Free radical

A free radical is an atom, molecule, or ion with at least one unpaired electron. The unpaired electron often makes it chemically reactive, though some radicals are relatively stable.

Homolytic cleavage: Bond breaking in which each bonded atom takes one electron from the shared pair. It is a direct way to create two radicals from a covalent bond.
Electron: A negatively charged elementary particle that occupies atomic orbitals and participates in chemical bonds. A radical's defining feature is an electron left unpaired in its electronic structure.
Combustion: A rapid exothermic reaction, commonly with oxygen, that releases heat and often light. Radical chain reactions drive many flame reactions and determine their speed.
Carbocation: An organic ion with a positively charged carbon atom that has only six valence electrons. A carbocation is electron-deficient but has no unpaired electron in its usual electronic state.
Moses Gomberg: An American chemist whose 1900 report established the existence of the triphenylmethyl radical. His work challenged the assumption that carbon compounds could not persist with unpaired electrons.
Radical chain reaction: A reaction sequence in which radicals initiate, propagate, and terminate chains of chemical transformations. Radicals can regenerate themselves during propagation, sustaining a reaction sequence.
Covalent bond: A chemical bond formed when atoms share one or more pairs of electrons. Homolytic bond breaking divides a shared pair and leaves one electron on each fragment.
Polymerization: A chemical process that joins small molecules into polymers. Radical initiators make chain-growth polymerization possible for many vinyl monomers.
Carbanion: An organic ion in which a carbon atom bears a negative charge and a lone pair. A carbanion has a paired lone pair where a carbon radical has one unpaired electron.
Triphenylmethyl radical: A carbon-centered radical stabilized by three phenyl groups, first reported by Moses Gomberg. Its relative persistence provided early experimental evidence for organic radicals.

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