Mendeleev’s Legacy: The Periodic System

Mendeleev, date unknown. Courtesy of the Edgar Fahs Smith Collection, University of Pennsylvania Library

By Eric R. Scerri

This year marks the 100th anniversary of the death of one of the most famous scientists of all time, the Russian chemist Dmitri Ivanovich Mendeleev (1834-1907). The periodic table that he introduced in 1869 was a monumental achievement—a wonderful mnemonic and a tool that serves to organize the whole of chemistry. No longer were students of chemistry obliged to memorize the properties of all the known elements; hereafter they could learn the properties of at least one element from each column and could in principle, make sound predictions about the other elements in the column.

Arguably, however, Mendeleev’s greatest achievement was not the periodic table so much as the recognition of the periodic system on which it was based. Of the nearly 1,000 variations that have been published since, all are attempts to represent the fundamental rule that after certain but varying intervals, the chemical elements show an approximate repetition in their properties.

Mendeleev was hardly the first to arrive at a periodic system. The observation that certain types of elements prefer to combine with certain other types prompted early chemists to classify the elements in tables of chemical affinities. In 1817 the German chemist Johann Wolfgang Döbereiner noticed the existence of groupings of elements in threes, subsequently called triads. The elements in these groupings displayed an important numerical relationship to each other: the equivalent weight (an early substitute for atomic weight) of the middle element had the approximate mean of the values of the two flanking elements. Although Döbereiner worked with the rather crude approximations of atomic weight available at the time, he successfully identified four such groups: calcium, strontium, and barium; iodine, bromine, and chlorine; lithium, sodium, and potassium; and sulfur, selenium, and tellurium. His triads—which would eventually appear on the periodic table in vertical columns—represented the first step in fitting the elements into a system that would account for their chemical properties and reveal their physical relationships.

By the 1860s a number of scientists had moved beyond the triad concept to produce some very respectable periodic systems. The French geologist Alexandre-Émile Béguyer de Chancourtois achieved the first true periodic system in 1862 by arranging the elements by atomic weight in a spiral line wrapped around a metal cylinder. Periodic relationships could be seen by moving vertically down the screw. In 1863 and 1864 two British chemists, John New-lands and William Olding (both born in the same London borough of Southwark), independently published periodic tables that used atomic weight to arrange the elements into groups with analogous properties. A more eccentric spiral periodic system was created by the Danish-born polymath Gustavus Hinrichs in 1864. Hinrichs was intrigued that atomic spectral frequencies, like planetary distances, show whole number ratios, and he concluded that atomic spectra must therefore be an indication of atomic size.

The closest precursor to Mendeleev’s table in both chronological and philosophical terms was developed by Julius Lothar Meyer, a German chemist, in 1864. Although Meyer stressed physical rather than chemical properties, his table bears remarkable similarity to the one that Mendeleev would develop five years later. For a number of reasons, Meyer’s prominence in the history books never matched Mendeleev’s. There was an untimely delay in the publication of his most elaborate periodic table, and, perhaps more important, Meyer—unlike Mendeleev—hesitated to make predictions about unknown elements.

Notwithstanding these earlier scientists’ contributions to the idea of periodicity, Mendeleev remains the undisputed champion of the periodic system as a defender, propagator, and elaborater. Mendeleev’s version of the periodic table left the greatest impact on the scientific community, both at the time it was produced and thereafter. In the popular imagination the periodic system invariably and justifiably connects to his name, to the same extent that the theory of evolution connects to Darwin’s name and the theory of relativity to Einstein’s. But what really set Mendeleev’s contribution apart?

Simple Substances to Abstract Elements

By organizing the elements as he did, Mendeleev took a stand on the centuries-old question of the philosophical status of the elements. Unlike some of his contemporaries, Mendeleev rejected the suggestion that the periodic system implied the existence of any form of primary matter of which all the elements were composed. He maintained that all elements were strictly individual, indestructible, and irreducible, yet he acknowledged the seeming challenge posed by chemical reactions. Consider the familiar example of sodium chloride: common white table salt does not seem to include either poisonous grey metallic sodium or poisonous green gaseous chlorine.

Aristotle had maintained that all matter was composed of some combination of four abstract elements: earth, fire, water, and air. Although the four elements were themselves unobservable, their relative proportions within a specific substance governed its properties. Antoine-Laurent Lavoisier and his contemporaries challenged this view in the 18th century with new concepts of simple substances. They created a list of 37 simple substances that could be isolated from the decomposition of compounds and could not be further decomposed by any known means. More than any of the other discoverers of the periodic system, Mendeleev was concerned with the philosophical status of the elements. At the beginning of the first volume of his landmark Principles of Chemistry (first English translation, New York, 1891; Osnovy Khimii, first edition, Saint Petersburg, 1869), he wrote, “It is useful in this sense to make a clear distinction between the conception of an element as a separate homogenous substance and as a material but invisible part of a compound” (p. 23). For Mendeleev an element was an entity that was essentially unobservable but formed the inner essence of simple bodies. Whereas a particular element was to be regarded as unchanging, its corresponding simple-body aspect could take many forms, such as charcoal, diamond, and graphite in the case of carbon. His periodic table classified abstract elements, not simple substances.

Mendeleev’s genius lay in recognizing that just as it was the element in the abstract sense that survived intact in the course of compound formation, so atomic weight was the only quantity that survived in measurable amounts. He therefore took the step of associating these two features: an element was to be characterized by its atomic weight. In a sense an abstract element had acquired a single measurable attribute that would remain unchanged in all its chemical combinations. Here, then, was a profound justification for using atomic weight as the basis for the classification of the elements, unlike any of the precursors to the periodic system.

Isotopes

Toward the end of Mendeleev’s life a growing body of evidence began to challenge his conception of the nature of the elements. Several revolutionary discoveries in physics showed that atoms were, in fact, reducible and that there was a sense in which all elements are composed of the same primary matter: protons, neutrons, and electrons. Most alarmingly, there was even evidence to suggest that certain elements could be transformed into others through radioactivity.

In 1879 J. J. Thomson identified electrons as the particles constituting cathode rays. By repeating his experiments with cathode rays produced by different elements, he concluded that the same particle was produced in every case and that this particle was therefore a fundamental constituent of all matter. Shortly thereafter Henri Becquerel and the Curies began to explore the phenomenon of radioactivity. One of the most talented researchers attracted to the study of radioactivity was Ernest Rutherford, who suggested in 1902 that radioactive reactions had the power to transform certain elements into entirely different elements. While fully aware of the possible criticism that such a notion might bring, Rutherford and his colleague Frederick Soddy went so far as to describe this new phenomenon as chemical transmutation, thus evoking the age-old dream of the alchemists.