Book excerpt: Valency and Molecular Structure, Fourth Edition provides a comprehensive historical background and experimental foundations of theories and methods relating to valency and molecular structures. In this edition, the chapter on Bohr theory has been removed while some sections, such as structures of crystalline solids, have been expanded. Details of structures have also been revised and extended using the best available values for bond lengths and bond angles. Recent developments are mostly noted in the chapter on complex compounds, while a new chapter has been added to serve as an introduction to the spectroscopy of complex compounds. Other topics include the experimental foundation of the quantum theory; molecular-orbital method; ionic, hydrogen, and metallic bonds; structures of some simple inorganic compounds; and electronic spectra of transition-metal complexes. This publication is a useful reference for undergraduate students majoring in chemistry and other affiliated science subjects.