Ketones are carbonyl compounds in which both of the groups attached to the carbonyl C-atom are hydrocarbon group. The general formula of a ketone is R-CO-R'. The hydrocarbon groups can be different. Acetone is the smallest ketone possible. In acetone both groups R and R' are methyl groups. Note the orientations of the methyl groups; the in-plane methyl C-H bonds and the C=O bond are on the same side.