Final answer:
Aldehydes and ketones share a carbonyl group as a common structural characteristic, where a carbon atom is double-bonded to an oxygen atom. This functional group defines the reactivity and nomenclature of these compounds, though their positioning differs between the two classes.
Step-by-step explanation:
The structural characteristic shared by both aldehydes and ketones is the carbonyl group. A carbonyl group consists of a carbon atom double-bonded to an oxygen atom. This functional group is crucial for both families of compounds and is central to their chemical behavior. However, aldehydes have the carbonyl group at the end of the carbon chain, whereas in ketones, the carbonyl group is located within the carbon chain. This difference in placement within the molecule's structure differentiates the two compounds and affects their nomenclature and reactivity.