IS THERE A QUANTUM DEFINITION OF A MOLECULE

The paper surveys how chemistry has developed over the past two centur ies starting from Lavoisier's classification of the chemical elements at the end of the eighteenth century; the subsequent development of th e atomic-molecular model of matter preoccupied chemists throughout the nineteenth century, while the results of the application of quantum t heory to the molecular model has been the story of this century. Where as physical chemistry originated in the nineteenth century with the me asurement of the physical properties of groups of chemical compounds t hat chemists identified as families, the goal of chemical physics is t he explanation of the facts of chemistry in terms of the principles an d theories of physics. Chemical physics as such was only possible afte r the discovery of the quantum theory in the 1920's. By then the first of the sub-atomic particles had been discovered and seemingly it is n o longer possible to discuss chemical facts purely in terms of atoms a nd molecules - one has to recognize the electron and the nucleus, the parts of atoms. The combination of classical molecular structure with the quantum properties of the electron has given us a tremendously suc cessful account of chemistry called 'quantum chemistry'. Yet from the perspective of the quantum theory the deepest part of chemistry, the e xistence of chemical isomers and the very idea of molecular structure that rationalizes it, remains a central problem for chemical physics.