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How does a carbonyl group form in an organic molecule?

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structural formulas of some organic compounds — organic compound The structures of organic compounds can be depicted in condensed, expanded, and three-dimensional structural formulas.

A carbonyl group is formed when an oxygen atom establishes a double bond with a carbon atom. This structural unit consists of a carbon atom bonded to an oxygen atom, along with two other atoms or groups attached to the carbon.123

The carbon-oxygen double bond is composed of a $σ$ (sigma) bond, where the electron pair resides between the atoms, and a $π$ (pi) bond, which occupies space on both sides of the $σ$ bond. Because oxygen is more electronegative than carbon, the electron pairs in the double bond are held closer to the oxygen atom, resulting in a polar functional group where the oxygen carries a partial negative charge and the carbon carries a partial positive charge.123

The specific identity of the molecule depends on the substituents attached to the carbonyl carbon. For example, in aldehydes, the carbon is bonded to at least one hydrogen atom, whereas in ketones, it is bonded to two other carbon atoms.12