The physico-chemical nature of the chemical bond: valence bonding and the path of physico-chemical emergence by Martha Lynn Harris A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of the Institute for History and Philosophy of Science and Technology University of Toronto © Martha Lynn Harris 2008 The physico-chemical nature of the chemical bond: valence bonding and the path of physico-chemical emergence Martha Lynn Harris Doctor of Philosophy, 2008 Institute for History and Philosophy of Science and Technology University of Toronto Abstract Through the development of physical chemistry and chemical physics over the late-nineteenth and early-twentieth centuries, the relationship between physics and chemistry changed to create a broad interdisciplinary framework in which chemists and physicists could make contributions to problems of common value. It is here argued that evolving disciplinary factors such as physical and chemical responses to the atomic hypothesis, the nature of disciplinary formation in Germany and the United States, the reception of quantum mechanics within physics and chemistry, and the application of quantum mechanics to the problem of chemical bonding by physicists and chemists, formed the chemical bond into a physico-chemical theory. In the late nineteenth-century context of early physical chemistry, the chemical bond was known as a physical link between atoms, which could not be studied by chemical means because of the lack of an adequate atomistic framework. Both chemists and physicists broadly accepted the atomistic hypothesis following the discovery of the electron at the turn of the twentieth century, which afforded theoretical study of chemical bonding. Between 1916 and 1919, Gilbert N. Lewis and Irving Langmuir proposed the valence bond to be a pair of electrons shared between two atoms, within the context of a ii cubic model of the atom. However, the lack of a physical mechanism for the shared electron pair prevented the formation of a fully physico-chemical view of bonding. In 1927, physicists Walter Heitler and Fritz London showed the stability of the valence bond was caused by the wave mechanical phenomenon of resonance. Chemist Linus Pauling extended their treatment of the valence bond to a theory of structural chemistry in The Nature of the Chemical Bond. His synthesis of the physical and chemical views, his value as a physico-chemical researcher during the 1930s, and the research of his contemporaries John Slater and Robert Mulliken show that a true physico-chemical blend was only realized within the amorphous discipline of chemical physics. Finally, it is seen that this interdisciplinarity of chemical bonding and its supporting framework force a re- evaluation of the reductionist criteria, and a re-definition of the chemical bond as a physico-chemical work. iii Acknowledgements I thank my parents, Dr. William Edgar Harris and Dr. Gretchen Luft Hagen Harris, for the many ways they have supported me during my undergraduate and graduate education. This thesis would not have been possible without their support and guidance. I thank my sister, Glenna Catherine Harris, for her support as a roommate, friend, and fellow doctoral candidate during the writing process. Thank you to my grandfather, Dr. Walter Edgar Harris, for sharing conversations about his experiences as a chemist, graduate student, and teacher as I wrote this thesis. Thank you to my aunt, Margaret Harris, for sharing her interest in the history and philosophy of science and tales of her graduate studies at the University of Toronto, and for always asking me for the latest in pop culture. I thank my committee members Dr. Margaret Morrison and Dr. Craig Fraser for their support during meetings, for their reading of earlier drafts of this thesis, and for their instrumental guidance during preparation for my doctoral specialist exam that formed the basis of this project. Thank you to Dr. Mary Jo Nye, Dr. Anjan Chakravartty and Dr. Donald Cotter, for their advice and comments on an earlier paper on reductionism and the chemical bond, which has been incorporated into this thesis. Thank you to friends and fellow students, Charissa Varma, Victor Boantza, Shana Worthen, Marionne Cronin, Jenny Crnac, Trevor Buttrum, Gill Gass, Isaac Record, Brigit Ramsingh, and the students of the IHPST common room for providing many welcome conversations, distractions and moral support during my time in Toronto. And finally, I thank my supervisor, Dr. Trevor Levere, for his unfailing support and guidance during my M.A. and Ph.D. studies, and for teaching me about many things. iv Table of Contents List of Figures................................................................................................................... vi Introduction....................................................................................................................... 1 Chapter 1: Physical chemistry and the origins of valency.......................................... 12 1. Physical chemistry and the Ionists. ................................................................................... 12 2. Nineteenth-century debates and the problem of the atom .............................................. 24 3. Growth of a new discipline: the institutional framework................................................ 37 Chapter 2: Atomism, chemical autonomy and the valence bond:.................................. Lewis, Langmuir and the cubic atom............................................................................ 48 1. Introduction......................................................................................................................... 48 2. The cubic atom and early valence bond theory................................................................ 51 2.1 Lewis’ academic influences .......................................................................................................... 51 2.2 Lewis, the atom, and the molecule................................................................................................ 57 3. Acceptance of the cubic atom............................................................................................. 68 3.1 Langmuir and the octet theory of valence..................................................................................... 68 3.2 Priority dispute: Lewis and the reception of the octet theory........................................................ 76 4. Emergence of chemical autonomy..................................................................................... 79 Chapter 3: Seeds of emergence: wave mechanics meets the valence bond................ 88 1. Introduction......................................................................................................................... 88 2. Wave mechanics meets the chemical bond ....................................................................... 89 2.1 Wave mechanics and the Heitler-London treatment ..................................................................... 89 2.2 The physico-chemical community responds................................................................................. 97 3. Pauling’s early career ....................................................................................................... 104 3.1 The ‘boy professor’ meets physical chemistry............................................................................ 104 3.2 Atoms in the grasp: physical chemistry meets the new physics.................................................. 109 3.3 The chemical bond leads to physics: Pauling as theoretical physicist ........................................ 115 4. The physical nature of the chemical bond ...................................................................... 127 4.1 Pauling’s program begins............................................................................................................ 127 4.2 The nature of the chemical bond................................................................................................. 133 4.3 The ‘physico-chemical’ nature of the valence bond ................................................................... 141 Chapter 4: Paths to interdisciplinarity: the chemical bond in context.................... 146 1. Introduction....................................................................................................................... 146 2. Reductionism vs. interdisciplinarity................................................................................ 150 3. Alternate avenues: the chemical bond elsewhere........................................................... 159 3.1 John Clarke Slater and the valence bond .................................................................................... 159 3.2. Molecular orbitals and Robert Mulliken .................................................................................... 169 4. Chemical bond in demand: Pauling as job candidate.................................................... 176 5. Chemical bond in context: reductionism, interdisciplinary, and the path of disciplinary change..................................................................................................................................... 189 Bibliography.................................................................................................................. 196 v List of Figures Page 121: Figure 1: Image of Linus Pauling in doctoral robes, 1925. Courtesy