and solvent. Identify the substance and conditions rather than relying on colour alone.
03Displacement tests the reactivity order
Reactivity decreases down Group 7. Extra occupied shells place the outer region farther from the nucleus and increase shielding, so attraction for an incoming electronAn extra electron that an atom gains when it forms an ion. becomes weaker.
A more reactive halogen can gain electrons from the halide ions of a less reactive halogen. Therefore chlorine displaces bromide and iodide, bromine displaces iodide but not chloride, and iodine displaces neither chloride nor bromide.
Halogens and their concentrated solutions can be harmful or corrosive. Classroom evidence should come from microscale teacher-controlled work, provided results or a simulation with suitable ventilation and protection.
Halogen displacement
A more reactive halogen displaces a less reactive one from a solution of its salt. Reactivity decreases down group 7: chlorine, then bromine, then iodine. Look for a colour change.
Controls
| Added ↓ / salt → | Chloride | Bromide | Iodide |
|---|---|---|---|
| Chlorine | — | Yes: bromine forms | Yes: iodine forms |
| Bromine | No reaction | — | Yes: iodine forms |
| Iodine | No reaction | No reaction | — |
Predict a bromine–iodide reaction
Bromine solution is added to potassium iodide solution. Predict the outcome and write a word equation.
- Order the relevant halogens: bromine is above iodine in Group 7.
- Reactivity decreases down the group, so bromine is more reactive than iodine.
- A more reactive halogen displaces a less reactive halogen from its halide.
- Write the substances carefully: bromine + potassium iodide → potassium bromide + iodine.
How to make the reasoning visible: A displacementA reaction where a more reactive element pushes a less reactive element out of its compound. reaction occurs because bromine gains electrons more readily than iodine under the same conditions.
Build and apply the halogen reactivity order
Physical data help reveal one set of trends. Use the established reactivity order F₂ > Cl₂ > Br₂ > I₂ for predictions; fluorine reactions are not suitable for ordinary classroom testing.
| Halogen | State at room temperature | Boiling point / °C | Relative reactivity |
|---|---|---|---|
| Fluorine, F₂ | Pale-yellow gas | −188 | Highest of these four |
| Chlorine, Cl₂ | Yellow-green gas | −34 | Lower than fluorine |
| Bromine, Br₂ | Red-brown liquid | 59 | Lower than chlorine |
| Iodine, I₂ | Grey solid | 184 | Lowest of these four |
Describe how boiling point and reactivity change from fluorine to iodine.
Predict whether iodine reacts with potassium bromide solution and explain why.
Check your understanding
Answer from memory, then check against the suggested answer.
How many outer-shell electrons does a Group 7 atom have?
What is a halide ion?
How does reactivity change down Group 7?
Chlorine is added separately to potassium bromide and potassium fluoride solutions. Predict each result.
Explain why reactivity increases down Group 1 but decreases down Group 7.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Chlorine solution is added to potassium bromide solution.
Predict whether a reaction happens. Explain using Group 7 reactivity.
(2)Describe the reaction in terms of electrons.
(1)Name the products and write the word equation.
(2)Total for this question: 5 marks
Command word: For predict and explainA command word pair: say what you think will happen, then give the scientific reason why., give the outcome first, then justify it with the reactivity order, displacement rule and electron transfer.