From Dalton to the Discovery of the Periodic System

Our story begins, somewhat arbitrarily, in the English city of Manchester around the turn of the nineteenth century. There, a child prodigy by the name of John Dalton, at the tender age of fifteen is teaching in a school with his older brother. Within a few years, John Dalton’s interests have developed to encompass meteorology, physics, and chemistry. Among the questions that puzzle him is why the various component gases in the air such as oxygen, nitrogen, and carbon dioxide do not separate from each other. Why does the mixture of gases in the air remain as a homogeneous mixture? As a result of pursuing this question, Dalton develops what is to become modern atomic theory. The ultimate constituents of all substances, he supposes, are hard microscopic spheres or atoms that were first discussed by the ancient Greek philosophers and taken up again by modern scientists like Newton, Gassendi, and Boscovich. But Dalton goes a good deal further than all of these thinkers in establishing one all-important quantitative characteristic for each kind of atom, namely its weight. This he does by considering quantitative data on chemical experiments. For example, he finds that the ratio for the weight in which hydrogen and oxygen combine together is one to eight. Dalton assumes that water consists of one atom of each of these two elements. He takes a hydrogen atom to have a weight of 1 unit and therefore reasons that oxygen must have a weight of 8 units. Similarly, he deduces the weights for a number of other atoms and even molecules as we now call them. For the first time the elements acquire a quantitative property, by means of which they may be compared. This feature will eventually lead to an accurate classification of all the elements in the form of the periodic system, but this is yet to come. Before that can happen the notion of atoms provokes tremendous debates and disagreements among the experts of Dalton’s day.

Element 61—Promethium

The last of our seven elements to be isolated was element 61, which is also the only rare earth among the seven. The problem with rare earths, which are 15 or even 17 in number depending on precisely how they are counted, is that they are extremely similar to each other and as a result are very difficult to separate. When the periodic table was first discovered in the 1860s only two or three rare earths even existed. As more of them turned up it became increasingly difficult to place them in the periodic system. Just like with all the other seven elements in our story, there were many false claims to its discovery. Moreover, the early claims must have seemed very plausible at the time because they appeared to draw support from X-ray evidence and Moseley’s law. Just like the priority dispute involving hafnium that took place in the early 1920s, the case of element 61 also involved an international controversy. This time one cannot entirely blame the aftermath of the Great War, as the two opponents consisted of Italians and Americans, with much of the scientific chicanery taking place, as was usual for the time, in the pages of London’s Nature magazine. But even though both sides of the priority dispute appealed to X-ray data and Moseley’s law, it turned out that neither side was right. In their own way, each side was working in complete delusion, since element 61 is highly radioactive and unstable, does not occur naturally on Earth, and could only be isolated in minute quantities by artificial means when such methods became sufficiently developed in the 1940s. Let us start at the beginning. In 1902, the Bohemian rare earth chemist Bohuslav Brauner was the first to suggest that an element lying precisely between neodymium and samarium remained to be discovered. He gave talks in his native Bohemia and published articles in some fairly obscure journals, all of which meant that few chemists in the wider arena became aware of his work.

Element 85—Astatine

The story surrounding element 85 is one of the most complex and interesting among our seven elements. The various claims for its discovery reveal many of the nationalistic traits that we have seen in the case of other elements, most notably the controversy surrounding the discovery of hafnium, element 72. But element 85 gives our study a greater depth than has yet been revealed by the already covered elements. What this story shows is that the nationalistic prejudices persist to this day in many respects and that the identity of the “discoverer” of the element very much depends on the nationality of the textbook that one might consult. It is also an element for which the majority of sources give an incorrect account in declaring Corson, MacKenzie, and Segrè as the true discoverers. The account I will detail owes much to the recent work of two young chemists, Brett Thornton and Shawn Burdette, whose 2010 article I have drawn heavily from. As in the case of many of the seven elements already surveyed, the view that Moseley’s experimental demonstration of the concept of atomic number resolved all issues in a categorical fashion is once again shown to be highly misleading. The position of element 85 in the periodic table shows it to lie among the halogens. Not surprisingly, therefore, the early researchers believed that they would find the element in similar locations to other halogens such as bromine and iodine, namely in the oceans or in sands washed up by oceans. Moreover, it was fully expected that the new element would behave like a typical halogen to form diatomic molecules and that it would have a low boiling point. The first major claim for the discovery of the element was made by Fred Allison, the same researcher who also erroneously claimed that he had discovered element 87. And just as in the case of element 87, Allison claimed to have found the new element using his own magneto-optical method, involving a time delay in the Faraday effect, which is to say the rotation of plane polarized light carried out by the application of a magnetic field to any particular solution of a substance.

Element 72—Hafnium

The story concerning the discovery and isolation of element 72 bears all the characteristics of controversy and nationalistic overtones that seems to characterize many of our seven elements. On one hand, it seems odd that there should be so much controversy associated with these elements given that Moseley’s method had apparently provided an unequivocal means through which elements could be identified as well as a way of knowing just how many elements remained to be discovered. On the other hand, perhaps it was precisely because the problem of the missing elements became so clearly focused on a few elements, with known atomic numbers, that the stakes became higher than they would have been if the number of elements remaining to be discovered had been uncertain, as they were in pre-Moseley times. Element 72 (fig. 4.1) was clearly anticipated, although not as such, even in Mendeleev’s earliest table of 1869. As fig. 4.2 shows, Mendeleev considered that an as yet undiscovered element with an atomic weight of 180 should be a homologue of zirconium (The modern accepted value is 178.50). This fact may not seem very significant and yet we will see, as the story of this chapter unfolds, that it amounts to Mendeleev predicting that this element would be a transition metal rather than a rare earth. But Mendeleev was not really in a position to make such a statement since the nature and number of rare earth elements was unknown in his day. Indeed, the problem of the rare earths was one of the most acute challenges to his periodic system and one that he personally never resolved. Sometime later, Julius Thomsen, a chemistry professor at the University of Copenhagen and incidentally the chemistry instructor to the physicist Niels Bohr, published a periodic table in which he too included a missing element that was a homologue of zirconium (fi g. 4.3). Suffice it to say that there was a general consensus among chemists that on the basis of the periodic table there should exist an element before tantalum that would be a homologue of zirconium.

Element 43—Technetium

Element 43 (fig. 6.1) holds a special distinction among the seven elements of this book. It was one of just four elements that Mendeleev first predicted in his famous table and article of 1871. This fact is not so well known, as most accounts mention just the three famous predictions, namely empty spaces to which Mendeleev gave atomic weights of 44, 68, and 72. These three elements were all discovered within a period of fifteen years and named scandium, gallium, and germanium, respectively. But in the same early table, Mendeleev assigned an atomic weight to just one more empty space, which he placed below manganese. Mendeleev predicted that it would have an atomic weight of 100, although he changed it slightly to 99 in his book, The Principles of Chemistry . Given the success of Mendeleev’s first three predictions it is hardly surprising that strenuous efforts were made, in many parts of the world, to find the fourth element. Little did these early chemists know the problems they would encounter in trying to isolate this particularly rare and unstable element. In the early twentieth century, several claims were made for the discovery of the element. But these alleged elements, given various names such as davyum, illenium, lucium, and nipponium all turned out to be spurious. Then, in 1925, as mentioned in the last chapter, Otto Berg, Walter Noddack, and Ida Tacke (later Ida Noddack), claimed to have discovered not just one but two new members of group 7, which they named masurium and rhenium. Although their discovery of rhenium was accepted, their claim for the element directly below manganese has been bitterly disputed ever since. The official discovery of element 43 is accorded to Emilio Segrè and coworkers. Technetium, as they eventually called it, had to be synthesized rather than isolated from naturally occurring sources. It is also the only element to ever be “discovered” in Italy—in Palermo, Sicily, to be more precise. Segrè , who had been a visitor at the Berkeley cyclotron facility in California, was sent some molybdenum plates that had been irradiated for several months with a deuterium beam. Various chemical analyses by the Italian team revealed a new element, which could be extracted by boiling with sodium hydroxide that also contained a small amount of hydrogen peroxide.

Element 75—Rhenium

Th e element rhenium lies two places below manganese in group VII of the periodic table (fi g. 5.1). Its existence was predicted by Mendeleev when he first proposed his periodic table in 1869. This group is rather unique because when the periodic table was first published, it possessed only one known element, manganese, with at least two gaps below it. Th e first gap was eventually filled by element 43, technetium, while the second gap was filled by rhenium. But rhenium was the first to be discovered, in 1925, by Walter Noddack and Ida Tacke (later Noddack) (fi g. 5.2) and Otto Berg in Germany. In the course of an arduously long extraction, they of the ore molybdenite. The German discoverers called their element “rhenium” after Rhenus, Latin for the river Rhine, which fl owed close to the place where they were working. They also believed that they had isolated the other element missing from group 7, or element 43, which eventually became known as technetium, but this was hotly disputed by several other researchers. As recently as the early years of the twenty-first century, research teams from Belgium and the United States reanalyzed the X-ray evidence from the Noddacks and argued that they had in fact isolated element 43. But these further claims have been debated by many radiochemists and physicists and now have been laid to rest, at least for the time being. By a further odd twist of fate, the Japanese chemist Masataka Ogawa believed that he had isolated element 43 and called it nipponium even earlier, in 1908. His claim too was discredited at the time but as recently as 2004 it has been argued that he had in fact isolated rhenium rather than element 43, well before the Noddacks and Berg. Otto Hahn’s first entry into the fi eld of radioactivity was as a student of Ramsay’s at University College, London, just after the beginning of the twentieth century.

The Invasion of the Periodic Table by Physics

Although John Dalton had reintroduced the notion of atoms to science, many debates followed among chemists, most of whom refused to accept that atoms existed literally. One of these skeptical chemists was Mendeleev, but as we saw in the previous chapter this does not seem to have prevented him from publishing the most successful periodic system of all those proposed at the time. Following the work of physicists like Einstein and Perrin, the atom’s reality became more and more firmly established starting at the turn of the twentieth century. Einstein’s 1905 paper on Brownian motion, using statistical methods, provided conclusive theoretical justification for the existence of atoms but lacked experimental support. The latter was soon provided by the French experimental physicist Jean Perrin. This work led in turn to many lines of research aimed at exploring the structure of the atom, and many developments that were to have a big influence on attempts to understand the periodic system theoretically. In this chapter we consider some of this atomic research as well as several other key discoveries in twentieth-century physics that contributed to what might be called the invasion of the periodic table by physics. The discovery of the electron, the first hint that the atom had a substructure, came in 1897 at the hands of the legendary J. J. Thomson, working at the Cavendish laboratory in Cambridge. A little earlier, in 1895, Wilhelm Conrad Röntgen had discovered X-rays in Würzburg, Germany. These new rays would soon be put to very good use by Henry Moseley, a young physicist working first in Manchester and, for the remainder of his short scientific life, in Oxford. Just a year after Röntgen had described his X-rays, Henri Becquerel in Paris discovered the enormously important phenomenon of radioactivity, whereby certain atoms break up spontaneously while emitting a number of different, new kinds of rays. The term “radioactivity” was actually coined by the Polish-born Marie Slodowska (later Curie).

Element 87—Francium

One of the most remarkable things about element 87 is the number of times that people claimed to have discovered it after it was predicted by Mendeleev in 1871 and given the provisional name of eka-caesium . It was recognized early on that the periodic table more or less fizzles out after element 83, or bismuth. All subsequent elements are radioactive and therefore unstable, with a few exceptions like uranium and thorium. But this fact did not deter a number of scientists from searching for element 87 among natural sources and in many cases from claiming to have isolated it. For example, Druce and Loring in England thought they had identified the element by using the classic method developed by Moseley for measuring the K α and K β lines of any element’s X-ray spectrum. But it was not to be. In the 1930s, it was the turn of Professor Fred Allison from the Alabama Polytechnic Institute (now Auburn University). Allison developed what he called a magneto-optical method for detecting elements and compounds based on a supposed time lag in the development of the Faraday effect, whereby the application of a magnetic field causes a beam of polarized light passing through a liquid solution to be rotated. Allison mistakenly thought that every element gave a particular time lag, which he claimed was observed with the naked eye, and that this effect could be used to identify each substance. He boldly claimed in a number of journal articles, and even a special feature in Time Magazine, that he had observed elements 87 and also 85, both of which were still missing at the time. Literally hundreds of papers were published on this effect, including a number of studies arguing that it was spurious. But these days the Allison effect is often featured in accounts of pathological science, alongside the claims for N-rays and cold fusion.

From Missing Elements to Synthetic Elements

The periodic table consists of about ninety elements that occur naturally, ending with element 92, uranium. One or two of the first ninety-two elements are variously reported either as not occurring on Earth or as occurring in miniscule amounts. To add to the complications in drawing a sharp line between natural and synthetic elements, the element technetium was first created artificially and only later found to occur naturally on Earth in minute amounts. As we have seen in previous chapters, chemists and physicists have succeeded in synthesizing some of the elements that were missing between hydrogen (1) and uranium (92), such as promethium and astatine. But in addition, a further twenty-five or so new elements beyond uranium have been synthesized, although again one or two of these, such as neptunium and plutonium, were later found to exist naturally in exceedingly small amounts. At the time of writing, the heaviest element for which there is good experimental evidence is element-118. All other elements between 92 and 118 have also been successfully synthesized including element-117, which was announced in April of 2010. The synthesis of this element means that for the first time, and probably the last, every single space in a contemporary periodic table has been filled, although some of these elements are still awaiting official ratification. The synthesis of any element involves starting with a particular nucleus and subjecting it to bombardment with small particles with the aim of increasing the atomic number and hence changing the identity of the nucleus in question. More recently, the method of synthesis has changed so that two nuclei of considerable weights are made to collide with the aim of forming a larger and heavier nucleus. In a sense in which all these syntheses are descended from a key experiment, conducted by Rutherford and Soddy in 1919 at the University of Manchester, Rutherford and Soddy bombarded nuclei of nitrogen with α particles (helium ions) with the result that the nitrogen nucleus was transformed into that of another element.

Element 91—Protactinium

The first of our seven elements, protactinium, was one of the many elements correctly predicted by Mendeleev even in his early publications. This is not true of the famous 1896 paper, where Mendeleev used incorrect values for both thorium (118) and uranium (116). A mere two years later, in 1871, Mendeleev corrected both of these values and indicated a missing element between thorium and uranium. But Mendeleev did not just indicate the presence of a missing element; he added the following brief paragraph in which he ventured to make more specific predictions: . . . Between thorium and uranium in this series we can further expect an element with an atomic weight of about 235. This element should form a highest oxide R 2 O5, like Nb and Ta to which it should be analogous. Perhaps in the minerals which contain these elements a certain amount of weak acid formed from this metal will also be found. . . . The modern atomic weight for eka-tantalum or protactinium is in fact 229.2. Mendeleev was somewhat unlucky regarding this case since he was not to know that protactinium is a member of only five “pair reversals” in the entire periodic table. This situation occurs when two elements need to be reversed, contrary to their atomic weights, in order to classify them correctly. The most clear-cut case of this effect was that of tellurium and iodine, as discussed in chapters 1 and 2. It was not until the work of Moseley in 1914 that a clear understanding of the problem was obtained. As Moseley showed, the more correct ordering principle for the elements is atomic number and not atomic weight. The justification for placing tellurium before iodine, as demanded by their chemical properties, is that tellurium has a lower atomic number. Returning to protactinium, it appears that Mendeleev’s brief predictions were broadly fulfilled since the element does indeed show an analogy with tantalum in forming Pa2 O5 as its highest and most stable oxide.