2.3.1Organic Chemistry
Group 7 Trends — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 10 of 42
Part 1 of 4Physical Properties
The colour becomes darker and the volatility falls as you go down the group.
Fluorine
- Fluorine is a pale yellow gas with a boiling point near −188 °C, so it is the most volatile halogen.
Chlorine
- Chlorine is a pale green gas at room temperature, with a boiling point near −34 °C.
Bromine
- Bromine is a red-brown liquid that gives off orange-brown vapour, boiling at about 59 °C.
Iodine
- Iodine is a grey-black solid that forms a purple vapour on gentle heating, and it is the least volatile halogen.
Diatomic
- Every halogen exists as a diatomic molecule such as Cl₂ or Br₂, with a single covalent bond between the atoms.
Part 2 of 4Explaining Boiling Points
1
Halogens are simple molecular substances, so only weak London forces act between the separate diatomic molecules when they melt or boil.
2
Going down the group each molecule contains more electrons, so the instantaneous dipoles it can form become larger.
3
Larger induced dipoles produce stronger London forces between molecules, which need more energy to overcome.
4
The covalent bond inside each molecule is not broken during boiling, so it plays no part in the trend.
5
Therefore melting points and boiling points increase down the group and volatility decreases.
Part 3 of 4Electronegativity and Oxidising Power
Electronegativity and Oxidising Power
Electronegativity
- The ability to attract bonding electrons falls down the group, from about 4.0 for fluorine to about 2.7 for iodine.
Atomic radius
- Atoms get larger because each element adds an extra electron shell below the one above it.
Shielding
- Inner shells shield the outer shell, so the nucleus attracts an incoming electron less strongly.
Oxidising power
- A halogen oxidises by gaining electrons, and this ability decreases from fluorine down to iodine.
Reactivity
- Reactivity of the halogens as oxidising agents decreases down the group, the opposite of Group 1 metals.
| Halogen | Colour and state | Approx. boiling point / °C |
|---|---|---|
| F₂ | Pale yellow gas | −188 |
| Cl₂ | Pale green gas | −34 |
| Br₂ | Red-brown liquid | 59 |
| I₂ | Grey-black solid | 184 |
Part 4 of 4Exam Checklist & Tips
2.3.1 – Key Points for the Exam
- Halogen An element in Group 7, existing as a diatomic molecule and having seven outer electrons.
- Volatility How easily a substance turns into a gas, which is greater for a lower boiling point.
- London forces Weak attractions between molecules caused by temporary and induced dipoles.
- Electronegativity The power of an atom to attract the bonding pair of electrons in a covalent bond.
- Oxidising agent A substance that gains electrons from another species and is itself reduced.
Common Mistakes
- Saying that covalent bonds break when halogens boil: only the weak London forces between molecules are overcome.
- Stating that fluorine has the highest boiling point because it is the most reactive.
- Confusing the trendhalogen reactivity decreases down Group 7 but metal reactivity increases down Group 1.
- Describing iodine vapour as blackthe solid is grey-black but the vapour is purple.
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