A hydroxyl or hydroxy group is a chemical functional group containing one oxygen atom connected by a covalent bonding to one hydrogen atom (-OH). It is sometimes called the alcohol functional group because when bonded to carbon in a molecule that otherwise contains only hydrogen and carbon the hydroxy (not hydroxyl) group defines the molecule as an alcohol, resulting in a name ending in -ol. A hydroxyl group bonded covalently to the carbon of a carbonyl group (C=O) produces a carboxyl group (C(O)OH) that is the defining group of a carboxylic acid. When the -OH group participates in an ionic bond, the [OH^-] anion is called the hydroxide ion. As a free radical, it is the hydroxyl radical.