Halogens – IGCSE Chemistry Definition

Group VII is tested almost as heavily as Group I. Examiners focus on halogen displacement reactions, the trend in reactivity, and the physical properties of the first four halogens. Displacement reactions in particular are worth 3–4 marks on Paper 4.

The 0620 definition

The halogens are the elements in Group VII of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). They all have seven electrons in their outer shell and exist as diatomic molecules (e.g. Cl₂, Br₂).

Physical properties

Halogen	Formula	Colour	State at 25 °C
Fluorine	F₂	Pale yellow	Gas
Chlorine	Cl₂	Green-yellow	Gas
Bromine	Br₂	Red-brown	Liquid
Iodine	I₂	Grey-black	Solid (sublimes to purple vapour)

Trend: melting and boiling points increase down the group because the molecules get larger and intermolecular forces (van der Waals) become stronger.

Chemical properties

All halogens:

• Have 7 outer electrons → gain 1 electron to form −1 ions (halide ions: Cl⁻, Br⁻, I⁻)
• Are oxidising agents (they gain electrons / oxidise other substances)
• React with metals to form ionic halides: 2Na + Cl₂ → 2NaCl
• React with hydrogen to form hydrogen halides: H₂ + Cl₂ → 2HCl

Displacement reactions

A more reactive halogen displaces a less reactive halide from solution:

Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)

Chlorine displaces bromine because chlorine is more reactive (smaller atom, stronger attraction for electrons).

Halogen added	KCl solution	KBr solution	KI solution
Cl₂	No reaction	Cl₂ displaces Br₂ (orange)	Cl₂ displaces I₂ (brown)
Br₂	No reaction	No reaction	Br₂ displaces I₂ (brown)
I₂	No reaction	No reaction	No reaction

Worked exam question

Bromine solution is added to potassium iodide solution. (a) State the observation. [1] (b) Write the balanced equation. [1] (c) Explain why this reaction occurs. [2]

Mark scheme

(a) Solution turns brown / dark brown [1]

(b) Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq) [1]

(c) Bromine is more reactive than iodine [1]; bromine has fewer electron shells / smaller atom, so it gains an electron more easily / has a stronger attraction for the incoming electron [1]

Common exam mistakes

• Saying reactivity increases down Group VII — it decreases (opposite to Group I metals).
• Writing halogen formulae as single atoms (Cl) instead of diatomic molecules (Cl₂).
• Describing iodine as brown — iodine is grey-black as a solid. The brown colour in displacement reactions is iodine dissolved in solution.