The carbonyl group (C=O) is found in several classes of molecules, including:

* Aldehydes: The carbonyl group is at the end of the carbon chain, meaning it's bonded to at least one hydrogen atom.

* Ketones: The carbonyl group is located within the carbon chain, meaning it's bonded to two other carbon atoms.

* Carboxylic acids: The carbonyl group is bonded to a hydroxyl group (-OH).

* Amides: The carbonyl group is bonded to a nitrogen atom.

* Esters: The carbonyl group is bonded to an oxygen atom, which is in turn bonded to another carbon atom.

So, there's not just one class of molecules that contains the carbonyl group. It's a key functional group found in a variety of organic compounds with diverse properties and functions.