Functional groups were introduced as a useful method for organizing this vast number of compounds because chemical reactions occur at the functional group and compounds with the same functional group undergo similar reactions.
Both aldehydes and ketones contain a C=O (carbonyl group. The aldehyde functional group contains a carbonyl group bonded to a hydrogen. The functional group of the ketone is a carbonyl group bonded to two carbon atoms. In the general structural formula of each functional group, we use the symbol R to represent other groups bonded to carbon to complete the tetravalence of carbon.