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That chart on the laboratory wall

Some of the basic substances In nature, which we call elements, have been known since early antiquity. But it is only during the past century that man has learned that there are about a hundred elements, and begun to understand how these are similar or different.

Guenrij Teterin and Claire Terlon

The work of Copernicus and Galileo had earlier brought order out of chaos in astronomy, Newton did much the same for mechanics, then Darwin and Pavlov for biology, while much later Bohr and Einstein were to make significant contributions to the physics of the atom. In chemistry, one of the great moments came in 1869 when the Russian researcher Dmitri Ivanovic Mendeleyev formulated the periodic law of the chemical elements.

The formulation of the periodic law marked the passing of the study of chemistry from almost medieval trial and error methods to a modern science capable of predicting elements not yet seen, heard, touched or smelled by man. The coherent arrangement of the elements by Mendeleyev crowned the efforts of men of science in many countries to discover a meaningful system in the properties of these basic substances.

Mendeleyev's idea was to be virtually a quantum jump from the simple table laid out In the eighteenth century by the French chemist Antoine Lavoisier who had Included, besides physical elements, what he called "imponderable fluids," such as light and the energy derived from heat. Although a far cry from Mendeleyev's rigorously logical approach, Lavoisier's effort succeeded in conditioning other scientists to reject the theory of phlogiston. This was the ancient chemical concept, dating from early Greek civilization, that fire in its various forms was a physical or material component of nature. Lavoisier's analysis was improved upon in 1803 by the British chemist John Dalton and his atomic theory which attributed a distinctive atomic "weight" to each of the 23 elements admitted by Lavoisier. Characteristics such as this and the "equivalent weight" concept of another English scientist, William Wollaston, were to pave the way for chemists later to perceive a cohesive order among all the elements found in nature. But until Mendeleyev's time, even the notions of what constituted an element were vague and subject to individual interpretation.

By 1850, an additional thirty elements had been recognized, bringing the total known to somewhat more than sixty. Tables of chemical order were offered by scientific thinkers such as Johann Döbereiner, Leopold Gmelin, E. Lennsen, Max von Pettenkofer, Jean-Baptiste Dumas, Willard Gibbs and John Gladstone to name but a few.

In 1817 Döbereiner's "triads," or groups of three, had been an attempt to correlate trios of elements according to neighbouring atomic weights. By 1852, Gmelin had adapted the triads to four-and five-element series (tetrads and pentads), ranged according to their atomic weights, in increasing order.

Among the researchers whom Dimitri Mendeleyev was later to single out as most influential on his work were the French scientists Dumas and Lennsen. Dumas' contribution had been a method to calculate the atomic weight of elements within a given group, whereas Lennsen's was a first attempt to interpolate the atomic weights of elements yet to be discovered.

During the 1860s, novel forms of classifying the elements were proposed. One of these was Alexandre Beguyer de Chancourtois' "telluric screw," a spiral arrangement wound round an imaginary cylinder. It strikes today's reader as curiously parallel to the "double helix" in modern genetic chemistry.

In another view held at that time by John Newlands of England, every eighth element in the ascending order of atomic weights repeated certain characteristics. This was called the theory of octaves, after the layout of the diatonic scale in western musical forms.

Probably the most provocative of the new ideas was that advanced by a German scientist, Lothar Meyer. In 1864 Meyer published a table listing forty-four of the sixty-two known elements, arranged according to their "valency" instead of atomic weight.

No longer of significance today, valence refers to the combining power of one atom of an element: in water, or H20, the valence of oxygen is two because one of its atoms can join with two others (in this case, two atoms of hydrogen). And the valence of hydrogen was the point of departure in Meyer's first table. A later table proposed by Meyer was to be based on atomic weights.

These endeavours and those of three scientists, William Odling, Gustavus Hinrichs and H. Baumhauer, were all steps in the right direction, even though a few men of learning considered them as little better than mental games. Some chemists viewed the correlation of properties of the elements grouped in triads, octaves or along a telluric spiral as nothing more than happenstance and therefore little better than superficial analogy. Even, as Newlands was presenting a paper before Britain's prestigious Chemical Society he was asked, ironically, if it would not be possible to obtain the same results by arranging the elements in their alphabetical order.

What, then, was Mendeleyev's theory all about? Briefly, he proposed arranging the elements in lines and columns (also called "periods" and "groups") inside a rectangle, with their atomic weights rising in number from left to right along the same line, one line following the other down the page.

The columns were determined by elements possessing analogous properties the same kind of combining oxide, for example. The fewest atoms of an element (R) combining with the fewest of oxygen (O) would appear in the first column, augmenting in combining proportions toward the seventh column.

Since only some sixty elements were then known, eight columns sufficed then and still do today. In fact, the lay-out of an entire system still in use was prescribed by Mendeleyev when only slightly more than half its components were known.

Mendeleyev knew from the outset that he had developed a scientific device to lay out the chemical elements in a convenient system. More than that, he realized he had discovered an objective, natural law. Yet just as Newton had been popularly said to understand universal gravitation when struck on the head by a falling apple (or Watt realized that a boiling kettle could be transformed into the steam engine), there were still those who thought that Mendeleyev had come upon the periodic law... as the result of a dream.

Man tends to overlook that while scientific truth may suddenly strike one man's mind as a flash of lightning, that same scientist may have spent years of arduous research on his subject. Indeed, it was Pasteur who later commented that "chance favours only the prepared mind." If we take a look at Mendeleyev's activities before 1869, it becomes fairly clear that the emergence of the periodic table was no mere accident.

Although Mendeleyev's table was considered by some scientists as simply another in an endless series of dubious hypotheses, one of its great merits was its boldness. The progress made in chemistry during the past century has proved Mendeleyev's theory to be correct on two additional counts. He said that new elements would be discovered to help fill the holes in the system he had devised, and that the atomic weights of some elements which did not fit into his table had been calculated erroneously.

In the latter case (concerning the atomic weights of the elements cerium, indium, titanium, uranium, yttrium and others), Mendeleyev was soon to be proved right as new research helped rectify weights which were incorrect. Where an element did not seem to fit (such as uranium with an atomic weight of 116), he speculated on the true value. He arbitrarily doubled, for instance, the weight of uranium to 232; today we know the actual weight of that element to be 238.04.

In the other and more important case the gaps in the periodic table Mendeleyev was to see three new elements identified and described within sixteen years of his epoch-making declaration before the assembly of Russian chemists. Among the elements he had foreseen are those at first called eka-aluminium, eka-boron and eka-silicon ("eka" means "one" in Sanskrit); they were later renamed to honour the countries where their discoveries had occurred.

Eka-aluminium, identified scientifically in 1875 by the Frenchman Paul-Emile Lecoq de Boisbaudran, became known as gallium (atomic weight 69.72). Gallium thus filled the "hole" in the table between aluminium and indium.

Eka-boron, which Mendeleyev had predicted would have an atomic weight midway between those of calcium and titanium (40 and 48), emerged in 1879 as scandium, honouring its Swedish discoverer Lars Frederick Nilson. The definitive atomic weight of this element, 44.956, was not established until 1955.

The third element, the so-called ekasilicon, became germanium on its discovery in 1886. Possessing an atomic weight of 72.59 and properties very close to those predicted by Mendeleyev, germanium was identified by Clemens Alexander Winkler, professor of chemistry at the Freiburg School of Mines, in Germany.

Besides being an excellent theorist, Mendeleyev proved to be a practical man. Before his death in 1907, he was to make chemical investigations of the oil fields at Baku in his native Russia and in Pennsylvania, U.S.A., as well as of the Caucasian springs of naphtha, a paraffin-like hydrocarbon mixture. (Hydrocarbons are chemical compounds consisting of carbon and hydrogen only.)

Long after his death, two more chemical elements whose existence was foretold by Mendeleyev were to be clearly identified. In 1925 the German husband-wife team of Walter and Ida Noddack isolated rhenium, which Mendeleyev had called bi-manganese.

A hard, grey metal often used in thermocouples, rhenium has an atomic weight of 186.20. Then, seventy years after Mendeleyev's discovery, Prof. Marguerite Perey, the distinguished French woman scientist, identified eka-cesium (renamed francium), at the Institut du Radium in Paris. Francium's atomic number is 87.

On the heels of the discovery of the periodic law of the elements came one of the scientifically sensational findings of the late 19th century, that of the "inert" gas argon. This was the work of both Sir William Ramsay and Lord Rayleigh. The former had suggested to Rayleigh in 1894, after careful experimentation, that "...there is room for gaseous elements at the end of the first column of the periodic table." The two chemists announced the new gas later at a meeting In Oxford. We now know .that argon and similar gases are not inert, but can combine with other elements.

Mendeleyev had not foreseen the inert gases simply because of their predominant "quality" of inactivity. Two years later (1896) Ramsay in England, and the Swedish chemist Per Theodor Cleve, working independently, discovered helium. The behaviour of helium (from the Greek word for "sun") had been observed spectroscopically for several years as one of the components of the solar atmosphere.

Based on Mendeleyev's reasoning, Ramsay was convinced that other similar gases existed. In 1898 he and Morris Travers identified three additional "Inert" gases neon, xenon and krypton. This family of elements came to form the "O" column of the periodic table.

In that same year, 1898, Pierre Curie and his Polish wife Maria Sklodowska identified the phenomenon of radioactivity, which shattered one of the foundations of Mendeleyev's law the invariability of the atom. Yet Mendeleyev could see no anomaly between his law and the existence of radioactive elements when he visited the Curies' Parisian laboratory in 1902.

Ten years later and after Mendeleyev's death, however, when the number of radioactive elements had risen to 37, scientists had some doubts about the adaptability of the periodic system. They wondered if Mendeleyev's arrangement could be considered valid if, as it appeared, there were no room on his table for the newly discovered elements.

It was in 1913, on the eve of the world's eruption into general war, that another modification clearly became necessary in the relationship between an element's structure and its position on the periodic table. Henry Moseley, a 25-years-old British physicist, had analyzed the X-ray spectra of fifty-one elements. He observed that there was a relation between an element's atomic number and the frequency of the X-rays it emits if bombarded by cathode rays. (An element's atomic number refers to how many electrons rotate about the nucleus of one of its atoms.)

As a result of the work of this brilliant young man (killed while on active service at the Dardanelles in 1915), seven more unfilled spaces on the periodic table were to gain new occupants. Besides the rhenium and francium already cited, these new elements were technetium, promethium, hafnium, astatine and protactinium. The uncovering of these new elements, found in effect by a technique, which Mendeleyev could not have known, in no way disrupted the original disposition of the elements on his table.

Shortly after Moseley's discovery, another Englishman, Frederick Soddy, introduced the notion of isotopes (from the Greek for "same places"). The isotopes of an element possess all the same chemical properties. Their physical properties are identical In every respect except that of the weight or "mass" of the atom. Most of the mass is situated in the atom's nucleus, consisting of the proton carrying a positive electrical charge and the electrically neutral neutron. (The negatively charged "cloud" of planetary electrons consists of particles each having a mass of about 1/1,836 the mass of the proton.)

Soddy demonstrated, as did the Polish chemist Kasimir Fajans in that same year, 1913, that as an element disintegrates through radioactivity, the new position it occupies on the periodic chart depends on the kind of radiation it gives off. Alpha-disintegration (decay through the loss of alpha-particles) moves an element two spaces to the left on the table, whereas beta-disintegration of electron emission moves the element one space to the right.

It is true that there were some anomalies in the method of aligning the elements as at first suggested by Mendeleyev. In a few cases he had listed slightly heavier elements before somewhat lighter ones. Tellurium (127.6) was placed before iodine (126.9), cobalt (58.9) ahead of nickel (58.7), and thorium (232.0) came before protactinium (231.0).

After the standard of atomic numbers had been adopted, it was observed that if the elements were classified according to their nuclear electrical charges, their positions on the periodic table were just as Mendeleyev had predicted. It is In this form that chemistry students today are familiar with "that chart on the laboratory wall."

Read also

Dmitry Mendeleev or the teachings of a prophet, UNESCO Courier, April-June 2019

Mendeleev’s periodic tableUNESCO Courier, January-March 2011

Dmitry Mendeleev, the man who brought law and order to chemistryUNESCO Courier, June 1971

Guenrij Teterin

He is head of the Scientific Laboratory at Odessa State University, in the Ukraine, where he has worked in the fields of physical and electrochemistry. He has also carried out research at Rensselaer Polytechnic Institute, in the U.S.A., under the U.S.S.R.-U.S.A. scientific exchange programme. Dr. Teterin worked with Unesco's Division of Science Teaching. He has written widely on physical chemistry and is the author of many popular science articles.

Claire Terlon

She is a specialist in physics as well as psychology. As a member of the Institut Pédagogique National in Paris, she is responsible for the preparation of special television refresher courses for chemistry and physics teachers in French lycées. She is a regular contributor to the French science magazine "La Recherche-Atomes".