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Elemental Deductions

Seymour Mauskopf

The Periodic Table: Its Story and Its Significance. Eric R. Scerri. xxii + 346 pp. Oxford University Press, 2007. $35.

From%20The%20Periodic%20TableClick to Enlarge ImageEric Scerri's new book is a most appropriate work to mark the centenary of the death of Dimitri Mendeleev. The title—The Periodic Table: Its Story and Its Significance—gives a fair idea of the book's contents, and the author's approach and perspective are captured by his statement that he is concentrating on "the fundamental scientific and philosophical ideas that underpinned the evolution of the system." This, then, is a book about scientific ideas. Scerri does provide brief biographical sketches of each of his scientific protagonists, but biographical, social and cultural context rarely intrude into the narrative.

A number of philosophical and historical concepts govern the narrative in the first half of the book, which covers the history of classification of the elements up through the development and reception of Mendeleev's periodic system. Scerri makes the point that Mendeleev's achievement was not a paradigm shift but a more gradual evolution, spanning much of the 19th century and picking up momentum in the 1860s in the wake of the clarification of atomic weights by Stanislao Cannizzaro. Indeed, Scerri goes so far as to characterize the history of the periodic system as "the supreme counter­example to Thomas Kuhn's thesis, whereby scientific developments proceed in a sudden, revolutionary fashion."

Another premise set forth is that scientific "mistakes" sometimes have good results and inspire creative thinking. The most important example Scerri gives of this process is the influence of William Prout's hypothesis that all atomic weights are integral multiples of hydrogen. Some philosophers in ancient Greece believed that an underlying primary form of matter, which they named protyle, was the basis of all matter, and Prout theorized in 1816 that hydrogen was it and thus underlay all apparent elemental diversity. This theory, although dismissed and "disproved" in the mid-19th century, led many scientists to look for relations between groups of elements based on atomic weight and chemistry. The notion that "mistaken" scientific ideas can play a positive role is nothing new to historians but may be more provocative to philosophers of science and scientists.

Analyzing the concept of an element, Scerri identifies three subcategories: property-bearing abstract elements (such as Aristotle's fire, earth, water and air); simple substances (the empirically defined elements of Lavoisian chemistry, which cannot be decomposed by any known means); and the material ingredients of substances (those which participate and persevere in a chemical compound). For Mendeleev, it was the abstract elementnow with the sole discernible and measurable property of atomic weight—that persevered in a chemical compound. Thus Scerri argues that atomic weight became the preeminent characteristic of chemical elements for Mendeleev. (Later in the book, Scerri employs the phrase basic substance to further differentiate between Mendeleev's elements, which were defined by atomic weight, and 20th-century elements, which are defined by atomic number.) This is an extremely interesting philosophical analysis, one that would have benefited from additional historical context.

Scerri argues that Mendeleev's periodic system was accepted primarily because it had the characteristic of "accommodation," by which he means "the ability of a new scientific theory to explain already known facts."He contrasts this with the more widely held view that it was Mendeleev's dramatically successful predictions of new elements and their properties that won the day for his system. Indeed, Scerri takes on recent assertions by two philosophers of science on just this point. Yet he provides almost no concrete historical evidence to support his contention—which is a shame, because the issue of what led to the acceptance of this comprehensive chemical system invites comparison with its nearly contemporary biological analogue: the reception accorded to Darwin's theory of evolution by natural selection.

In the second half of the book, Scerri turns to developments in physics early in the 20th century that are often taken as providing the theoretical basis for the periodic ordering of the chemical elements. These include the discovery of the electron, the delineation of a "solar system" model of the atom and its quantization by Niels Bohr, the enunciation of the concept of the isotope, and the recognition that elements are defined (and differentiated from one another) by atomic number (eventually identified as the number of protons in the atomic nucleus) rather than by atomic weight.

By 1920, Bohr and his colleagues had devised models of atoms comprised of nuclei surrounded by multishelled electronic configurations. These models offered insightful, although incomplete, theoretical explanations for the periodic relationships between the elements. Soon the development of quantum mechanics provided refinements to and extensions of the explanatory power of quantum physics.

Indeed, as Scerri notes, one of the principal architects of quantum mechanics, Paul Dirac, declared in 1929 that "The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known" (emphasis mine).Dirac's dictum could serve as something of an epigraph for the second half of the book, which I found more enthralling than the first half's description of the development of the periodic system.

Scerri's persona as a philosopher of chemistry really comes into its own in the later chapters. The main thrust of his argument is to counter the deductive and "reductive" (to use his word) dicta espoused by physicists such as Dirac and by many authors of chemical textbooks. Although Scerri is not as explicit as he might be, he is attempting to show that chemistry, particularly as embodied in the periodic system, is profoundly different from physics and in particular is resistant to the kind of deductive generalities that physics offers. For at least 300 years (since the criticism of Bernard de Fontenelle in 1699 that, compared with physics, "the spirit of chemistry is more confused, more shrouded"), chemists have often suffered from a sense of scientific inferiority and consequently have felt the need to legitimate their science using the theoretical basis of physics.

Without denying that 20th-century quantum mechanics has provided a great deal of theoretical insight for the periodic system, Scerri champions chemistry as a science in its own right—a richly empirical one that deals with the material world in all of its variety and complexity. Behind this effort lies a didactic purpose. Scerri notes that most textbooks imply that quantum mechanics satisfactorily explains the periodic system:

This, in turn, fuels the general impression that chemistry is fully explained by quantum physics and has a negative effect on chemical education. Instead of starting from chemical facts, and the properties of the elements, the modern tendency is to expose students to the rules for electronic configurations in the belief that the chemistry will somehow follow.

Presumably as a corrective, Scerri provides a most interesting chapter on periodic systems that chemists have constructed in the 20th century with little or no reference to quantum theory or quantum mechanics. A chapter on the complex relation between quantum mechanics and the explanation of chemical periodicity follows, providing many anomalies that complicate a straightforward reduction. Indeed, Scerri characterizes the explanatory success of quantum mechanics for the periodic table as "something of a miracle." But he concludes his book with a positive nod to physics in a chapter on astrophysical theories of nucleosynthesis.

The classic predecessor of The Periodic Table is J. W. van Spronsen's 1969 book The Periodic System of Chemical Elements: A History of the First Hundred Years. The material covered is quite similar, but van Spronsen's account, much more iterative than Scerri's, provides more examples of the great diversity of periodic systems that have been constructed, particularly in the 20th century. Scerri, by contrast, is much more attuned to the kind of philosophical issues outlined above, and he gives a much deeper and more comprehensive study of the developments of 20th-century atomic physics and quantum mechanics.

The Periodic Table is written in a straightforward style. However, Scerri has an annoying tendency to extol his own virtue in uncovering hitherto ignored historical personages and data. Another minor caveat is that the name of the very important 19th-century French chemist Jean-Baptiste-André Dumas, although given correctly on first reference, is twice mis-styled in the text as "Alexandre Dumas" and appears in the index as "Dumas, André." Notwithstanding these imperfections, this book is a fine addition to the history and philosophy of chemistry, fields that Scerri himself has played an important role in developing.

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