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S1.1 - Introduction to the particulate nature of matter S1.2 - The nuclear atom S1.3 - Electron configurations S1.4 - Counting particles by mass - The mole S1.5 - Ideal gases S2.1 - The ionic model S2.2 - The covalent model S2.3 - The metallic model S2.4 - From models to materials S3.1 - The periodic table - Classification of elements S3.2 - Functional groups - Classification of organic compounds R1.1 - Measuring enthalpy changes R1.2 - Energy cycles in reactions R1.3 - Energy from fuels R1.4 - Entropy and spontaneity AHL R2.1 - How much? The amount of chemical change R2.2 - How fast? The rate of chemical change R2.3 - How far? The extent of chemical change R3.1 - Proton transfer reactions R3.2 - Electron transfer reactions R3.3 - Electron sharing reactions R3.4 - Electron-pair sharing reactions

R3.3 - Electron sharing reactions

3.3.1 Radicals 3.3.2 Homolytic Fission 3.3.3 Practical Substitution Reaction

Radicals

Specification Reference R3.3.1

Quick Notes

  • A radical is a species with an unpaired electron.
  • Radicals are neutral but highly reactive due to the presence of the unpaired electron.
  • Represented by a dot (•) next to the atom or group containing the unpaired electron.
  • Common examples include:
    • •CH3 (methyl radical)
    • Cl• (chlorine radical)

Full Notes

A radical (also called a free radical) is any atom, ion, or molecule with one or more unpaired electrons. These species are highly reactive and often appear as intermediates in reactions.

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Matt’s exam tip

Radicals are defined by the presence of an unpaired electron – not their charge. That’s why they can be neutral, cations (positively charged), or anions (negatively charged).

Representation of Radicals

Radicals are shown with a dot (•) to indicate the unpaired electron.

Summary

Linked Course Question

Structure 2.1 — Linked Course Question

How is it possible for a radical to be an atom, a molecule, a cation or an anion?

1. Atomic Radical – a single atom with an unpaired electron
Example: Cl•, formed when chlorine gas undergoes homolytic fission under UV light:
Cl2 → 2Cl•

2. Molecular Radical – a neutral molecule with an unpaired electron
Example: NO• (nitric oxide), a stable molecule in which one electron is unpaired.

3. Radical Cation – a positively charged species with an unpaired electron
Example: CH4+, often formed in mass spectrometry when CH4 loses an electron.

4. Radical Anion – a negatively charged species with an unpaired electron
Example: O2•⁻ (superoxide ion), formed when O2 gains an electron.