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

Abstract

Arguments are presented that the theoretical definition of an atom in a molecule or of a functional grouping of atoms that derive from experimental chemistry must be unique. Definitions based on the orbital model or, as recently proposed, in terms of domains defined by isovalued density envelopes fail for a number of reasons, among them being their failure to enable a quantum mechanical description of the atomic or group properties. Chemistry is concerned with the observation and measurement of properties. Definitions that do not predict the measurable, additive properties found for atoms in molecules 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 for an atom, free or bound, that meets this essential requirement.Key words: atoms in molecules, proper open systems.