How can a functional group be defined?

A functional group is best defined as a cluster of atoms with specific chemical properties. This is because functional groups are distinctive groups of atoms that impart particular characteristics and reactivity to a molecule. They determine the behavior and chemical reactions of organic compounds because they participate in chemical reactions in a predictable manner. For example, the hydroxyl group (-OH) in alcohols is responsible for their solubility in water due to its polar nature.

Functional groups also serve as the basis for classifying organic compounds. Different functional groups lead to different classes of compounds, such as alcohols, acids, and amines, each with unique properties and reactions. This understanding is fundamental in organic chemistry, as it allows chemists to predict how different compounds will interact based on their functional groups.

The other options do not accurately capture the essence of a functional group. Chemical bonds describe the interactions between atoms rather than a specific grouping of atoms. A set of reactions does not define a functional group but rather describes a sequence of chemical processes. Meanwhile, isomers refer to compounds that have the same molecular formula but different structural arrangements, which is distinct from the concept of functional groups.