Week 7, Session 3: Group 7 Halogens and Displacement Reactions

Lesson route

Estimated study time: 55–70 minutes

Learning goals

You will learn to:

• describe chlorine, bromine and iodine at room temperature;
• state and explain the Group 7 reactivity trend;
• predict halogen displacement reactions;
• write balanced symbol equations for simple displacements.

1. Shared Group 7 features

The halogens are reactive non-metals with seven outer electrons. As elements they form diatomic molecules:

\[ \mathrm{F_2,\ Cl_2,\ Br_2,\ I_2} \]

At room temperature:

• chlorine is a pale green gas;
• bromine is a red-brown liquid;
• iodine is a grey solid that forms purple vapour when heated.

Melting and boiling points increase down the group because larger molecules have stronger intermolecular attractions.

2. Reactivity trend

Reactivity decreases down Group 7. A halogen atom reacts by gaining one electron.

Lower in the group:

• the outer shell is farther from the nucleus;
• shielding is greater;
• attraction for an incoming electron is weaker.

Therefore chlorine gains an electron more readily than bromine, and bromine more readily than iodine.

3. Displacement rule

A more reactive halogen displaces a less reactive halogen from a halide compound.

\[ \mathrm{Cl_2(aq) + 2KBr(aq) \rightarrow 2KCl(aq) + Br_2(aq)} \]

Chlorine displaces bromide because chlorine is more reactive. Iodine cannot displace chloride because iodine is less reactive than chlorine.

4. Prediction table

"No displacement" for the same halogen and halide is not a new chemical change.

Safety

Halogens are hazardous. Chlorine and bromine must not be handled in an unsupervised setting. Self-study should use data, diagrams or teacher-approved videos.

Self-check

Independent study

Write balanced equations for chlorine with potassium iodide and bromine with sodium iodide. For each, identify the displaced halogen.

Session summary

Halogens have seven outer electrons and form diatomic molecules. Reactivity decreases down the group because attraction for an incoming electron weakens. A more reactive halogen displaces a less reactive halogen from its halide.

Deepen your understanding

A. Halogens are diatomic non-metals

Elemental fluorine, chlorine, bromine and iodine exist as diatomic molecules:

\[ \mathrm{F_2,\ Cl_2,\ Br_2,\ I_2} \]

Each atom has seven outer electrons and tends to gain one electron, forming a \(1-\) halide ion. Element names end in -ine, while ion names end in -ide: chlorine forms chloride, bromine forms bromide and iodine forms iodide.

B. Physical trends down Group 7

At room temperature:

• chlorine is a pale green gas;
• bromine is a red-brown liquid with hazardous vapour;
• iodine is a grey-black solid that forms purple vapour when warmed.

Melting and boiling points increase down the group because larger molecules have more electrons and stronger temporary intermolecular attractions. This physical trend is different from the chemical reactivity trend.

C. Reactivity decreases down the group

Halogens react by gaining an electron:

\[ \mathrm{X_2+2e^-\rightarrow2X^-} \]

Down the group, the outer shell is farther from the nucleus and more shielded. Attraction between the nucleus and an incoming electron becomes weaker. Electron gain is less favourable, so reactivity decreases:

\[ \mathrm{F_2>Cl_2>Br_2>I_2} \]

D. Displacement reactions

A more reactive halogen displaces a less reactive halogen from a halide compound.

\[ \mathrm{Cl_2+2KBr\rightarrow2KCl+Br_2} \]

Chlorine gains electrons from bromide ions. Bromide ions lose electrons and form bromine. The net ionic form is:

\[ \mathrm{Cl_2+2Br^-\rightarrow2Cl^-+Br_2} \]

If iodine is added to potassium chloride, no reaction occurs because iodine is less reactive than chlorine and cannot displace chloride.

E. Use a prediction matrix

"No displacement" means no new halogen forms. Colour observations can be difficult because halogen and halide solutions may have overlapping colours and depend on solvent and concentration.

F. Electron-transfer reasoning

In chlorine plus potassium bromide:

• chlorine molecules gain electrons, so chlorine is reduced;
• bromide ions lose electrons, so bromide is oxidised.

The terms oxidation and reduction are GCSE extension ideas here. The core KS3 explanation is that chlorine has a stronger tendency to gain electrons than bromine.

G. Safety and environmental control

Halogens and their vapours can be toxic, corrosive or irritating. Bromine should not be handled in an open classroom. School work usually uses very dilute solutions or microscale methods in a fume cupboard under trained supervision. Never mix household chlorine products with acids or ammonia because dangerous gases can form.

Worked prediction

Question: What happens when bromine water is added to potassium iodide solution?

Answer: Bromine is above iodine in Group 7 and is more reactive. It displaces iodide ions to form bromide ions and iodine:

\[ \mathrm{Br_2+2KI\rightarrow2KBr+I_2} \]

Further self-check

Displacement reasoning drill

For every proposed reaction, ask two questions: Which halogen is free, and which halide ion is in the compound? The free halogen reacts only if it is higher in Group 7 than the halogen represented by the halide.

Example: \(\mathrm{Br_2+2NaCl}\). Bromine is below chlorine, so no displacement occurs. Example: \(\mathrm{Cl_2+2NaI}\). Chlorine is above iodine, so iodine forms and the balanced equation is \(\mathrm{Cl_2+2NaI\rightarrow2NaCl+I_2}\).

Create three more examples and justify each result before writing an equation. This prevents the common habit of balancing an impossible reaction. First decide whether the chemistry occurs, then represent it symbolically.

Academic sources

• Royal Society of Chemistry, Groups 1, 7 and 0 poster and guidance
• OpenStax Chemistry 2e, Section 6.5: Periodic Variations in Element Properties

The lesson paraphrases and adapts these sources for Key Stage 3. OpenStax material is used with attribution under its published licence.