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Unformatted text preview: Chapter 7 Introduction to Quantum Theory 7.1 Introduction Toward the later part of the 19 th century, several new observations caused people to question the basic ideas that were the cornerstone of the physics of the time. Primary among these was the growing acceptance of an atomic theory of matter. Successful predictions in chemistry and the development of a statistical particle basis for thermal phenomena being key. To us today, the atomic basis of matter is so obvious that we do not question it. On the other hand to the physicist of the early 19 th century, the continuous nature of matter was obvious. Given the technology of the day, any attempt to measure the size of the atom was impossible. The scale of phenomena at which the discreetness of the atoms could be observed was inconceivably small, see Sec 1.4.2 on Things That Everyone Should Know. Even Dal- ton, the father of Chemistry, had his doubts about the atomic nature of matter. Although his model of atoms described with great success the rules of chemical composition, he could not understand chemical structures like gaseous O 2 . If two one oxygen atom was attracted and bound to another why wouldnt two O 2 s be even more attracted to one another and form an O 4 ? Continuing this line of reasoning, he would believe that oxygen should be a solid and not a gas. Regardless of these conceptual difficulties, by the later part of the 19 th century, because of its success in chemistry and statis- tical mechanics, the atomic theory became dominant and, with it, the idea that the atom had definite properties and a definite size. At this same time, the discover of the electron provided an opportunity for a model of atoms 203 204 CHAPTER 7. INTRODUCTION TO QUANTUM THEORY based on bound electrons. The development of a model of atoms became a dominant effort of the period. We now know that all efforts based on a classical model were destined to failure. It would take the development of quantum mechanics to solve this problem and it would be some time after the first successes of quantum mechanics before a satisfactory model of the atom was possible. Today, the success of quantum mechanics in explaining chemistry is phenomenal. We can compute complex reactions and propose molecular configurations before they are seen in the laboratory. Although it was the successful application of quantum mechanics to atomic theory in the early parts of 20 th century the that lead to its ac- ceptance, the conceptual development of quantum mechanics begins much earlier and deals with a much simpler system light. We will follow this pre- atom development not only because it is the historically correct approach to the study of quantum mechanics but also because, in its simplicity, it makes the conceptual basis of the theory most clear. This will require that we understand some basic elements of the nature of thermal systems. We will also base most of the development on our understanding of the phenom-...
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- Spring '07