Chemists' Ongoing Search for Order

THE PERIODIC TABLE: Into the 21st Century, edited by Dennis H. Rouvray and R. Bruce King, Research Studies Press, 2004, 396 pages, $68, £40 (ISBN: 0-86380-292-3)

If some universal catastrophe were to engulf the world, and humankind could retain one scientific concept in order to rebuild civilization, what would that concept be?

The periodic table encapsulates the concept of elements, organizes physical and chemical trends of substances, and compares the structure of the different atoms--an enormous amount of information in a small space.

But which periodic table? Ever since geologist Alexandre Émile Beguyer de Chancourtois' first listing of elements according to atomic weight, his "telluric screw" version of 1862, there have been dozens of periodic systems proposed. Most of these have evolved from the prototype tabular presentations of Dmitrii Mendeleev and Lothar Meyer in 1868-69.

And what are the underlying principles of the periodic table? Can we unite all the chemical, physical, and structural trends by a few simple scientific axioms? What are the reasons for all of the variations and exceptions to the periodic table as we know it today?

In an attempt to answer these questions, a scientific conference was organized in the summer of 2003 that resulted in a set of essays collected and published as "The Periodic Table: Into the 21st Century," edited by Dennis H. Rouvray and R. Bruce King, both chemistry professors at the University of Georgia. The 13 essays begin with the history of the periodic table and continue into discussions of new patterns and theories, leading to full discussions of the lanthanide and actinide elements, relativistic effects, quantum effects, and a search for "the" fundamental periodic system.

Other forms of the periodic table that have evolved over the decades include pyramidal or "spread-out" arrangements that emphasize electronic configurations but in which chemical relationships are lost. There have been compact tabular arrangements, for example, with zinc below magnesium, but these deemphasize the separateness of the transition elements.

There have been "information-overload" periodic tables published by scientific houses, which serve mainly as repositories of data. There are three-dimensional scrolls, towers, and trees that attempt to present all possible periodic relationships, which can be confusingly complex. There is the "left-step" or "Janet" periodic table that fixes helium with the alkaline earth group, to the delight of spectroscopists. There are "modified" tables with arrows showing special patterns, such as the "Knight's move," referring to a chess piece moving two squares horizontally and one square vertically. One example is cadmium to lead, both of which are poisonous.

And still there are additional trends not visible in any variation yet invented. These include geologist's groupings, such as magnesium and iron in basalts, or the discontinuities in physical properties, such as the boiling points of the oxygen group hydrides. It's all enough to make one wonder if the perceived relationships are forced.

If there are so many variations of the periodic table, doesn't this imply that its form is arbitrary with no deeper significance? One can almost sympathize with one critic, physicist George Foster, who asked sarcastically at a London Chemical Society meeting in 1866 that if perhaps an alphabetical order would be more meaningful! Indeed, at its inception, many people foretold a short life for the periodic table.

But the periodic table has survived, now adorning the walls of chemistry lecture halls everywhere, and will soon celebrate its sesquicentennial.

The current popular form has been a useful one, lending itself to easy modification. It has evolved to include a whole new group, the noble gases; a whole new system of ranking, by atomic number; and a rearrangement to explain the behavior of the transition-metal and rare-earth elements. The table has been simplified to accommodate isotopes, and it has been recognized to reflect the electronic structure of the atom itself. Obviously, one would think, there is an underlying order to this system and that chemists, either implicitly or explicitly, accept this underlying order with unshakable faith.

At first glance, each essay chapter in the book appears disjointed. But upon deeper inspection, this adds to the strength of the overall presentation by allowing a multifaceted approach to the study. For example, there are five chapters devoted to history, each with a different slant; some sections deal objectively with Mendeleev's specific accomplishments, and another discusses the Newtonian philosophy responsible for his ideas. Later chapters are quite mathematical and would be difficult for the average reader. But the range of difficulty and complexity is a plus for the book because the reader can go into any depth he or she wants.

Perhaps the best feature of the book is that so many questions are answered. Why do gold and mercury behave so strangely? Why should gold be so beautifully colored, and why should mercury, one of the heaviest metals, be liquid? It's all because of relativistic effects of these heavier elements.

Ever wonder how the tangled mess of identifying the radioactive elements was resolved? There was a flood of "different" elements discovered in the beginning of the 20th century as products of radioactive decay.

Why was Mendeleev so bold in his prognostications while Meyer was reticent? Did Mendeleev make any errors in his predictions? He made many mistakes, and some of them were real howlers.

Can analogs of the periodic table be extended to the subatomic world? How can group theory and quantum mechanics be used to explore the theoretical basis of the periodic table?

There is something useful in every chapter, and there are chapters for everyone--from the elementary school science teacher to the university theoretician.

So what are the deepest principles underlying the periodic table? As admitted by the authors, there is no "perfect" arrangement of the elements. This may be the most unsatisfying aspect of the discussion, that the elements of the periodic table can't be reduced to simple groupings the way physicists have done with the symmetrical families of subatomic particles.

The "finest periodic table of all may lie, like the abstract elements, in the realm of unobservables ... the platonic periodic system," notes one passage in the book. But the authors state it as they see it, and they must be admired for tackling a most formidable task to explain, as comprehensively as possible, the periodic classification of the elements with all of its meanings, trends, variations, and exceptions.

The book suffers from the low-quality reproduction of some figures and the lack of an index. These faults, while frustrating, are more than compensated for by the richness of the information. In one book, an interested reader can explore questions on a multitude of different levels regarding the periodic--or unperiodic--behavior of an element.

James L. Marshall is a chemistry professor at the University of North Texas. Marshall and his wife, Jenny, have collected samples of all elements up to uranium from around the world and charted a history of their discovery (www.jennymarshall.com/rediscovery.htm).