What defines a functional group in organic chemistry?

A functional group in organic chemistry is indeed defined as a structural feature or group of atoms that influences the chemistry of a compound. Functional groups determine the characteristics and chemical reactivity of the molecules they are part of. For instance, the presence of a hydroxyl group (-OH) in alcohols makes them polar and capable of hydrogen bonding, while the carboxyl group (-COOH) in carboxylic acids imparts acidic properties.

Functional groups serve as the basis for classifying and naming organic compounds, allowing chemists to predict the reactivity and properties of these compounds based on the functional groups they contain. This is crucial for understanding how different organic molecules will behave in chemical reactions.

Other choices do not accurately capture the concept of functional groups. A single atom does not typically have the influence to determine the properties of a compound in the way functional groups do. Additionally, the description of a molecule existing only in the gas phase is not relevant to the definition of functional groups. Finally, categorizing organic compounds generally does not equate to the specific defining characteristics brought by functional groups.