The purpose of this book is to present an outline of the principles of quantum mechanics that are fundamental to understanding the properties of molecules. The text is structured in such a way as to guide an undergraduate reader from the point where the weaknesses of classical physics are recognized in the construction and application of a theory that appears sufficient for interpreting the spectra and electrical and magnetic properties of molecules. The emphasis of the latter parts of the book is on properties rather than the structure of molecules, as there are already a large number of excellent books that address structure. Similarly, little attention has been given to the theory of chemical reactions, as it is still in a relatively early stage of development. The theory of chemical bonding cannot be ignored, but the chapter in which it is included aims to present the principles of chemical bonding rather than detailed explanations. [...]

(from the preface of the first edition)