nitro group, in chemistry, functional group that consists of a nitrogen atom joined to two oxygen atoms. Compounds that contain a nitro group, e.g., picric acid and trinitrotoluene (TNT), are called nitro compounds; many nitro compounds are unstable and are used as explosives. As a result of the chemical bonding in the nitro group, the nitrogen atom is positively charged and each oxygen atom has a partial negative charge. For this reason the nitro group has a powerful attraction for electrons. Many of the chemical reactions of nitro compounds are due to this electronegativity of the nitro group.