A functional group is a recurring arrangement of atoms that gives a molecule a characteristic pattern of bonding and chemical behavior. Organic molecules can have very different carbon skeletons yet react similarly when they contain the same functional group.

Common functional groups include:

Here carbonyl group means a C=O unit. Aldehydes and ketones contain carbonyl groups, while carboxylic acids, esters and amides contain a carbonyl attached to another heteroatom-containing group.

The symbol R is often used for an unspecified carbon-containing substituent. For example, an alcohol may be represented as R-OH and an ether as R-O-R'. The symbols do not imply that the two carbon groups are identical.

Functional groups are local features, and one molecule can contain several of them. Lactic acid,

$$\mathrm{CH_3CH(OH)CO_2H},$$

contains both an alcohol group and a carboxylic-acid group. Amino acids typically contain at least an amine-related nitrogen group and a carboxylic-acid group.

Recognizing functional groups is more useful than memorizing whole molecules because existing ideas about electronegativity, bond polarity, lone pairs and bond multiplicity can be applied to these recurring local structures. A carbonyl C=O bond, for example, is polarized toward oxygen, while an amine nitrogen commonly carries a lone pair.

Functional groups do not make every molecule containing them behave identically. Nearby substituents, formal charge, three-dimensional accessibility and the surrounding chemical environment can modify their behavior. Functional groups provide the first reusable classification layer for reasoning about large organic and biological molecules.