Both aldehydes and ketones are organic compounds with carbonyl functional groups, but they have different molecular structures. In methanol (always called formaldehyde), the simplest aldehyde, the carbonyl group is bonded to two hydrogen atoms. The simplest aldehyde, H2C=0, methanal (formaldehyde), has two hydrogens on the carbonyl carbon atom.
The simplest aldehydes are methanal (formaldehyde, CH2O) and ethanal (acetaldehyde, CH3CHO). Ketones contain the group R-CO-R, where the -R group may be either aliphatic or aromatic. Probably the best known and simplest ketone is acetone CH3COCH3. Acetone, a common name of the simplest ketone, is also called propanone.