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Subject: Cambridge International AS Level Chemistry (9701) Topic: Group 17
By the end of these notes, you must be able to:
The Group 17 elements are called the halogens. They sit in the second-to-last column on the right side of the periodic table. The three halogens you need to know for this syllabus objective are chlorine (Cl₂), bromine (Br₂), and iodine (I₂).
All three halogens exist as diatomic molecules — this means each molecule is made of exactly two atoms bonded together (for example, Cl₂ means two chlorine atoms joined as one molecule).
Each halogen has its own distinct colour. A key pattern to remember is: the colours get darker as you go down the group.
| Halogen | Formula | Colour | Physical State at Room Temperature |
|---|---|---|---|
| Chlorine | Cl₂ | Yellow-green | Gas |
| Bromine | Br₂ | Orange-brown | Liquid |
| Iodine | I₂ | Grey-black solid / purple vapour | Solid |
💡 Memory tip: Going down the group — yellow → orange-brown → grey/black. The colour deepens at each step.
Volatility means how easily a substance evaporates (turns from a liquid or solid into a gas). Think of it like this: a highly volatile substance "wants" to escape into the air very easily.
Look at how the boiling points change going down the group:
| Halogen | Boiling Point (°C) | Volatility |
|---|---|---|
| Chlorine (Cl₂) | −34 | Most volatile of the three |
| Bromine (Br₂) | +59 | Less volatile than chlorine |
| Iodine (I₂) | +184 | Least volatile of the three |
The trend: Going down the group from chlorine to iodine, the boiling point increases, so the halogens become less volatile.
⚠️ Note for the syllabus: You only need to know chlorine, bromine and iodine for Objective 1. Fluorine is not required here.
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