Radicals
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.

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.
- Cl• (chlorine radical)
- •CH3 (methyl radical)
Summary
- Radicals are species with unpaired electrons, represented by a dot (•).
- They can be atoms, molecules, cations, or anions.
- Highly reactive and often appear as intermediates in reactions.
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.