This work provides an introduction to the principles of quantum mechanics needed in physical chemistry. Mathematical tools are presented and developed as needed and only basic calculus, chemistry, and physics is assumed. Applications include atomic and molecular structure, spectroscopy, alpha decay, tunneling, and superconductivity. New edition includes sections on perturbation theory, orbital symmetry of diatomic molecules, the Huckel MO method and Woodward/Hoffman rules as well as a new chapter on SCF and Hartree-Fock methods. This revised text clearly presents basic quantum mechanics for students in chemistry. Separate sections treat needed mathematical techniques. It presents complete mathematical details of derivations. It contains applications of quantum mechanics to a broad range of problems in spectroscopy and molecular structure. New in this Edition are: a new chapter on molecular orbital calculations (extended Huckel and self-consistent field); a significant number of additional figures and improvements to existing figures; new exercises, plus answers for selected problems; includes the photoelectric effect, the perturbation treatment of the helium atom, orbital symmetry and chemical reactions, and molecular term symbols; and careful and extensive edits throughout the text improve clarity and correct minor errors.