6.3.1Additional lessons
Radicals — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 147 of 172
Part 1 of 4What Is a Radical
Radicals form when a covalent bond breaks and each atom keeps one of the shared electrons.
Definition
- Definition A radical is an atom or group of atoms that has an unpaired electron, written with a dot such as Cl•.
Examples
- Examples Common radicals include the chlorine atom Cl•, the methyl radical •CH₃ and the hydroxyl radical •OH.
Not an ion
- Not an ion A radical can be electrically neutral, whereas an ion carries an overall charge.
Notation
- Notation The dot is placed next to the symbol of the atom bearing the unpaired electron.
Part 2 of 4Why Radicals Are Reactive
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An unpaired electron makes the species unstable, because atoms prefer to complete their bonding pairs.
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A radical therefore reacts rapidly by pairing its electron with one from another molecule.
3
That reaction often removes an atom from the second molecule and leaves behind a new radical.
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The new radical can continue the sequence, so a single radical can start a long chain of reactions.
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The chain ends only when two radicals meet and combine to form a stable molecule.
Part 3 of 4Radicals in Everyday Chemistry
Radicals in Everyday Chemistry
Alkane halogenation
- Alkane halogenation Chlorine radicals attack methane in sunlight, leading to chloromethane by free-radical substitution.
Ozone depletion
- Ozone depletion In the stratosphere UV light breaks C–Cl bonds in CFCs, releasing Cl• radicals that destroy ozone.
Combustion
- Combustion Many combustion steps involve radicals such as •OH and H•, which carry the flame through the fuel.
Storage
- Storage Radicals are too reactive to store, so they exist only briefly in reaction mixtures.
| Species | Unpaired electron? | Example |
|---|---|---|
| Radical | Yes | Cl•, •CH₃ |
| Anion | No | Cl⁻ |
| Molecule | No | Cl₂ |
Part 4 of 4Exam Checklist & Tips
6.3.1 – Key Points for the Exam
- Radical A species with an unpaired electron that is highly reactive.
- Unpaired electron An electron in an orbital without a partner of opposite spin.
- Chain reaction A reaction in which each step makes a species that starts the next step.
- Free-radical substitution A reaction in which radicals replace atoms in a molecule.
Common Mistakes
- Writing the dot on a bond rather than on the atom that carries the unpaired electron.
- Confusing a radical with a negative ion; a radical has no extra charge.
- Saying radicals are stable because they are neutral.
- Forgetting that radicals are formed by homolytic fission.
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