functional group, in organic chemistry, group of atoms within a molecule that is responsible for certain properties of the molecule and reactions in which it takes part. Organic compounds are frequently classified according to the functional group or groups they contain. For example, methanol, ethanol, and isopropanol are all classified as alcohols since each contains a functional hydroxyl group.

In psychology, a broad school of thought that originated in the U.S. in the late 19th century and emphasized the total organism in its endeavours to adjust to the environment. Reacting against the school of structuralism led by Edward Bradford Titchener, functionalists such as William James, George Herbert Mead, and John Dewey stressed the importance of empirical, rational thought over an experimental trial-and-error philosophy. The movement concerned itself primarily with the practical applications of research (see applied psychology) and was critical of early forms of behaviourism.

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In molecules, any of numerous combinations of atoms that undergo characteristic chemical reactions themselves and in many cases influence the reactivity of the rest of the molecule. Organic compounds are often classified according to the functional groups they contain. Common functional groups include hydroxyl (singlehorzbondOH), in alcohols and phenols; carboxyl (singlehorzbondCOOH), in carboxylic acids; carbonyl (singlehorzbondCdoublehorzbondO), in aldehydes, ketones, amides, carboxylic acids, esters, and quinones; and nitro (singlehorzbondNO₂) and amino (singlehorzbondNH₂), in certain organic nitrogen compounds.

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