4CH1
Group 7: The Halogens
Inorganic Chemistry
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
stable
Low
Stable6%
Displacement reactions and physical property trends tested consistently.
What the halogens are
- Group 7 contains the halogens: fluorine, chlorine, bromine, iodine and astatine
- They are reactive non-metals, all toxic, and many are coloured
- Each halogen atom has seven electrons in its outermost shell, which gives the group its family resemblance
- Halogens exist as diatomic molecules (F2, Cl2, Br2, I2) — a pair of atoms held together by one single covalent bond, sharing a pair of electrons
Physical state and appearance at room temperature
| Halogen | State at room temperature | Colour | Note |
|---|---|---|---|
| Fluorine, F2 | Gas | Pale yellow | Most reactive of all elements |
| Chlorine, Cl2 | Gas | Pale yellow-green | Dense, pungent |
| Bromine, Br2 | Liquid | Red-brown | Volatile; gives orange-brown vapour |
| Iodine, I2 | Solid | Dark grey, shiny crystals | Sublimes on warming to a violet vapour |
- The colour deepens as you go down the group (pale yellow → yellow-green → red-brown → dark grey/violet)
Exam tip
Identifying iodine at room temperature
What comes up: a multiple-choice or short-answer question asks for the colour or state of solid iodine at room temperature.
Write: dark grey (shiny) solid. If asked for its vapour on warming, write violet.
Watch out: the mark scheme explicitly rejects "dark brown solid" and "purple gas" as descriptions of the solid at room temperature — brown and purple are the two most common wrong answers, so be precise.
Trend in melting and boiling points
- Both the melting point and the boiling point increase steadily down the group
- F2 boils at −188 °C, Cl2 at −34 °C, Br2 at 59 °C, I2 at 184 °C
- The halogens are simple molecular structures, so it is the intermolecular forces between molecules that are overcome on melting or boiling — not the strong covalent bond inside each molecule
- As the molecules get larger going down the group, the intermolecular forces become stronger and more energy is needed to overcome them — so the melting and boiling points rise
