Group VII PropertiesCambridge IGCSE Chemistry: Revision notes

Section 1

What are the Group VII halogens and their basic properties?

Group VII elements (also called the halogens) are a family of highly reactive non-metals located in Group 17 of the periodic table. The three most important halogens for IGCSE are:

Halogen	Molecular formula	State at r.t.p.	Appearance at r.t.p.
Chlorine	Cl₂	Gas	Pale yellow-green gas
Bromine	Br₂	Liquid	Red-brown liquid
Iodine	I₂	Solid	Grey-black solid

All halogens exist as diatomic molecules (two atoms bonded together), which is why their formulae are written as Cl₂, Br₂, and I₂. This diatomic nature affects their physical states and explains why they have relatively low melting and boiling points compared to metals.

Key termshalogensdiatomic moleculesGroup VII

Exam tip

Always write halogen formulae as diatomic: Cl₂, Br₂, I₂. Examiners check this carefully. The physical state (gas, liquid, solid) at room temperature and pressure (r.t.p.) is a standard comparison across the group.

Section 2

What are the trends in density and reactivity down Group VII?

As you move down Group VII from chlorine to iodine, two important trends emerge:

1. Density increases

Chlorine: low density gas
Bromine: denser than chlorine (but still volatile)
Iodine: densest of the three (solid)
This trend reflects the increasing atomic mass and larger atomic radius down the group

2. Reactivity decreases

Chlorine is the most reactive halogen
Bromine is less reactive than chlorine
Iodine is the least reactive halogen
This occurs because electrons are further from the nucleus as atomic radius increases, making it harder for the atom to attract additional electrons

The decreasing reactivity is the key trend to remember for predicting halogen behaviour and displacement reactions.

Key termsreactivitydensityatomic radius

Think of it like this

Think of reactivity like a magnet losing strength as you move away from it: chlorine (close to the nucleus) 'pulls' electrons strongly, but iodine (further away) has a weaker 'pull', making it less reactive.

Common mistake

Students often confuse which halogen is most reactive. Remember: chlorine is most reactive because it is smallest and its electrons are closest to the nucleus, giving it the strongest attraction for additional electrons.

Section 3

How do halogen displacement reactions work?

A displacement reaction occurs when a more reactive halogen displaces (removes) a less reactive halogen from a halide salt solution.

General rule: A more reactive halogen will displace a less reactive halogen from its salt.

Example displacement reactions:

Chlorine displacing bromine:
- Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
- Chlorine is more reactive, so it removes bromine from potassium bromide
- The solution changes from colourless to orange-brown (colour of bromine)
Chlorine displacing iodine:
- Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
- The solution changes from colourless to brown (colour of iodine)
Bromine displacing iodine:
- Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
- Bromine is more reactive than iodine, so displacement occurs
- The solution changes to brown (iodine colour)

Why does this happen? The more reactive halogen has a stronger attraction for electrons, so it pulls electrons from the halide ion of the less reactive halogen, forming a new halide salt and releasing the less reactive halogen as a diatomic molecule.

Key termsdisplacement reactionhalide ionreactivity series

Exam tip

When writing displacement equations, remember: the halogen added goes on the left, the halide salt is the other reactant, and the products are a new halide salt and the halogen that was displaced. The colour change of the solution (orange-brown for Br₂, brown for I₂) is crucial evidence for the reaction occurring.

Example

Worked example: Chlorine is added to potassium iodide solution. Will a displacement reaction occur? Yes, because chlorine is more reactive than iodine. The equation is: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq). The solution turns brown, confirming iodine has been released.

Section 4

How can you predict properties of other Group VII elements?

Using the periodic trends you have learned, you can predict the properties of unfamiliar halogens (such as fluorine or astatine) without being explicitly told them.

Predicting properties:

Fluorine (F₂) – above chlorine:
- Must be more reactive than chlorine (smallest halogen, electrons closest to nucleus)
- Would be a gas at r.t.p. (following the trend that small halogens are gases)
- Less dense than chlorine
Astatine (At₂) – below iodine:
- Must be less reactive than iodine (largest halogen, electrons furthest from nucleus)
- Would be a solid at r.t.p. (following the trend of increasing density)
- Denser than iodine

Method for prediction:

Identify where the unknown halogen sits in relation to chlorine, bromine, and iodine
Apply the trend: reactivity decreases down the group, density increases down the group
Predict intermediate properties if the element is between known halogens
Use the diatomic nature of all Group VII elements in your predictions

This approach demonstrates understanding of periodic trends and allows you to apply knowledge to unfamiliar contexts – a key skill examiners assess.

Key termsperiodic trendsextrapolationgroup properties

Exam tip

Examiners often test your ability to predict properties of unfamiliar elements. Always explain your answer using the trends: 'Fluorine is above chlorine, so it must be more reactive because electrons are closer to the nucleus.' This demonstrates understanding, not just memorisation.

Example

Worked example: What can you predict about fluorine's reactivity and physical state? Fluorine is above chlorine in Group VII. Since reactivity increases towards the top of the group, fluorine must be more reactive than chlorine. Since density decreases up the group and chlorine is a gas, fluorine must also be a gas at r.t.p.

Section 5

What evidence confirms halogen displacement reactions?

When studying displacement reactions experimentally, colour changes provide clear evidence that a reaction has occurred:

Halogen	Colour in solution	Disappears when displaced?
Cl₂	Pale yellow-green	Not easily seen
Br₂	Orange-brown	Yes – solution becomes colourless
I₂	Brown	Yes – solution becomes colourless

Using colour to identify reactions:

If you add chlorine water to potassium bromide solution and the solution turns from colourless to orange-brown, displacement has occurred (Br₂ formed)
If you add chlorine water to potassium iodide solution and the solution turns brown, displacement has occurred (I₂ formed)
If you add bromine solution to potassium iodide and the orange-brown colour fades and is replaced by brown, iodine has been displaced

Important: The colour you observe indicates which halogen has been released by the displacement reaction. This is a reliable, observable way to predict and confirm whether a displacement reaction will occur before you even write the equation.

You can also use starch solution to test for the presence of iodine – iodine turns starch blue-black, providing additional confirmation.

Key termscolour changeevidencestarch test

Exam tip

In exam questions about displacement, always reference the colour change to show you understand the evidence: 'The solution turns orange-brown, confirming that bromine (Br₂) has been displaced and is present.' Examiners reward students who link observations to the chemistry.

Must Know

All Group VII halogens are diatomic non-metals with the formulae Cl₂, Br₂, and I₂
Physical states at r.t.p.: chlorine is a pale yellow-green gas, bromine is a red-brown liquid, iodine is a grey-black solid
Reactivity decreases down the group: chlorine (most reactive) > bromine > iodine (least reactive) because electrons are further from the nucleus and harder to attract
Density increases down the group: as atomic mass and atomic radius increase, density of the halogen increases
More reactive halogens displace less reactive halogens from halide salts: e.g. Cl₂ + 2KBr → 2KCl + Br₂ (solution turns orange-brown)
Colour changes confirm displacement reactions: Br₂ is orange-brown, I₂ is brown; these colours appear in solution when the halogen is released
Predict properties of unknown halogens using periodic trends: fluorine is more reactive and less dense than chlorine; astatine is less reactive and denser than iodine

That's the notes covered.

Carry on to the next subtopic.