How is a ketone characterized?

A ketone is characterized by the presence of a carbonyl group (C=O) that is situated between two carbon chains. This positioning is crucial because it differentiates ketones from other carbonyl-containing compounds, such as aldehydes, where the carbonyl group is located at the end of a carbon chain. The structure of a ketone allows for a variety of interesting chemical behaviors and reactivity due to the carbonyl's position, making it a key functional group in organic chemistry.

In further context, the other options highlight different functional groups or molecular structures. For instance, a carbonyl group at the end of a molecule describes an aldehyde, while a carbonyl group flanked by two hydrogen atoms does not accurately represent any relevant functional group within common organic chemistry frameworks. Lastly, a compound containing sulfur and nitrogen does not pertain specifically to ketones and represents an entirely different class of compounds. Thus, the defining feature of a ketone is indeed the carbonyl group located between two carbon chains.