Ketones (pronounced like "key tone") are organic molecules with the general chemical formula R1R2C=O where R1 and R2 are alkyl or aromatic (hydrocarbon) substituents. The two R groups may be the same or different and the carbon atom is bound to the oxygen atom with a double bond (the C=O unit is called a carbonyl unit). The carbonyl oxygen atom has two lone pairs of electrons. Aldehydes are similar to ketones, but one of the R groups is replaced by a hydrogen atom giving a general formula of HRC=O.