This book explores the way in which quantum theory has become central to our understanding of the behaviour of atoms and molecules and the way in which this underlies so many of the experimental measurements we make, how we interpret those experiments and the language which we use to describe our results. It attempts to provide an account of the quantum theory and some of its applications to chemistry. The subject matter develops as follows: Chapter 1 considers the place of theory in science, emphasising in particular the significance of hypotheses, postulates and laws; Chapter 2 gives an account, in approximately historical sequence, of the development of the quantum theory paying particular attention to the emerging experimental data and the new theoretical concepts developed for their interpretation; Chapters 3 and 4 describe some fundamental details of the theory with explanations and simple, chemically–relevant examples. Emphasis is laid on what we can and...