Functional Group
An atom or group of atoms within a molecule that determines its characteristic chemical behaviour.
Unit
Dimensionless
In depth
Because the carbon skeleton is largely inert, reactivity is concentrated at the functional group, so compounds sharing a group react in the same way. This is what makes organic chemistry systematic rather than a catalogue of individual substances, and it drives the whole IUPAC naming scheme.
Examples in the real world
–OH in alcohols, –COOH in carboxylic acids, C=O in carbonyls, and –NH₂ in amines.