Chemical Education Today
Editorial
Turning the (Periodic) Tables From the iodine thermometer on the cover, through an encounter with Oliver Sacks and a collection of elements (p 879), to an article about using chemical demonstrations in support of inquiry-based, group learning (p 962), this issue abounds with descriptive chemistry. An enormously successful way of summarizing and classifying the physical and chemical properties of the elements and many of their compounds is to display the element symbols in a format that emphasizes similarities and differences by means of graphic design—a periodic table. Notice that I said a periodic table, not the periodic table. Articles in this issue by Cronyn (p 947) and by Jensen (p 952), and the unbridled enthusiasm of Oliver Sacks for the fascinating diversity of the elements, underscore the likelihood that there is no single best way to represent the properties of the elements in graphic form. Those who have written about representations of chemical periodicity tend to avoid reference to the periodic table, referring instead to “the periodic system” (1, 2) or “graphic classification of the elements” (3). Even Mendeleev was undecided about how best to arrange the elements graphically. In his first paper on the periodic system (4) Mendeleev’s preferred version was Figure 12 (from reference 2), and he described three others in footnotes. In one footnote he said that Figure 13 (from reference 2) would not be a convenient choice, but it is closest to the one commonly used today. In another footnote, he described a helical or zig-zag table. By 1870 his preferred version was the one shown in Figure 17 (from reference 2), which is a predecessor of the table that I used in high school. Mendeleev’s great contribution was not that he discovered the periodic table, but that he used a periodic classification to predict that there were undiscovered elements and to estimate their properties. Reference 1 includes about 60 representations of chemical periodicity and reference 2 about 140, indicating the persistence of attempts to create better periodic tables in two or even three dimensions. This provides even more evidence that there is no single best organization. Nevertheless, textbooks seem to have converged only a very few of the many possibilities. This may be unfortunate. There is good reason to believe that a periodic table serves not only as a reminder of facts already known, but also as an influence on how both faculty and students think. Cronyn points out, for example, that hydrogen’s typical position at the top of the alkali-metal group leads some to read far more than they ought into the fact that when compressed to more than a million bars a sample of hydrogen conducts electricity. Boron, oxygen, sulfur, selenium, tellurium, and phosphorus all can be made conductive under pressure, but only in the case of hydrogen is metallization thought to vindicate its predicted properties. There is another reason to include more than just a single periodic table in our teaching. Each of the many different arrangements proposed has advantages and drawbacks, and we can use those to improve pedagogy. Asking students to argue pro or con for a particular representation
…Mendeleev’s great contribution was not that he discovered the periodic table, but that he used a periodic classification…
of periodicity can be a challenging and instructive exercise. It requires that they know enough about properties of the elements to make convincing arguments, and it points out that science does not always arrive at a single, best, correct answer to a complicated question. First-year students, for example, should be able to make reasoned arguments for placing hydrogen above lithium, above carbon, and above fluorine. Having three groups of students consider this issue and then debate it would expose them to an important aspect of science that is often neglected—the fact that there is not necessarily one correct answer to every question. Read the articles by Cronyn and Jensen and see if you don’t come up with some good ideas for turning periodic tables to even more effective use in the classroom. Literature Cited 1. van Spronsen, J. W. The Periodic System of Chemical Elements; Elsevier: Amsterdam, 1969. 2. Mazurs, Edward G. Graphic Representations of the Periodic System During One Hundred Years; University of Alabama Press: University, AL, 1974. 3. Quam, G. N.; Quam, Mary Battell. J. Chem. Educ. 1934, 11, 27–32, 217–223, 288–297. 4. Mendeleev, D. I. Zh. Russk. Khim. Obshch. 1869, 1, 60–77.
JChemEd.chem.wisc.edu • Vol. 80 No. 8 August 2003 • Journal of Chemical Education
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