in a covalent chemical compound, a group or chain of atoms bearing valence electrons that are not engaged in single-bond formation and that modify the behaviour of each other. If, for example, a carbonyl group (C ∶ O) and a hydroxyl group (OH) are widely separated in a molecule, each has distinctive properties, but in combination they form the carboxyl group (COOH), which has an entirely different character. Similarly, isolated double bonds contribute no colour to a hydrocarbon, but, in a molecule containing a chain of alternating single and double bonds, intense coloration results from conjugation.