The concepts of an atom and chemical bond in physics and chemistry: the role of approximations

It is well known from the history of science, that almost all crucial concepts are subject to change during the evolution of scientific theories. There is, however, another (often forgotten or omitted) aspect of this process, i.e. a role of some approximations, necessarily applied to any system of interest, in practically every single calculation carried out within quantum mechanical formalism, which can also result in change of the meaning of some of these concepts. In this paper, the two concepts of great importance in chemistry and physics, namely the concept of an atom, and the concept of a chemical bond are analyzed, both from the point of view of classical chemistry and quantum mechanics. The evolution of their meaning, due to the approximations, resulting in some important differences is highlighted.