Chemical Education Today edited by
Norbert J. Pienta
When Constant Values Meet the Process of Science: Recent Changes in Atomic Weights and Hydrogen Bonds It did not appear in headlines when the announcement was made in the middle of December 2010, perhaps because the press was not sure what to make of it. And in the scheme of things to acknowledge in the International Year of Chemistry, it is not likely to be widely celebrated or reviled. But the scope is global because the announcement came from IUPAC, the International Union of Pure and Applied Chemistry, a group of scientists concerned with standards and definitions on behalf of all of us in the world. The news is listed on IUPAC's Web site: Atomic Weights of Ten Chemical Elements About to Change (1). The report announces that the atomic weights of 10 elements (hydrogen, lithium, boron, carbon, nitrogen, oxygen, silicon, sulfur, chlorine, and thallium) will now be reported as a range rather than a single value. Modern analytical techniques can measure atomic weight very precisely, and those values are important in defining standards in research and industry. The ranges are expected “to decode the origins and the history of a particular element in nature” according to Michael Wieser of the University of Calgary, the lead author of the report (1, 2). It seems to me that there are several potential messages here, especially if you think of the various perspectives of chemists and chemical educators. In these difficult economic times, the need to reprint all of the periodic tables in the world is either a tremendous financial opportunity or something approaching a tragedy. For those who might take a conservative view of this, the periodic table is not really supposed to change. Atomic weights are constants, a number that you can count on. Indeed, such changes do not happen very often. Depending on what one defines as a change, this might be the first time in history. I am certain that “corrections” have appeared over time. For example, a large lecture auditorium at my institution that was originally built ca. 1930 had a magnificent illuminated periodic table with the element name “columbium” in the place where we would now expect the updated version, niobium. The IUPAC seemed appropriately concerned for both educators and students. In a statement accompanying the release of the report, Fabienne Meyers, associate director of IUPAC, said, “Though this change offers significant benefits in the understanding of chemistry, one can imagine the challenge now to educators and students who will have to select a single value out of an interval when doing chemistry calculations” (2). It certainly may annoy those folks who have the values committed to memory, after having done those sample calculations “at the board” for much of their professional careers. I look at this as another teachable moment and an important one. The atomic weights on a periodic table are experimentally determined, averages of the masses of each pure isotope. What we are experiencing with this report from IUPAC is the process of science. Chemistry can and does adjust what our disciplinary community believes and teaches, as better informa-
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Department of Chemistry University of Iowa Iowa City, IA 52242-1294
tion becomes available. Students need to know this about how science is supposed to work. And perhaps that lesson is more important than the skill to do the calculation mentioned in the quote. Calculations using atomic weights are better used as means to accompany conceptual understanding, not as a goal. And the clever ones among you are already devising a whole new category of questions to challenge students' cognitive abilities: “Calculate the percentage difference in a set of samples that contain sulfur at the lowest and highest values of atomic mass.” But also remember to ask them where those values arise and what those differences are likely to mean. The answer may be harder to grade, yet the outcome is likely to be more satisfying. If you think that the IUPAC has only (inadvertently or intentionally) targeted the first-semester course in chemistry, I am happy to remind you that they have also been busy redefining the hydrogen bond (3). Earlier in 2010, a committee submitted a provisional report that better defines the various interactions, also based on the latest experimental evidence. It does not become an actionable item until this spring. Like much of what we teach in the introductory courses, the original model was too simple. This case provides another teachable opportunity, both for instructors and for students. A novice must start with a simple understanding, often derived from a simple model. We can continue to add complexity (and accompanying accuracy), but that takes time and effort. Again, it is important for students to understand what they know, where it comes from, and why their knowledge might be limited. It is equally important for us as educators to focus on what we can and should do because it is safe to assume that the IUPAC reports will keep up with the progress of science. Norbert J. Pienta Editor in Chief
Literature Cited 1. For the IUPAC announcement, see http://www.iupac.org/web/nt/ 2010-12-13_2009-atomic-weights (accessed Jan 2011). 2. For a report on the disclosure of the IUPAC report in the ChemistryTimes, see http://www.chemistrytimes.com/research/ Atomic_weights_of_10_elements_on_periodic_table_about_to_ make_an_historic_change.asp (accessed Jan 2011). 3. For the IUPAC committee report on the hydrogen bond, see http:// media.iupac.org/reports/provisional/abstract11/arunan_prs.pdf (accessed Jan 2011).
Norbert J. Pienta is a professor in the Department of Chemistry, University of Iowa, Iowa City, IA 52242-1294; norbert-pienta@ jce.acs.org.
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r 2011 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 3 March 2011 10.1021/ed200016z Published on Web 01/21/2011
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Journal of Chemical Education
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