Chemical Education Today
Letters Instructional Scanning Tunneling Microscope While I am delighted to see scanning tunneling microscopy brought into the high school classroom, the article by Carl Rapp (1) perpetuates a common misconception about the beautiful images of the graphite surface that are obtained by STM. Inspection of Figures 4–7 shows that these images cannot be a true picture of the way atoms are disposed on the surface of graphite. First, the distance between the “graphite atoms” is too large, about 2 Å; the C–C bond length in bulk graphite is known from crystallographic data to be only 1.415 Å (2). Second, the six-membered rings in graphite do not contain central atoms! This oddity in STM images of graphite was noted by Binnig et al. in 1986 (3). A simple explanation is based on the fact that STM does not produce pictures of atoms, but of the tunneling current. In the most common form of graphite, the sheets of six-membered rings are offset, so that half of the atoms in the top sheet lie atop the center of a six-membered ring in the sheet underneath. Thus, graphite contains two inequivalent types of carbon atoms, which happen to transmit different tunneling currents. A more rigorous treatment of electronic structure effects is given by Tersoff (4), who concludes that for one- or two-dimensional semiconductors such as graphite, “the image has no direct relation to the positions of the atoms within the unit cell.” Literature Cited 1. Rapp, C. S. J. Chem. Educ. 1997, 74, 1087–1089. 2. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; pp 236–238. 3. Binnig, G.; Fuchs, H.; Gerber, C.; Stoll, E.; Tosatti, E. Europhys. Lett. 1986, 1, 31. See also Park, S.-I.; Quate, C. F. Appl. Phys. Lett. 1986, 48, 112. Sonnenfeld, R.; Hansma, P. K. Science 1986, 232, 211. 4. Tersoff, J. Phys. Rev. Lett. 1986, 57, 440–443. Marya Lieberman Department of Chemistry and Biochemistry University of Notre Dame Notre Dame, IN 46556
v The article “Getting Close with the Instructional Scanning Tunneling Microscope (J. Chem. Educ. 1997, 74, 1087) contains errors with regard to the resolution of the ISTM. As the critical reader will have realized, the surface structures shown in Figures 4–7 of this article do not show the arrangement of the atoms expected for the well-known graphite structure. Each white spot seen in the figures does not represent a single carbon atom, but a complete hexagon of carbon atoms. It is unfortunate that the two companies who are known to me as producers of low-cost STMs claim atomic resolution in their promotional material showing the graphite surface as the prime example. This may be misleading and arouse expectations that can actually not be fulfilled. Albert Lötz Institut für Physikalische Chemie Universität München 80333 München, Germany
The author replies: Thank you for pointing out the relationship between scanning tunneling images and the positions of atoms in graphite. Unfortunately the companies producing low-cost STMs had led me to believe that we could see atoms in the graphite structure. I agree, it does seem they have raised expectations of both me and my students. In the study of the already confusing world of atomic chemistry, I certainly would not want to add to my student’s confusion. This is a point, however, that I have now clarified for my students. Thanks for setting us straight. Carl Steven Rapp University School East Tennessee State University Johnson City, TN 37614
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Spontaneity and Log K The article by Frances H. Chapple (J. Chem. Educ. 1998, 75, 342) clarifies a paradox that has bothered many of us for some time. It is puzzling that the degree of spontaneity and the equilibrium constant should have different temperature dependences. However, the paradox disappears when we use the Planck function Y = S – H/T = –G/T as our criterion of spontaneity instead of G. It is a more primitive criterion, since ∆Y = ∆Ssystem + ∆Ssurroundings. Finally,
∂∆Y ∂T
P
= ∆H2 T
∂ln K ∂T
P
= ∆H2 RT
and
Irving M. Klotz and Robert M. Rosenberg Chemistry Department Northwestern University Evanston, IL 60208-3113
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Reforming High School Chemistry Textbooks Ron Gillespie’s (1) call for additional textbooks for introductory chemistry courses and the supportive letters in the January issue (2) point to a major educational need. The letters from university-level chemists suggest that introductory college chemistry needs new textbooks, teaching approaches, and supportive multimedia materials. However, some diversity exists at the introductory college level, and more courses are currently being developed. But the greater problem exists at the high school level. During a recent district science curriculum evaluation, we found that all available high school chemistry textbooks in the United States, except ChemCom, share the same traditional format and teaching approach. No longer available is the teaching format introduced by ACS and NSF in CHEM
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Chemical Education Today
Letters Study (3) in the 1960s and further developed by Marjorie Gardner’s IAC (4) group in the 1970s, where students observe chemical phenomena and then develop explaining theory. That “science approach” provided students with a fundamental understanding of the connection between observations and theory. Now virtually all high school chemistry textbooks utilize the “philosophical approach”, where students are asked to assume the theory and occasionally see the phenomena later. This is most clearly evident where every textbook presents the “nuclear atom”, complete with protons and neutrons, in the early chapters as the basis for developing further understanding. A typical high school student has never considered any evidence for protons, let alone neutrons. Presented before many students have digested the evidence for elements or set combining ratios, such a format asks students to learn chemistry as dogma rather than as science! This is probably most comfortable for students and teachers who live with television presentations. But it is not the most effective way to teach an understanding of science. Perhaps this helps explain the fact, as noted by Richard Zare (5), that two-thirds of the increase in American science and engineering doctoral awards were earned by non-U.S. citizens. ChemCom presents a clear advantage over the “traditional chemistry” textbooks by presenting students with realistic problems needing scientific solutions. This “engineering approach” provides motivation for seeking solutions provided by science. But it might not be ideal for students who wish to develop an understanding of scientific research. ChemCom may be ideal for citizens who will only use science, but an alternative high school course should be available for those who (broadly) will do science. High schools badly need a course that teaches chemistry by having students do chemistry. In an age where students get progressively fewer “real life” chemical experiences, the course should be not just “descriptive” but actually “hands on”, with sixty or more chemical experiences for students in a year. Multimedia should provide additional experiences that are hard
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to achieve in high school laboratories. If the teacher is competent to supervise, students should do several research projects as well. Every experience should have multiple objectives. While seeing the phenomena, students should also be learning typical chemical skills and techniques. While some may need to be microscale, others need to be macro (how many cooks use exclusively microscale chemistry?). Theory should help students understand the observations. Since historical developments often parallel the development of the abstract theory, history should be utilized in “multimedia” to help students understand the development of the theory. Computer animations should help visualize the molecular level and connect that to the observations. High school chemistry content should be broad and interesting, but basic and fundamental. There is little reason why it should be a 180-day version of introductory college chemistry. While a team of high school teachers might use email to develop such a course, the chemical expertise of university faculty will be beneficial as in past high school curriculum projects. As other writers concur, development of such a course will need ACS and NSF support. Literature Cited 1. Gillespie, R. J. J. Chem. Educ. 1997, 74, 484–485. 2. Gillespie, R. J. and colleagues. Letters. J. Chem. Educ. 1998, 75, pp 10, 26–32. 3. Seaborg, G. T.; Campbell, J. A.; Pimentel, G. C.; et al. Chemical Education Material Study; Freeman: San Francisco, 1960. 4. Gardner, M.; Heikkinen, H.; et al. Interdisciplinary Approaches to Chemistry; Chemistry Associates of Maryland, 1973. 5. Zare, R. N. J. Chem. Educ. 1998, 75, 16–17. David W. Trapp Sequim High School 601 N. Sequim Ave. Sequim, WA 98382
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