Can there be more than as single definition of an atom in a molecule?

Arguments are presented that the theoretical definition of an atom in a mol ecule or of a functional grouping of atoms that derive from experimental ch emistry must be unique. Definitions based on the orbital model or, as recen tly proposed, in terms of domains defined by isovalued density envelopes fa il for a number of reasons, among them being their failure to enable a quan tum mechanical description of the atomic or group properties. Chemistry is concerned with the observation and measurement of properties. Definitions t hat do not predict the measurable, additive properties found for atoms in m olecules fail to recover the essence of the atomic concept and can play no operational or predictive role in chemistry. Atoms exist in real space and their form determines their properties. There is but a single definition fo r an atom, free or bound, that meets this essential requirement.