The Seven Last Infra-Uranium Elements to Be Discovered

The term “infra-uranium,” meaning before uranium, is one that I have proposed by contrast to the better-known term transuranium elements that are discussed in the following chapter. The present chapter concerns the last seven elements that formed the missing gaps in the old periodic table that ended with the element uranium. After Moseley developed his X-ray method, it became clear that there were just seven elements yet to be isolated among the 92 naturally occurring elements or hydrogen (#1) to uranium (#92). This apparent simplicity is somewhat spoiled by the fact that, as it turned out, some of these seven elements were first isolated from natural sources following their being artificially created, but this raises more issues that are best left to the next chapter of this book. The fact remains that five of these seven elements are radioactive, the two exceptions being hafnium and rhenium, the second and third of them to be isolated. The first of the seven final infra-uranium elements to be discovered was protactinium, and it was one of the lesser-known predictions made by Mendeleev. In his famous 1896 paper, Mendeleev indicated incorrect values for both thorium (118) and uranium (116). (See figure 1.6.) A couple of years later, he corrected both of these values and showed a missing element between thorium and uranium (figure 4.4). In doing so, Mendeleev added the following paragraph, in which he made some 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 R2O5, 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 229.2. The apparent inaccuracy in Mendeleev’s prediction is not too surprising, however, since he never knew that protactinium is a member of only four “pair reversals” in the entire periodic table.

Astrophysics and Nucleosynthesis

Having now examined attempts to explain the nature of the elements and the periodic system in a theoretical manner, it is necessary to backtrack a little in order to pick up a number of important issues not yet addressed. As in the preceding chapters, several contributions from fields outside of chemistry are encountered, and the treatment proceeds historically. So far in this book, the elements have been treated as if they have always existed, fully formed. Nothing has yet been said about how the elements have evolved or about the relative abundance of the isotopes of the elements. These questions form the contents of this chapter. It also emerges that different isotopes show different stabilities, a feature that can be explained to a considerable extent by appeal to theories from nuclear physics. The study of nucleosynthesis, and especially the development of this field, is intimately connected to the development of the field of cosmology as a branch of physical science. In a number of instances, different cosmological theories have been judged according to the degree to which they could explain the observed universal abundances of the various elements. Perhaps the most controversial cosmological debate has been over the rival theories of the big bang and the steady-state models of the universe. The proponents of these theories frequently appealed to relative abundance data, and indeed, the eventual capitulation of the steady-state theorists, or at least some of them, was crucially dependent upon the observed ratio of hydrogen to helium in the universe. Chapters 2, 3, and 6 discussed Prout’s hypothesis, according to which all the elements are essentially made out of hydrogen. Although the hypothesis was initially rejected on the basis of accurate atomic weight determinations, it underwent a revival in the twentieth century. As mentioned in chapter 6, the discoveries of Anton van den Broek, Henry Moseley, and others showed that there is a sense in which all elements are indeed composites of hydrogen.

Quantum Mechanics and the Periodic Table

In chapter 7, the influence of the old quantum theory on the periodic system was considered. Although the development of this theory provided a way of reexpressing the periodic table in terms of the number of outer-shell electrons, it did not yield anything essentially new to the understanding of chemistry. Indeed, in several cases, chemists such as Irving Langmuir, J.D. Main Smith, and Charles Bury were able to go further than physicists in assigning electronic configurations, as described in chapter 8, because they were more familiar with the chemical properties of individual elements. Moreover, despite the rhetoric in favor of quantum mechanics that was propagated by Niels Bohr and others, the discovery that hafnium was a transition metal and not a rare earth was not made deductively from the quantum theory. It was essentially a chemical fact that was accommodated in terms of the quantum mechanical understanding of the periodic table. The old quantum theory was quantitatively impotent in the context of the periodic table since it was not possible to even set up the necessary equations to begin to obtain solutions for the atoms with more than one electron. An explanation could be given for the periodic table in terms of numbers of electrons in the outer shells of atoms, but generally only after the fact. But when it came to trying to predict quantitative aspects of atoms, such as the ground-state energy of the helium atom, the old quantum theory was quite hopeless. As one physicist stated, “We should not be surprised . . . even the astronomers have not yet satisfactorily solved the three-body problem in spite of efforts over the centuries.” A succession of the best minds in physics, including Hendrik Kramers, Werner Heisenberg, and Arnold Sommerfeld, made strenuous attempts to calculate the spectrum of helium but to no avail. It was only following the introduction of the Pauli exclusion principle and the development of the new quantum mechanics that Heisenberg succeeded where everyone else had failed.

The Periodic System

In ancient Greek times, philosophers recognized just four elements—earth, water, air, and fire—all of which survive in the astrological classification of the 12 signs of the zodiac. At least some of these philosophers believed that these different elements consisted of microscopic components with differing shapes and that this explained the various properties of the elements. These shapes or structures were believed to be in the form of Platonic solids (figure 1.1) made up entirely of the same two-dimensional shape. The Greeks believed that earth consisted of microscopic cubic particles, which explained why it was difficult to move earth. Meanwhile, the liquidity of water was explained by an appeal to the smoother shape possessed by the icosahedron, while fire was said to be painful to the touch because it consisted of the sharp particles in the form of tetrahedra. Air was thought to consist of octahedra since that was the only remaining Platonic solid. A little later, a fifth Platonic solid, the dodecahedron, was discovered, and this led to the proposal that there might be a fifth element or “quintessence,” which also became known as ether. Although the notion that elements are made up of Platonic solids is regarded as incorrect from a modern point of view, it is the origin of the very fruitful notion that macroscopic properties of substances are governed by the structures of the microscopic components of which they are comprised. These “elements” survived well into the Middle Ages and beyond, augmented with a few others discovered by the alchemists, the precursors of modern-day chemists. One of the many goals of the alchemists seems to have been the transmutation of elements. Not surprisingly, perhaps, the particular transmutation that most enticed them was the attempt to change the base metal lead into the noble metal gold, whose unusual color, rarity, and chemical inertness have made it one of the most treasured substances since the dawn of civilization.

Prediction and Accommodation: The Acceptance of Mendeleev’s Periodic System

Although periodic systems were produced independently by six codiscoverers in the space of a decade, Dmitri Mendeleev’s system is the one that has had the greatest impact by far. Not only was Mendeleev’s system more complete than the others, but he also worked much harder and longer for its acceptance. He also went much further than the other codiscoverers in publicly demonstrating the validity of his system by using it to predict the existence of a number of hitherto unknown elements. According to the popular story, it was Mendeleev’s many successful predictions that were directly responsible for the widespread acceptance of the periodic system, while his competitors either failed to make predictions or did so in a rather feeble manner. Several of his predictions were indeed widely celebrated, especially those of the elements germanium, gallium, and scandium, and many historians have argued that it was such spectacular feats that assured the acceptance of Mendeleev’s periodic system by the scientific community. The notion that scientific theories are accepted primarily if they make successful predictions seems to be rather well ingrained into scientific culture, and the history of the periodic table has been one of the episodes through which this notion has been propagated. However, philosophers and some scientists have long debated the extent to which predictions influence the acceptance of scientific theories, and it is by no means a foregone conclusion that successful predictions are more telling than other factors. In looking closely at the bulk of Mendeleev’s predictions in this chapter, it becomes clear that, at best, only half of them proved to be correct. This raises a number of questions. First of all, why is it that history has been so kind to Mendeleev as a maker of predictions? As historian of chemistry William Brock has pointed out, “Not all of Mendeleev’s predictions had such a happy outcome; like astrologers’ failures, they are commonly forgotten.”

Mendeleev

Dmitri Ivanovich Mendeleev is the undisputed champion of the periodic system in at least two senses. First of all, he is by far the leading discoverer of the system. Although he was not the first to develop a periodic system, his version is the one that created the biggest impact on the scientific community at the time it was introduced and thereafter. His name is invariably and justifiably connected with the periodic system, to the same extent perhaps as Darwin’s name is synonymous with the theory of evolution and Einstein’s with the theory of relativity. Although it may be possible to quibble about certain priority aspects of his contributions, there is no denying that Mendeleev was also the champion of the periodic system in the literal sense of propagating the system, defending its validity, and devoting time to its elaboration. As discussed in chapter 3, there were others who produced significant work on the system, but many of them, such as Alexandre-Émile Béguyer De Chancourtois, William Odling, and Gustavus Hinrichs, moved on to other scientific endeavors. After publishing their initial ideas, these contributors devoted their attention to other fields and never seriously returned to the periodic system to examine its full consequences to the extent that Mendeleev did. This is not to suggest that Mendeleev himself worked only on the periodic system. He is also known for many other scientific contributions, as well as for working in several applied fields, such as the Russian oil industry and as the director of the Russian institute for weights and measures. But the periodic system remained Mendeleev’s pride and joy throughout his adult life. Even toward the end of his life he published an intriguing essay in which he returned to the periodic system and, among other speculations, attempted to place the physicist’s ether within the periodic system as a chemical element. Much has been written on Mendeleev, and it would be impossible to do justice to his contributions in the space of a few pages.

Synthetic Elements

The periodic table consists of about 90 elements that occur naturally ending with element 92 uranium. This lack of precision is deliberate since one or two elements such as technetium were first created artificially and only later found to occur naturally on earth. This kind of occurrence provides a foreshadowing of things to come when we begin to discuss the transuranium elements, meaning those beyond uranium that have been artificially synthesized. Chemists and physicists have succeeded in synthesizing some of the elements that were missing between hydrogen (1) and uranium (92). In addition, they have synthesized a further 25, or so, new elements beyond uranium, although, again, one or two of these elements, like neptunium and plutonium, were later found to exist naturally in exceedingly small amounts. The existence of superheavy elements raises a number of interesting questions that pertain to the field of philosophy of science and also sociology of science. In fact, the very question of whether these superheavy elements actually exist needs to be dissected further, as it will be in the course of this chapter. The synthetic elements are extremely unstable, and only the lightest ones among them have been created in amounts large enough to be observed. Roughly speaking, the heavier the atom, the shorter its lifetime is. For example, the heaviest element for which there is now conclusive evidence is element 118, a few atoms of which have been created in just one single isotope form and with a half-life of less than a millisecond. Laypersons and specialists alike have asked themselves in what sense these elements can really be said to exist. The superheavy elements also have philosophical implications for the study of the periodic system as a whole and the question of whether there is a natural end to chemical periodicity. A related question, which has now become quite pressing, is the possible extension of the periodic table to include a new g-block which in formal terms should begin at element 121.

The Electron and Chemical Periodicity

J.J. Thomson’s discovery of the electron is one of the most celebrated events in the history of physics. What is not so well known is that Thomson had a deep interest in chemistry, which, among other things, motivated him to put forward the first explanation for the periodic table of elements in terms of electrons. Today, it is still generally believed that the electron holds the key to explaining the existence of the periodic table and the form it takes. This explanation has undergone a number of subtle changes. The extent to which the modern explanation is purely deductive or whether it is semiempirical is examined in this chapter. While Dmitri Mendeleev had remained strongly opposed to any attempts to reduce, or explain, the periodic table in terms of atomic structure, Julius Lothar Meyer was not so averse to reduction of the periodic system. The latter strongly believed in the existence of primary matter and also supported William Prout’s hypothesis. Lothar Meyer did not hesitate to draw curves through the numerical properties of atoms, whereas Mendeleev believed this to be a mistake, since it conflicted with his own belief in the individuality of the elements. This is how matters stood before the discovery of the electron, three years prior to the turn of the twentieth century. The atom’s existence was still very much a matter of dispute, and its substructure had not yet been discovered. There appeared to be no way of explaining the periodic system theoretically. Johnston Stoney first proposed the existence and name for the electron in 1891, although he did not believe that it existed as a free particle. Several researchers discovered the physical electron, including Emil Wiechert in Königsberg, who was the first to publish his findings. Because these early researchers did not seriously follow up on their results, it was left to the British physicist Thomson to capitalize upon and establish the initial observations.

Discoverers of the Periodic System

The periodic system was not discovered by Dmitri Mendeleev alone, as is commonly thought, or even just by Mendeleev and Julius Lothar Meyer. It was discovered by as many as five or six individuals at about the same time, in the decade of the 1860s, following the rationalization of atomic weights at the Karlsruhe conference. It became apparent by the middle of the nineteenth century that something needed to be done to resolve the widespread confusion over equivalent and atomic weights. Amedeo Avogadro had already proposed a solution to Gay-Lussac’s law that preserved John Dalton’s indivisible elemental particles. Recall that Gay-Lussac had observed that volumes of gases entering into chemical combination and their gaseous products are in a ratio of small integers. Dalton had refused to accept this viewpoint because it implied that atoms appeared to divide in some instances, such as the combination of hydrogen and oxygen to create steam. Avogadro had suggested that such “atoms” must be diatomic; that is, in their most elemental form they must be double. Thus, the oxygen atom was not dividing; rather, it was an oxygen molecule, which consisted of two oxygen atoms, that was coming apart. Unfortunately, the terms in which Avogadro expressed his views were rather obscure and failed to make much impression on the chemists of the day. Two exceptions were the French physicist and chemist André Ampère and the Alsatian chemist Charles Gerhardt, both of whom adopted the view that elemental gases were composed of diatomic molecules. One consequence of the general refusal to recognize the existence of diatomic molecules as the ultimate “atoms” of gaseous elements was that, as mentioned in chapter 2, the confusion between equivalent weights and atomic weights continued to reign. Although the relative weights of oxygen to hydrogen in water are approximately 8 to 1, the relative weight of the oxygen atom to the hydrogen atom takes on values of 8 or 16 depending on what one considers the correct formula for water to be.

Quantitative Relationships among the Elements and the Origins of the Periodic Table

Elements within a vertical group on the periodic table share certain chemical similarities, but the modern periodic system is not derived purely from descriptive characteristics. If chemical similarities were the sole basis for their classification, there would be many cases where the order and placement of the elements would be ambiguous. The development of the modern periodic system began when it was recognized that there are precise numerical relationships among the elements. Its subsequent evolution has also involved contributions from physics, as described in subsequent chapters. But whereas the latter contributions drew on fundamental physical theories, the ones that are examined in this chapter do not share this aspect. Instead, they involved looking for patterns among the numerical properties, such as equivalent weight or atomic weight, associated with each element. Throughout its history, the development of the periodic table has required a delicate interplay between two contrasting approaches: discerning quantitative physical data, on one hand, and observing qualitative similarities among the elements as a form of natural history, on the other. Both approaches are essential, and the balance that has been struck between them has been of crucial importance at various stages in our story. Whereas attention to qualitative aspects has always been an essential part of chemistry, the use of quantitative data has been a relatively new addition. The time when chemists began to pay attention to quantitative aspects of chemical reactions and chemical substances has been the source of much debate among historians. The traditional view has been that this step was taken by Antoine Lavoisier , who is regarded as the founder of modern chemistry. The more recent historical account is that Lavoisier made few original contributions and that much of his fame lay in his abilities as an organizer and presenter of chemical knowledge. Nevertheless, Lavoisier was able to dispel some of the vagueness and confusion that dogged the field of chemistry as he found it.