Group 7 Halogens

Summary: Group 7 elements (F2, Cl2, Br2, I2) are diatomic non-metals. Reactivity decreases down the group. A more reactive halogen displaces a less reactive one from its halide salt. Tags: igcse chemistry periodic-table Created: 2026-07-14 Last Updated: 2026-07-16

Physical Properties and Appearance

All halogens exist as diatomic molecules: F2, Cl2, Br2, I2. They are covalently bonded (sharing one pair of electrons between the two atoms: X-X).

Properties of the first four halogens (IGCSE focus):

Trend in melting and boiling points down the group:

• Melting points and boiling points increase down Group 7.
• Reason: As the molecules get larger (more electrons), the intermolecular forces (van der Waals forces) between molecules get stronger. More energy is required to overcome these forces.
• This is why F2 and Cl2 are gases, Br2 is a liquid, and I2 is a solid at room temperature and pressure (RTP).

Example: I2 molecules have more electrons than Cl2 molecules. The intermolecular forces between I2 molecules are stronger. More energy is required to overcome the intermolecular forces, so iodine has a higher boiling point.

Electronic Structure

All halogens have 7 electrons in their outer shell:

Why they form 1- ions: Halogens need to gain 1 electron to achieve a full outer shell (stable noble gas configuration). For example, Cl (2.8.7) gains 1 electron → Cl- (2.8.8), which is the electron configuration of argon. Gaining 1 electron is much more favourable than losing 7.

General equation: X2 + 2e- → 2X- (where X is a halogen atom)

Reactivity Trend

Reactivity DECREASES down Group 7 (F2 is the most reactive, then Cl2, then Br2, then I2). This is the opposite trend to Group 1.

Why reactivity decreases down Group 7:

1. As you go down the group, the outer shell is further from the nucleus.
2. There are more inner electron shells, providing more shielding.
3. The nuclear attraction for an incoming electron is therefore weaker.
4. It is harder to gain an electron, so the atom is less reactive.

Example: Chlorine has its outer shell in the 3rd energy level; iodine’s outer shell is in the 5th energy level. The outer shell of chlorine is closer to the nucleus than iodine’s outer shell. There is less shielding in chlorine (fewer inner electron shells). The nuclear attraction for an incoming electron is stronger in chlorine, so it gains an electron more easily than iodine.

Displacement Reactions

A more reactive halogen will displace (push out) a less reactive halogen from its halide salt. This is a redox reaction.

General pattern: more reactive halogen + metal halide of a less reactive halogen → metal halide of the more reactive halogen + less reactive halogen

Key displacement reactions for IGCSE:

Using displacement reactions to determine reactivity order: By testing whether a halogen can displace another, the order of reactivity can be established. The experimental results above confirm: Cl2 > Br2 > I2 (i.e., reactivity decreases down the group).

Why the colour changes occur:

• The original potassium halide solutions (KCl, KBr, KI) are colourless.
• When bromine is displaced, it appears orange/brown in aqueous solution.
• When iodine is displaced, it appears brown in water (or purple if extracted into an organic solvent like hexane).

Redox interpretation: In Cl2 + 2KBr → 2KCl + Br2:

• Chlorine is reduced: Cl2 + 2e- → 2Cl- (oxidation state decreases from 0 to -1)
• Bromide ions are oxidised: 2Br- → Br2 + 2e- (oxidation state increases from -1 to 0)
• Chlorine is the oxidising agent; bromide ions are the reducing agent.

Test for Halide Ions (Cl-, Br-, I-)

Method:

1. Add a few drops of dilute nitric acid (HNO3) to the test solution. This removes any carbonate or hydroxide ions that might interfere by forming other precipitates.
2. Add a few drops of silver nitrate solution (AgNO3).
3. Observe the colour of the precipitate formed.
4. (Optional) Test the solubility of the precipitate in ammonia.

Equations:

• Ag+(aq) + Cl-(aq) → AgCl(s) — white precipitate
• Ag+(aq) + Br-(aq) → AgBr(s) — cream precipitate
• Ag+(aq) + I-(aq) → AgI(s) — yellow precipitate

Why nitric acid is added first: To remove carbonate ions (which would form a white precipitate of Ag2CO3, giving a false positive for chloride) and hydroxide ions.

Uses of Halogens and Their Compounds

Key Points

• Group 7 = halogens: F2, Cl2, Br2, I2 (IGCSE does not require astatine, At)
• All are diatomic molecules (X2); all are non-metals
• Colours: F2 = pale yellow gas, Cl2 = greenish-yellow gas, Br2 = red-brown liquid, I2 = dark grey/purple solid (sublimes to purple vapour)
• Reactivity decreases down the group: F2 > Cl2 > Br2 > I2
• Displacement: a more reactive halogen displaces a less reactive one from its halide salt
• Chlorine displaces both Br- and I-; bromine displaces I- only; iodine displaces nothing
• Halide test: AgNO3 + dilute HNO3 → AgCl (white, soluble in dilute NH3), AgBr (cream, soluble in conc NH3), AgI (yellow, insoluble in NH3)
• All halogens form 1- ions by gaining 1 electron to fill their outer shell

Key Concepts from Past Papers

• Bromine water test: unsaturated → decolourises; saturated → remains orange/brown
• Reactivity decreases down Group 7 due to increased shielding and distance from nucleus

Keywords from Past Papers

bromine, aqueous, water, remains, colourless, colour, saturated, orange, change, unsaturated, decolourises, turns, goes, brown, loses

Related Notes

• Periodic Table Trends
• Group 1 Alkali Metals
• Group 0 Noble Gases
• Transition Metals
• Atomic Structure

Sources

• OpenStax Chemistry 2e — Chapter 6: Electronic Structure and Periodic Properties, Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — Group 7 Halogens, BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus topic 8 (The Periodic Table), Cambridge Assessment International Education
• CK-12 Chemistry for High School — Halogens, CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q55(c)(ii) (1m), Q55(c)(ii) (1m)
• 0620/32 Feb/March 2020: Q44(a)(iii) (3m), Q44(a)(iii) (3m)
• 0620/32 Feb/March 2023: Q66(d)(iii) (1m)
• 0620/33 May/June 2021: Q44(a)(i) (2m), Q44(a)(i) (2m)
• 0620/33 May/June 2023: Q77(a)(iii) (3m), Q77(a)(iii) (3m)
• 0620/33 October/November 2020: Q11(a)(iv) (1m), Q11(a)(iv) (1m)

Common Misconceptions