Organic element symbol "R" I keep hearing different things about what exactly the letter R represents in organic chemical structures. Does the R always represent an alkyl group or can it represent groups with heteroatoms? For example http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/Aldehyde2.png/60px-Aldehyde2.png http://upload.wikimedia.org/wikipedia/commons/thumb/4/48/Ketone-general.png/60px-Ketone-general.png theres the structure of an aldehyde and a ketone. Can I safely assume that the R's attached of the ketone will be Carbon atoms and will be attached to other C or H atoms. In other words will a ketone always have alkyl groups attached to its O=C or can a ketone have any elements in it?