Issue Date: November 12, 2007
Reviewing The Periodic Table
Eric Scerri's delightful "The Periodic Table: Its Story and Its Significance" follows the 1969 classic by J. W. Spronsen, "The Periodic System of Chemical Elements," but is a different treatment of this familiar topic. In addition to the usual historical outline of the evolution of the periodic table, Scerri includes a philosophical assessment of its deeper significance. He includes unusual topics and tackles questions that are generally ignored in similar treatments. For example, he includes a discourse on the philosophy of Dimitri Mendeleev, who is generally regarded as the architect of the present periodic table, and an impressive, expanded treatment of the quantum mechanical issues of today's periodic table.
Discussions of periodicity of the elements, whose physical and chemical properties repeat in a cyclical fashion as one progresses through the series, generally include a listing of possible graphical designs or textual representations of this periodic behavior, and Scerri's book is no exception. My first experience with new forms for the periodic table occurred nearly half a century ago, in 1949, when Life magazine presented a photographic essay titled "The Atom: A Layman's Primer On What The World Is Made Of" (Life, May 16, 1949, page 68) and included a beautiful multicolored elliptical periodic chart patterned after chemist John Clark's spiral design (J. Chem. Ed. 1933, 10, 675).
Previously, the only periodic table I had seen was the prosaic short-form table hanging on the classroom wall at the local college where my father taught chemistry in the 1940s and '50s and which persisted for another two decades. In the decades since, the classroom periodic table has evolved into the "medium-long" table where the transition metals do not double back but instead are arranged in their own block. This form has become so universally accepted that we see it everywhere, in any university and in any country.
But many other periodic systems have been suggested. Since the mid-1800s, more than 100 forms have appeared, including two-dimensional and three-dimensional representations; continuous and discontinuous; tables, charts, and complex architectures of screws, cones, spirals, spheres, helices, pretzels, ellipses, arenas, wedges, cubes, trees, and laminae. From time to time, one sees a new system in the Journal of Chemical Education or elsewhere. Perhaps the latest, most eye-arresting is Philip J. Stewart's "Chemical Galaxy," a sweeping multicolored rotating sea of element "stars." So many ways to represent relationships among the elements are possible that one wonders whether there is a "correct" one. But deep down inside, probably everybody believes there is an internal order that the periodic table represents, a fundamental truth that is the bedrock of chemistry.
But is there such a fundamental truth to the periodic table? Is it possible that all of these different representations of the periodic table only reflect the human drive to see relationships that, in fact, may be only coincidental?
Georges Cuvier, the French naturalist who founded vertebrate paleontology, warned against "abstractions of man" that do not accurately reflect nature. Such perceived elemental relationships of questionable significance might include, for example, German chemist Ernst Lenssen's observation in 1857 that the spectral flame colors of triads—three elements of the same chemical family—can be complementary. And organizational schemes have failed before for chemistry. Carl Linnaeus attempted to apply his system of nomenclature to chemistry, but it was not successful because external characteristics of inanimate objects—the appearances of minerals, for example—do not reflect an inner pattern as they do in zoology, where external characteristics of an animal reflect the genetic code.
It is to be remembered that the first periodic tables of the independent codiscoverers of periodicity among elements, Mendeleev and Lothar Meyer, were designed merely as a convenient means to organize their textbooks. Treatises discussing the periodic system tend to gloss over the point that perceived relationships could be arbitrary. In his book, for example, Spronsen, after reviewing scores of different systems, perfunctorily states that the preferred one is the two-dimensional table where "all the elements have their rightful place by reason of their properties."
But which properties? And what is the ultimate reality that is being reflected in a periodic system? Or is there an ultimate reality at all?
In Scerri's book, these questions are addressed throughout. The first half of the book is devoted to the development of chemical periodicity through the 19th century, at the end of which Mendeleev's table was generally accepted. Scerri not only discusses the straight-line development of science leading to the periodic table, but he also reviews incidental work. He believes missteps are important in the telling of a story, because they help describe the complete thinking process, allowing the reader to understand the dynamic nature of the scientific method. Thus, he discusses not only Antoine Lavoisier's Table of the Elements and Johann Wolfgang DÖbereiner's triads but also, for example, the less well known pyramid tables of Leopold Gmelin or the difference relationships of group elements by Max von Pettenkofer.
Scerri is not afraid to debate widely accepted axioms, such as the commonly held view that Mendeleev's successful predictions of the elements gallium, scandium, and germanium are the reason his name is so famous. Scerri points out that Mendeleev made many predictions and that at most half of them were correct. Mendeleev also made other bizarre suggestions, such as the possible existence of six new elements between hydrogen and lithium.
Scerri also details how "the acceptance of Mendeleev's system was not a simple matter," how some chemists, such as Charles-Adolphe Wurtz, were not impressed by the periodic table at all. Scerri analyzes deeply why "history has been so kind to Mendeelev" and concludes that historical events "belie the notion that what counted most in the acceptance of the periodic system were Mendeleev's successful predictions." For example, in the Davy Medal Citation (awarded jointly to Mendeleev and Meyer), not one mention is made of predictions; instead, it includes specific observations such as "periodic re-appearance of analogous properties of ... melting points and atomic volumes." Furthermore, Meyer's periodic table appeared earlier and was more accurate than Mendeleev's, and the periodicity of the elements is more obvious from Meyer's plot of atomic volume, where they can be seen "at a glance."
In part, Mendeleev was so successful because he was the champion of propagating the periodic system, defending its validity and devoting time to its elaboration. But Scerri goes deeper; he suggests that Mendeleev's advantage lay in his philosophical approach that allowed him to differentiate "simple substances" (Lavoisier's isolable elements) from "basic, or abstract, substances" (unobservable, or property-bearing entities) and thus achieve a deeper understanding of just what the periodic table represents.
Even more thought-provoking is the second half of Scerri's book, where he takes up radioactivity, isotopes, and quantum mechanics. Scerri's detail in tracing the evolution of the periodic system of the 20th century is exquisite. For example, he carefully traces the development of atomic number, pointing out the great deal of background history in place before Henry Moseley performed his X-ray research: Anton van den Broek proposed the concept with his idea of a charge on the nucleus, and Ernest Rutherford actually coined the term "atomic number."
But the most engrossing discussion Scerri provides is that of the impact of quantum mechanics on the periodic table. The prevalent view among chemistry educators is that Niels Bohr, the 1922 physics Nobel Laureate, finally validated the long form of the periodic table by his quantum mechanical treatment that independently elaborated the electronic structure of the atom. Scerri argues against this. Through a full history and discussion of the quantum mechanical development of the periodic table, Scerri shows that instead of quantum mechanics predicting the periodic table, it is the physical properties of the periodic table that show the way for quantum mechanics.
For example, textbooks generally do not emphasize or even mention that the lengths of successive periods of the periodic table have not yet been strictly deduced from quantum theory, or that the rules for building up the electronic configuration are fundamentally empirical. The problem is, of course, that no exact quantum mechanical solution can be made for anything more complex than the hydrogen atom. So far, no calculation has shown, for example, why the valence electronic configurations of the nickel-palladium-platinum group are not identical. Quantum mechanics simply cannot predict accurately from first principles how valence shells are to be filled.
Staunch defenders of the quantum mechanical method argue that all electronic configurations and/or energies can be calculated correctly if one merely advances to a sufficiently sophisticated level. But the fact is, no one knows he has arrived at that level until he compares the calculated values with the experimental ones. And additional second-order relationships, such as the "knight's move" relationship (move down one element, then to the right two spaces, to see similar behavior; for example, Zn and Sn, or Ag and Tl) are far beyond the power of the calculations and remain strictly empirical observations.
Scerri concludes his book with a fine discussion of how the elements are synthesized in the universe, differentiating between Big Bang and supernova nuclear chemistry, and a discussion of the ultimate question, "Is there a best form for the periodic table?" Scerri returns to Mendeleev's original question: Should elements be considered "simple substances" or "basic substances"? Scerri optimistically believes there is a Platonic ideal with "basic substances" that chemists can approach through reductionism, which leads to a left-step table where blocks of elements are right-addressed.
But he realizes also that "simple substances" perhaps should be represented as well, and so doing leads to a form more like the familiar medium-long table. Still, hydrogen and helium are an enduring problem; they just won't fit ideally in any scheme. Thus, pragmatism rules the day: a mineralogist, a biochemist, a theoretician—all might choose different representations of the elements.
So what are the answers to the really bothersome questions: Why doesn't the periodic table work perfectly? Why are there so many versions? Is a deeper truth reflected in the periodic table? Or are the various periodic tables artificial to a certain extent? Scerri bravely pursues these problems with a deeper treatment that gives at least partial answers and that furnishes us with an important contribution to our understanding of this topic.
Scerri's style is just right—readable and fluid, but deep and provocative. The book has been written carefully and is well-organized. It has a full index, covering not only the body of the book but, impressively, also the footnotes. This should prove to be an important reference book and a welcome addition to the library of the research scientist, student, and chemistry educator.
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