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🧪 Group 7 — Halogens

Spec 5.1.2.6 📙 Higher
📖 In-Depth Theory

Properties of the Halogens

GROUP 7 contains the HALOGENS:
Fluorine (F) — pale yellow gas.
Chlorine (Cl) — yellow-green gas.
Bromine (Br) — red-brown liquid.
Iodine (I) — grey-black solid (purple vapour).
Astatine (At) — radioactive, very rare.
All Group 7 elements:
Have 7 OUTER ELECTRONS — they gain ONE electron to form −1 ions (halide ions: F⁻, Cl⁻, Br⁻, I⁻).
Exist as DIATOMIC MOLECULES — F₂, Cl₂, Br₂, I₂ (each molecule is two atoms bonded together).
Are NON-METALS — poor conductors, low melting/boiling points (except iodine which is a solid).
React with metals to form SALTS (compounds containing halide ions) — hence 'halogens' means 'salt-forming'.
Are TOXIC — they are all poisonous in varying degrees.

Physical and Chemical Trends Down Group 7

Going DOWN Group 7 (F → Cl → Br → I):
PHYSICAL TRENDS:
MELTING AND BOILING POINT INCREASE:
F₂: −188°C, Cl₂: −101°C, Br₂: 59°C, I₂: 184°C.
Larger molecules → stronger London dispersion forces → higher boiling point.
COLOUR BECOMES DARKER:
Fluorine: pale yellow, Chlorine: yellow-green, Bromine: red-brown, Iodine: grey-black.
CHEMICAL TREND:
REACTIVITY DECREASES going down — OPPOSITE to Group 1.
Fluorine is the MOST REACTIVE non-metal known.
Iodine is much less reactive.
Why reactivity decreases:
Going down, atoms get LARGER — more electron shells.
The outer shell (where the incoming electron goes) is further from the nucleus.
Weaker attraction for an incoming electron → less easily gained → less reactive.

Displacement Reactions of Halogens

A MORE REACTIVE halogen will DISPLACE a LESS REACTIVE halogen from its salt solution.
This demonstrates the reactivity order: F > Cl > Br > I.
EXAMPLES:
Chlorine water + sodium bromide solution:
Cl₂ + 2NaBr → 2NaCl + Br₂
Colour change: colourless → orange/brown (bromine appears).
Chlorine DISPLACES bromine because Cl is more reactive than Br.
Chlorine water + potassium iodide solution:
Cl₂ + 2KI → 2KCl + I₂
Colour change: colourless → brown (iodine appears).
Bromine water + potassium iodide solution:
Br₂ + 2KI → 2KBr + I₂
Bromine displaces iodine — bromine is more reactive than iodine.
NO REACTION: bromine water + sodium chloride — bromine CANNOT displace chlorine as Cl is more reactive.
OIL LAYER TRICK: Add a non-polar solvent (e.g. cyclohexane) which forms a separate layer — iodine in the oil layer turns VIOLET/PURPLE, bromine turns ORANGE.
⚠️ Common Mistake

Reactivity DECREASES down Group 7 — the OPPOSITE of Group 1. In Group 1, going down makes metals MORE reactive (outer electron easier to LOSE). In Group 7, going down makes non-metals LESS reactive (harder to GAIN an extra electron when the outer shell is further away).

📐 Key Equations
Cl₂ + 2NaBr → 2NaCl + Br₂
Cl₂ + 2KI → 2KCl + I₂
Br₂ + 2KI → 2KBr + I₂
📌 Key Note

Group 7: halogens, 7 outer electrons, form −1 ions, diatomic molecules (X₂). Boiling point increases down group. Reactivity DECREASES down group (outer shell further away, weaker pull on incoming electron). More reactive halogen displaces less reactive from solution.

🎯 Matching Activity — Match the Halogen Displacement Reaction

Predict whether a reaction occurs and match to the correct outcome. — drag the symbols on the right to match the component names on the left.

Reaction occurs
Drop here
Reaction occurs
Drop here
No reaction
Drop here
Reaction occurs
Drop here
No reaction
Drop here
Cl₂ + NaBr solution — chlorine displaces bromine (Cl more reactive than Br)
Br₂ + NaCl solution — bromine cannot displace chlorine (Cl is MORE reactive)
I₂ + NaCl solution — iodine cannot displace chlorine (Cl is much more reactive)
Cl₂ + KI solution — chlorine displaces iodine (Cl more reactive than I)
Br₂ + KI solution — bromine displaces iodine (Br more reactive than I)
⭐ Higher Tier Only

Quantitative explanation: increasing atomic radius down group → outer shell further from nucleus → weaker attraction for incoming electron → lower electron affinity → less reactive. Astatine predicted to be solid, dark coloured and least reactive halogen. Displacement reactions as reactivity evidence.

🎯 Test Yourself
Question 1 of 3
1. Chlorine water is added to a solution of potassium bromide. What is observed?
2. Why is fluorine more reactive than chlorine?
3. Iodine solution is added to sodium chloride solution. What happens?
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