CC5.2 — Group 7 halogens (Edexcel 1SC0)

Halogens: F, Cl, Br, I, At

All Group 7 elements have 7 outer electrons → gain 1 electron → form 1− ion (halide). React as diatomic molecules (F₂, Cl₂, Br₂, I₂).

Physical properties trend (down Group 7)

• Colour: F₂ = pale yellow; Cl₂ = yellow-green gas; Br₂ = red-brown liquid; I₂ = grey-black solid.
• Boiling point increases (stronger intermolecular forces in larger molecules).
• Reactivity decreases (harder to gain an extra electron as the outer shell is further from the nucleus).

Displacement reactions

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

Example: Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromide → bromine produced, orange-brown colour appears).

Uses of halogens

• Chlorine: water treatment (kills bacteria), making plastics (PVC).
• Iodine: antiseptic; test for starch.

Properties of hydrogen halides

HCl, HBr, HI — dissolve in water to form acids (hydrochloric, hydrobromic, hydroiodic acid).