2.2Inorganic Chemistry
Group 7 (Halogens) — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 8 parts Lesson 9 of 23 Unit quiz
Group 7: The Halogens — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the properties, trends and displacement reactions of chlorine, bromine and iodine and explain them using electronic structure.Group 7 elements are fluorine, chlorine, bromine, iodine and astatine.
- seven electrons in its outer shell
- diatomic molecules
- Reactivity decreases down the group
- Worked example 1
- Worked example 2
- Worked example 3
Part 1 of 6The halogen family and its physical trends
Group 7 elements are fluorine, chlorine, bromine, iodine and astatine. Each atom has seven electrons in its outer shell, so one electron is needed to complete the shell. They exist as diatomic molecules (F2, Cl2, Br2, I2) held by a single covalent bond.
| Element | State at room temperature | Colour |
|---|---|---|
| Chlorine | Gas | Pale yellow-green |
| Bromine | Liquid (volatile) | Red-brown, orange-brown vapour |
| Iodine | Solid | Grey-black, purple vapour |
Down the group, melting point, boiling point and colour depth increase because larger molecules have stronger intermolecular forces (attractions between molecules), so more energy is needed to separate them. These forces are weak compared with covalent bonds, so halogens have low melting points and do not conduct electricity.
Part 2 of 6Chemical properties and reactivity trend
Halogens are non-metals that gain an electron to form halide ions (Cl-, Br-, I-) with a stable noble-gas arrangement. They react with metals to form ionic salts: 2Na + Cl2 → 2NaCl, and with hydrogen to form hydrogen halides, which are covalent gases that dissolve in water to form acids (hydrochloric acid, hydrobromic acid).
Reactivity decreases down the group. A smaller atom has its outer shell nearer the nucleus and less shielding, so it attracts an extra electron more strongly. Fluorine is therefore the most reactive and iodine the least of the common halogens. This is the opposite of the trend in Group 1.
Chlorine is used to sterilise drinking water and swimming pools and to make bleach; bromine and iodine have uses in medicine and photography. Chlorine water first turns damp blue litmus red and then bleaches it white.
Part 3 of 6Displacement reactions of halogens
A more reactive halogen displaces a less reactive halogen from a solution of its halide salt. The halogen that is displaced is formed as the free element and gives a colour change.
| Halogen added | Potassium chloride | Potassium bromide | Potassium iodide |
|---|---|---|---|
| Chlorine | No reaction | Orange (bromine formed) | Brown (iodine formed) |
| Bromine | No reaction | No reaction | Brown (iodine formed) |
| Iodine | No reaction | No reaction | No reaction |
Word equationchlorine + potassium bromide → potassium chloride + bromine.
Part 4 of 6Predicting properties and safety
Astatine, below iodine, would be a dark solid with a high melting point and be the least reactive halogen; it would not displace iodine. Fluorine, above chlorine, is a very reactive pale yellow gas. Halogens are toxic and corrosive, so experiments are done in a fume cupboard with small quantities, and chlorine is handled as a dilute solution.
Part 5 of 6Uses, hazards and the link to the periodic table
The halogens show a clear pattern that lets you predict an unfamiliar member.
- so the state changes from gas to liquid to solid; the colours get darker;
- and the elements become less reactive because the outer shell is further from the nucleus.
- as in hydrogen chloride, HCl,
- and chloromethane.
Uses and hazards
- chlorine kills bacteria in water supplies and is used to manufacture plastics such as PVC and solvents;
- iodine solution is an antiseptic;
- silver bromide is the light-sensitive compound in black and white photographic film.
Part 6 of 6Worked examples
Describe what you would see when chlorine water is added to potassium iodide solution and explain it. [3]3 marks
The colourless solution turns brown as iodine forms. Chlorine is more reactive than iodine so it displaces it: Cl2 + 2KI → 2KCl + I2.
Explain why the boiling point of the halogens increases down the group. [2]2 marks
The molecules get larger, so the intermolecular forces between them are stronger and more energy is needed to overcome them.
Will bromine react with sodium chloride solution? Explain. [2]2 marks
No. Bromine is less reactive than chlorine, so it cannot displace chlorine from its salt.
Explain why fluorine is more reactive than chlorine. [3]3 marks
Fluorine has fewer shells so its outer shell is closer to the nucleus and less shielded. It attracts an incoming electron more strongly, so it gains an electron more readily.
Key terms
| Term | Meaning |
|---|---|
| Halogen | Group 7 non-metal with seven outer electrons |
| Diatomic | Molecule made from two atoms, such as Cl2 |
| Halide ion | Negative ion formed when a halogen gains one electron |
| Displacement reaction | A more reactive element replaces a less reactive one in a compound |
| Intermolecular forces | Weak attractions between molecules |
| Oxidation | Loss of electrons |
Avoid these mistakes
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