MO Theory and the Freshman
To The Editor: Loud cheers are due any forthright and telling statement such as that of Schubert ["Should MO Theory be Taught in Freshman Chemistry-No!" [J. CHEM. Enuc., 47, 626 (1970)l. It is thus doubly unfortunate that Dr. Schuhert identifies what he does wish taught as VB (Valence Bond)theory. What he actually favors is "the concept of a molecule as a collection of atoms held toget,her by definite bonds." This is no more VB theory than it is 110 theory. That so many teachers and texts persist in the idea that VB theory has a monopoly on the localized bond may be due to the long domination of bonding theory (at least in this country) . . by . Pauling's - "The Nature of the Chemical Bond." MO theory is readily related t o localized bonds. I don't know how this is to be put across any more strongly-the point has been made very clearly at least twice in your pages within the past few months [46,746 (1969); 46,487 (1969)l. Nor is the point new: see C. A. Coulson's "Valence," (1st ed.), 1952, Ch. VII. I strongly agree with Dr. Schuhert that freshman chemistry is no place to trot out our entire theoretical arsenal. It would seem far better t o develop theory as a unifying basis for the discussion of descriptive material as it is presented, attaching no labels other than those clearly meaningful in context.
To the Editor: With the notion that the column "Provocative Opinion" is intended to provoke discussion, I wish t o submit that Leo Shubert's article banning MO Theory from freshman courses [J.CHEM.EDUC.47, 626 (1970)l is a prime example of the attempt by all of us in some degree t~ freeze our enthusiasms on the learning that was important to us in the early stages of our development both because the enthusiasms stay with us and because it's less work and more comfortable to not have to move with the times by making way for new facts, new models, and new methods of instruction. Such movement requires deft cutting and/or trimming of that which was and is so important to our ancient competence and spiritual well being. Theoreticians do not write freshman chemistry books as Leo contends. Even Linus Pauling and Harry Gray are primarily experimentalists who use theory effectively in the best traditions of an experimental science. A194
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Journal of Chemical Education
I would prefer to view Mahan's thermodynamic emphasis and Gray's (let's face it, Harry wrot,e that magnificent chapter) molecular orbitals for beginners as experiments in teaching which must he tried and modified and honed to useful proportions, or perhaps even exorcised eventually as Leo would have it. I don't think the returns are yet in on either experiment. We should meanwhile try to provide the newly educated teacher with modern tools for presenting his discipline. Certainly teachers must seek and obtain competence in what they teach. But the choices in chemistry are large and it is up to each teacher to construct his course with emphasis on his own competences. However, i t serves no purpose to have him sit back and complain because authors of some textbooks choose other materials than he. Nor does it behoove educationists and chemical politicians to blame textbook authors because people improperly or insufficiently trained select those textbooks for their classes. Leo argues for truth yet is willing to settle for two stationary dots (perhaps identified as negative charges) as "visualizable" bonds. If "the passions and convictions" of most freshmen dictated what we teach, we could eliminate t,he entire one-year service course for non-majors. Chemistry is a physical science (what does "middle science" mean?) and some physical ideas must be employed to describe atoms and molecules. Any description of atomic and molecular structure is physics-not just A40 theory. X suspect that. valence bond theory is easier for Leo to imagine, not necessarily for the student. I will freely admit to this in my own cases prejudice I'm trying to overcome. I frankly don't see how valence bond orbitals relate to the experience of an average freshman more than molecular orbitals. The reference to MOT making better sociologists is fatuous. Is the consequence of bathing in H2S0, productive of better sociology? A few years ago I taught (or t,ried to teach) t,he mole concept at a summer institute to would be teachers of advance placement chemistry courses. Fully half of the teachers showed what I felt to be a permanent difficulty in handling the mole concept in simple problem solving. Would Leo have us abandon teaching the "theoretical construct" of the mole because many existing teachers cannot handle it. Name any topic ever introduced to freshmen and somebody, if not most of us, have either overdone it or decried its appearance on the scene. (cf., multistep equilibrium problems, thermodynan~ics,reduction potentials, descriptive chemistry of any given element, atomic structure, etc.) Chemistry is a dynamic subject and its rate of change constantly forces on the freshman teacher the problem of restructuring his course, maintaining balance between theoretical framework and descriptive detail. I remember when Hubert Alyea was criticized for putting a diagram of a reaction coordinate in a freshman book. Such diagrams are now routine. We've learned what is useful about them to introduce and the limitations which must he maintained to keep them useful for beginners. (Continued on page Am$)
LETTERS (Cdinusd from page A194)
I'll paraphrase Leo's final five point paragraph by saying: An experiment in introducing constructs from MO theory to freshmen is worth a try by interested teachers because: 1) Many young instructors do most of their thinking about molecules in this theoretical framework and should try to formulate the basic ideas for beginning students. 2 & 3) MOT e m ~ l "o v sa. thwretical construct which can be used to explain ehemioel structures and reactivity. Emphasis can and should be on what is being explained by whatever theory is introduoed. (No two people can agree on what is isnduc emphaaiq m rlwory or #,"rxprrinwntnl fxcr.) 4 ) I rernernbrra lot olnly f d k w students h e i w ulirnurrd hy a welter of "fnvuU-or rarhrr ~ ) I I I I I I I W and ewations devoid of rhcoretical unity. A valid theoretioal frame&ork relating facts is essential to the subject. 5 ) We should present the state of thinking in our subject in light of our incomplete and imperfeot understanding as an incentive t o students to join up and help further such knowledge and understanding.
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I have no objection whatever to Leo's choice of VB over h 4 0 hut I do strongly object to his discouraging experiments with material to be taught. Students were alienated by chemistry-a difficult subject-long before MO's, and no individual topic--especially a recent arrival-can be singled out as a whipping boy for the continued frustration of our own ent.husiasms as we try to reach students with whatever program we have designed.
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lournol of Chemical Education
