You're describing a carbonyl group, a functional group that plays a critical role in many organic molecules. Here's a breakdown:

Structure:

* Carbonyl group: A carbon atom double-bonded to an oxygen atom (C=O).

* In the middle of a chain: This means the carbonyl group is not at the end of a carbon chain, but rather somewhere within it.

Examples:

* Ketones: The carbonyl group is attached to two carbon atoms within the chain. The simplest ketone is acetone (CH₃COCH₃).

* Aldehydes: The carbonyl group is attached to one carbon atom and a hydrogen atom at the end of the chain. The simplest aldehyde is formaldehyde (HCHO).

Important points:

* Polarity: The carbonyl group is polar because oxygen is more electronegative than carbon. This creates a partial positive charge on the carbon and a partial negative charge on the oxygen.

* Reactivity: The polarity of the carbonyl group makes it reactive. It can participate in various chemical reactions, such as nucleophilic addition, condensation, and oxidation.

Let me know if you'd like more detail on any aspect of carbonyl groups!