MOLECULES AND THE CHEMICAL BOND Chemistry Simplified This highly original book by a famous chemistry teacher about general chemistry in a new key may change how teachers teach - • Atomic Theory • The Mole Concept and Avogadro's Constant • The Gas Laws • Solving Problems in Chemical Stoichiometry • The Saturation and Directional Character of Chemical Affinity • The Pauli Exclusion Principle • Linnett's Double Spin Set Theory • Pauling's Rules of Crystal Chemistry • The Octet Rule • Lewis Structures for O2, NO, CO, SO2 and SO3 • Construction of Bond Diagrams • VSEPR Theory • Dative Bonding • Multicenter Bonding • Bonding in Metals • pH Calculations • The Periodic Table • The Energy Function and the First Law of Thermodynamics • The Entropy Function and the Second Law of Thermodynamics • How an Inductive Science Advances
MOLECULES and The Chemical Bond
By Henry A. BentTrafford Publishing
Copyright © 2011 Henry A. Bent
All right reserved.ISBN: 978-1-4269-6299-8Foreword
William B. Jensen
In 1970 I completed my B.S. degree in chemistry at the University of Wisconsin-Madison. Tired of endless course work and exams, and having had many frustrated and unhappy graduate students as teaching assistants, I found that I had developed grave reservations about continuing directly on to graduate school myself. In the end, I compromised by signing on for a two-year joint M.S. Program in Chemistry and Education, which would automatically lead to certification as a high-school chemistry teacher and to some form of gainful employment. Thus it was that, during the fall and early winter of 1971, I found myself serving time as a student teacher at Park High School in Cottage Grove, Minnesota, just south of St. Paul. This brief encounter with the American secondary school system had two lasting consequences for me — first, and foremost, it provided the strongest motivation I ever had to return to graduate school and to complete a doctorate in chemistry and, second, it led to my discovery of the writings of Henry A. Bent.
During my brief respite from courses and exams, I began to voraciously read many of the chemical monographs I had collected during my undergraduate career but had never had time to explore in detail, as well as what little journal literature I could lay my hands upon. Needless to say, the opportunities presented by Park High School with respect to either category of reading material were strictly limited. However, one of the senior chemistry teachers at the school had been a faithful subscriber to the Journal of Chemical Education for many years and had an office filled with back issues. These I began to systematically work through during my daily free period.
It was not long before I discovered a wonderful six-part series of articles by Henry in the journal under the general title of "Tangent-Sphere Models of Molecules", which I devoured with great enthusiasm. Here was a simple, easily visualizable, and physically understandable way of inflating flat topological Lewis diagrams into 3D structures with endless stereochemical consequences. Where had this model been during my undergraduate training? Like all chemistry majors I had suffered through a semester course in quantum chemistry, but this had consisted of endless mathematical exercises in differential equations and matrix algebra with a very low yield of useable chemical consequences at the other end. As part of the course requirements for the M.S. degree I had also taken, shortly before beginning my student teaching, a second graduate course in advanced chemical bonding models. But this seemed to consist only of clever gimmicks and rules — again with little or no intuitive physical justification — that were applicable only to conjugated organic molecules and of little or no interest to the blossoming inorganic chemist that I had begun to imagine myself to be.
Ironically, these not overly fond memories were being daily reinforced during my student teaching as, in order to upgrade their certification, the two permanent chemistry teachers at the school were being forced to commute twice a week to Stout State across the border in Wisconsin to take an evening course in quantum mechanics — indeed the very same course I had taken back at Madison as an undergraduate! Since it had been nearly two decades since either of them had taken a math course, I soon found that my job description had been expanded to include homework tutor and, now knowing from personal experience the realities of the actual chemistry course we were teaching to the students at Park High School, I could not but marvel at the utter irrelevance of it all. Even more so, I could not help but marvel at the irony — nay I should say the tragedy — that these teachers were being told nothing of the ideas in Henry's wonderful series on the tangent-sphere model, though these would have been of far greater relevance to their actual teaching situation.
Following the leads in Henry's references, I eventually discovered that these ideas had in fact been made the basis of a high-school chemistry text published in 1964 under the title of the CBA or Chemical Bond Approach and that this was, in turn, based on the so-called charge-cloud model of the chemical bond developed in a series of Ph.D. theses done under the direction of the quantum chemist George Kimball, at Columbia University, during the years 1951-1957. I was also led to several more advanced and equally exciting reviews on both tangent-spheres and closely related subjects done by Henry in other journals, including one on donor-acceptor interactions in Chemical Reviews and one on the isoelectronic principle, which also appeared in the Journal of Chemical Education.
Indeed the 1960s and 1970s proved to be an exciting time in chemical education. In addition to Henry's articles on tangent spheres, there were related articles by Ronald Gillespie on the prediction of molecular geometries using the VSEPR or valence-shell electron-pair repulsion model (which is in fact a direct consequence of the tangent sphere model), on new dynamic varieties of mechanical models suitable for illustrating these ideas (made either from rubber bands and Styrofoam balls or plastic eggs or from clusters of inflated balloons), articles by William F. Luder on Linnett double-quartet theory (of which the tangent-sphere model is a special case), as well as articles on new ways of teaching entropy and thermodynamics (an area in which Henry has also made a significant contribution). Many of these ideas also became the subject of small supplementary paperback books, which were published in great abundance during this same time frame.
All of this is now but a faint memory. Gone are the supplementary paperbacks and gone are the introductory textbooks willing to take a chance on new and creative conceptual approaches to the teaching of chemistry. Only the VSEPR model has survived as a standard topic in the Freshman textbook where, shorn of its intellectual underpinnings, it is now presented, like the MO filling diagrams for diatomics and hybridization labels, as yet another set of lifeless rules to be memorized, sans context and sans integration with the other random topics which now comprise our typical textbooks. Chemistry is no longer the exciting central science, but rather a drab service course whose purpose is to train, rather than to educate, [students for] other professions and whose content is determined by the marketing departments of the book publishers and by the schools of nursing and engineering rather than by its own instructors.
I have long know that, since the publication of his original series of articles on the tangent-sphere model, Henry has kept extensive notebooks in which he has extended and applied the model to many areas of chemistry. Consequently, after the passage of so much time, it is a delight that he has finally decided to share some of them in this book. Over the years I have observed that Henry's writing style has become ever more terse and metaphorical so that he now writes on what might almost be described as a form of chemical haiku. Each word and picture counts, and each contains a provocative insight into the nature of chemical bonding and structure. One might not always agree with what he says, but careful readers will always find it stimulating and exciting, if for no other reason than it will force them to think about why they disagree.
No one pretends that the tangent-sphere model is a substitute for the results of modern computational chemistry [anymore than one supposes that the latter are a substitute for the images, concepts, and terminology generated by the former]. It is [from a purely computational point of view] basically a zeroth-order, semi-quantitative approximation to what may best be described as a set of idealized (zero overlap) localized MOs. It will never replace the sophisticated computerized quantum mechanical calculations which the theoretician and, to an increasing degree, the average practicing chemist, have now come to expect on a daily basis. But it is also, without doubt, a great advance on the circa 1916 Lewis diagrams still used in all introductory chemistry texts and, as such, is intended to serve as a pedagogical bridge between classical structure and Lewis dot chemistry, on the one hand, and low-level quantum mechanical calculations on the other. In recognition of this pedagogical function, Henry has now chosen to abandon the term tangent sphere and to speak instead of electron-pair domains, valence-sphere models, and conceptual valence-bond theory [in order to emphasize the isomorphism that exists between valence stroke diagrams and charge cloud models of molecules].
I have always told my students that the sophisticated use of bonding models does not lie in always using the most complex and sophisticated model available, but rather in learning how to choose the right approximation for the right situation. No matter how accessible and how automated computerized quantum mechanical calculations become, I am convinced that chemists, and especially introductory students, will always have a psychological need for a simple, qualitative — and above all — easily visualizable model [emphasis added] that they can carry around in their heads [or shirt pockets, in the form of a simple "starter kit"] and play with in moments of idle creative thought. We need to form a pedagogical hierarchy of bonding models in which we introduce progressively more sophisticated models at each successive stage of the overall chemical curriculum — from high school through graduate school — instead of the current largely random approach which I experienced as both a student and a teacher. The electron-domain model has much to offer in this regard, so read and, above all, think about what Henry has to say.
Department of Chemistry University of Cincinnati March 2011
Excerpted from MOLECULES and The Chemical Bondby Henry A. Bent Copyright © 2011 by Henry A. Bent. Excerpted by permission of Trafford Publishing. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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