This book was written after one of us published two editions of a smaller book (Atomic Structure and Valency by R. D. Brown, 1961 and 1966) and from the experiences of all three of us teaching a first-year university course in valency at Monash University. Our object has been to give enough of an historical introduction to quantum mechanics to enable a student to grasp the fundamental ideas without being subjected to much mathematical formalism. We have also tried to avoid making erroneous statements in the interest of simplicity (e. g" the widespread tendency to ignore the difference between...