We Await the Magic New Textbook To the Editor:
My comments are stimulated by those of Bent [1986,63, 8901, Sanderson [1986,63,845], Zuckerman [1986,63,829]. I write as a . ohvsical chemist who has been eniovina . . - a threeyear stint in the teaching of a freshman chemistry course, as one who has recentlv reviewed a number of books and manuscripts in the field bf freshman chemistry, and also as one who has some familiarity with modern theories of valency and molecular structure-(at least in passing), as well as the other accoutrements of physical chemistry and chemical physics (statistical mechanics, quantum and classical mechanics, thermodynamics, electrodynamics, e t ~ . ) . I welcome the views of each of these persons with regard to the half-baked repertory of "tbeory" which is foisted upon chemistry students. Zuckerman in particular does a very well-reasoned job of exposing the bankruptcy of traditional ~edaaoav.a least as i t aouears to be represented in the &ekglut of textbooks. B& the problem;sn't with theory. I t stems more from the fact that textbooks and teachers have not kept up very well with modern pedagogical ideas designed to make molecular structure understandable. One can travel from Berlin (1,2) to Deb (3-5) to Bader (6) (with a large number of stops in between) and accomplish much more (and more correctly) than by using the traditional aufbau based on hydrogenic bound states, followed by some ill-defined minimal basis LCAO-MO model conjured up by hand waving. As Zuckerman fails to note in his comments
about spectroscopy and X-ray diffraction, molecular charge distributions are ohservables (7). Of course, they are subject to the same sorts of uncertainties as one suffers with other exoerimental ouantities h e . . alwavs more uncertain than one would likej, but obseivables thky are, nonetheless. The fundamental dogma of chemistry is that chemical properties, like physical properties, flow from the charge distrihutions of the interactina species. This seems excruciatinalv obvious, but it appears to have escaped the attentionVo> those who write our textbooks. Complaints about the presentation of other parts of "theory" are also justified. Witness in our current textbooks the use of sections on atomic tbeory, chemical equilibrium, and electrochemistry as padding to provide a discrete separation between the first law of thermodynamics and the second! Or the utter neglect of the hard-won material in "kinetic-molecular theorv" when it comes to the teachine" of chemical kinetics. These are blunders of missed opportunity, and they overshadow completely the factual errors which abound in these texts. One is tempted cynically to observe that authors avoid building cross-links between concepts because it interferes with a modular presentation of the material and spoils the linear form of the syllabus. The magic new textbook which we await (with Zuckerman) will eventually emerge. Let us hope that it will be more than a CRC and book set in text foim, as these authors seem to argue. I t should include a balanced view of chemistry from a theoretical perspective-after all, the universe is not governed by whimsy!-and should eschew dogged devo-
Volume 65
Number 8
August 1988
745
tion to models that no longer correspond to current theoretical thought. I t should seek to use science which is as invariant as oossible when formulatine- "ex~lanations". In addi. tion, whrn recourse ro modrl:, ii made, it ought to be honest enough toemohasi~ethe defects ola i~articularmodel and to exp& why chat model was chosen in spite of its failings. On the other hand, when it turns to a discussion of measured quantities, it should give a suitable critical explanation of the manner in which the data were obtained. In short, mastery of empirical fact goes hand in hand with critical thinking and with correlation and generalization. It is amusing to wonder what our role as teachers would he if we were to turn to an encyclopedia of descriptive chemistrv. -, devoid of d o t . Would we oreach from the text hut provide our ow; exegesis? Or m&t we stick to the written word? The latter course suggests we would he teachingread-
.
Acknowledgement I would like to acknowledge the helpful suggestion of a reviewer. W. Danlel Goodman College of Great Falls @eat Fails, MT 59405
MOLDOT To the Editor: I was interested in reading about the program MOLDOT that produces space-filling diagrams of molecules [1986,63, 3261. However, I was disappointed that the figures omitted hydrogen atoms, which constitute much of the outer surface of organic molecules and thus determine much of their "visible" form. i.e.. their steric orooerties. I t is our ex~erience that many postgraduate stuienis learning crystal structure determination. if asked to eenerate mace-filline diaerams, tend to omit hydrogen atoms and &us to produce such "unrealistic" diagrams. I t is a paradox of X-ray structure determination that the hydrogen atoms, which contribute so little to the diffraction pattern, play a major role in determining the molecular packing in the crystal. Clearly, the omission of hydrogen atoms reduces computing time. Equally clearly, we should not encourage the misconception that a molecular model without hydrogens is realistic. ~~~
John L. Ragle University of MaSSaChUSenS Amherst, MA 01003
Llterature Cited 1. Rer1in.T.
J.Chem. Phys. 1951,19.208.
2. Hirschfeider. J. 0.: Curtis, C. F.;Bird, R. 9. Molocvlor Theory o/ Cases end Liquids:
Wiley: New York, 195kpp932-937. 8. 9amrai.A. S.: Deb, B. M, Re". Mod.Phys. 1981,53,95-126. 4 . Par.. R. G.Ann. Rsu. Phys. Chem. 1983,34.631-656. 5 . Deb, 9 . M. ( E d . ) . The Force Concept in Chemistry; V a n Nostrand-Reinhold: New York, 1981. 6. Bader. R. F.: Nguyen-Dang,T.T.:Tal. Y.Repta,Plog.Phys. 1381.44 893-946. 7. StewarLR. F. C h ~ m . P h y s . L a l l .1379.65.335-342.
~
Peter G. Jones institlR fur anoraanische Chemie ~&mannstrasse 4 3400 Gdhingen Fed. Rep, of Germany
Cool But Not Quick To the Editor:
To the Editor:
I read Cohen's article, "Temperature, Cool but Quick" [1986,63,1038], photocopied it for the students of my physi-
The program MOLDOT draws whatever atoms are specified in the data file. Two of the four data files supplied with the MOLDOT disk include hydrogen atoms. (See structure of cubane, below, in which hydrogen atoms are included.)
cal chemistry class, and introduced Cohen's article for discussion. I focused the discussion hy offering a slightly altered interpretation of the information and by asking the studenu to rhoow. In thought experiments, we looked at the exnansion of eases. the distribution of velocitv curves, and a t &hen's ambLguity. To try to understand the ambiguity, we used a supersonic jet plane analogy. First, we agreed with Cohen that the cooling of a gas by expanding it from a region of high pressure into a region of low pressure is a routine procedure. Next, we agreed with Cohen's suggestion that the width of the Maxwell-Boltzmann distribution is identified with temperature. By the use of a jet plane and exhaust analogy, we noticed, however, that we could understand the source of the Cohen ambiguity. For instance, viewed from inside a jet plane, the exhaust would appear to be speeding away from the plane; from earth, however, the plane would appear to be moving away from its exhaust. T o measure directly the temperature of the plane exhaust, we decided, one would position a thermometer in the exhaust molecules. We decided to specify the frame of referenre tor the supersonic jet and molecules PIueriment as the tmmerarure meas~rinathr distribution of molecules. With the thermometer as the reference, we found that ambiguity about the distribution vanished for us in Cohen's jet-and-molecules thought experiment. In particular, we experienced ambiguity when we visualized the collection of molecules a t 10 K as speeding away from the stationary jet. However, because the laws of physics must be the same for all frames of reference, i t is equally valid, but unambiguous, to consider the jet source as moving away from the stationary thermometer and the collection of molecules at 10 K, in the manner of a jet plane speeding away from its exhaust. 746
Journal of Chemical Education
Omission of the hydrogen atoms in the figure used to illustrate my article describing MOLDOT does not greatly reduce computing time, which, as explained in the article, depends chiefly on the number of dots, rather than on the number of atoms. Steven Brumby me Australian Natlonsl University G.P.O. E m4 Canberra. ACT 2601. Australia
