An atom or group of atoms, such as a carboxyl group, that replaces hydrogen in an organic compound and that defines the structure of a family of compounds and determines the properties of the family. 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 (OH), in alcohols and phenols; carboxyl (COOH), in carboxylic acids; carbonyl (CO), in aldehydes, ketones, amides, carboxylic acids, esters, and quinones; and nitro (NO2) and amino (NH2), in certain organic nitrogen compounds