by Isaac Asimov
However, it occurred to the German physicist Max Theodore Felix von Laue that crystals are a natural diffraction grating far finer than any artificial one. A crystal is a solid with a neat geometric shape, with its plane faces meeting at characteristic angles, and with a characteristic symmetry. This visible regularity is the result of an orderly array of atoms making up its structure. There were reasons for thinking that the space between one layer of atoms and the next was about the size of an X-ray wavelength. If so, crystals would diffract X rays.
Laue experimented and found that X rays passing through a crystal were indeed diffracted and formed a pattern on a photographic plate that showed them to have the properties of waves. Within the same year, the English physicist William Lawrence Bragg and his equally distinguished father, William Henry Bragg, developed an accurate method of calculating the wavelength of a particular type of X ray from its diffraction pattern. Conversely, X-ray diffraction patterns were eventually used to determine the exact orientation of the atom layers that do the diffracting. In this way, X rays opened the door to a new understanding of the atomic structure of crystals. For their work on X rays, Laue received the Nobel Prize for physics in 1914, while the Braggs shared the Nobel Prize for physics in 1915.
Then, in 1914, the young English physicist Henry Gwyn-Jeffreys Moseley determined the wavelengths of the characteristic X rays produced by various metals, and made the important discovery that the wavelength decreased in a regular manner as one went up the periodic table.
This pinned the elements into definite position in the table. If two elements, supposedly adjacent in the table, yielded X rays that differ in wavelength by twice the expected amount, then there must be a gap between them belonging to an unknown element. If they differ by three times the expected amount, there must be two missing elements. If, on the other hand, the two elements’ characteristic X rays differ by only the expected amount, one can be certain that there is no missing element between the two.
It was now possible to give the elements definite numbers. Until then there had always been the possibility that some new discovery might break into the sequence and throw any adopted numbering system out of kilter. Now there could no longer be unsuspected gaps.
Chemists proceeded to number the elements from 1 (hydrogen) to 92 (uranium). These atomic numbers were found to be significant in connection with the internal structures of the atoms (see chapter 7) and to be more fundamental than the atomic weight. For instance, the X-ray data proved that Mendeleev had been right in placing tellurium (atomic number 52) before iodine (53), in spite of tellurium’s higher atomic weight.
Moseley’s new system proved its worth almost at once. The French chemist Georges Urbain, after discovering lutetium (named after the old Latin name of Paris), had later announced that he had discovered another element which he called celtium. According to Moseley’s system, lutetium was element 71 and celtium should be 72. But when Moseley analyzed celtium’s characteristic X rays, it turned out to be lutetium all over again. Element 72 was not actually discovered until 1923, when the Danish physicist Dirk Coster and the Hungarian chemist Georg von Hevesy detected it in a Copenhagen laboratory and named it hafnium (from the Latinized name of Copenhagen).
Moseley was not present for this verification of the accuracy of his method; he had been killed at Gallipoli in 1915 at the age of twenty-eight—certainly one of the most valuable lives lost in the First World War. Moseley probably lost a Nobel Prize through his early death. The Swedish physicist Karl Manne George Siegbahn extended Moseley’s work, discovering new series of X rays and accurately determining X-ray spectra for the various elements. He was awarded the Nobel Prize for physics in 1924.
In 1925, Walter Noddack, Ida Tacke, and Otto Berg of Germany filled another hole in the periodic table. After a three-year search through ores containing elements related to the one they were hunting for, they turned up element 75 and named it rhenium, in honor of the Rhine River. This left only four holes: elements 43, 61, 85, and 87.
It was to take two decades to track those four down. Although chemists did not realize it at the time, they had found the last of the stable elements. The missing ones were unstable species so rare on the earth today that all but one of them would have to be created in the laboratory to be identified. And thereby hangs a tale.
Radioactive Elements
IDENTIFYING THE ELEMENTS
After the discovery of X rays in 1895, many scientists were impelled to investigate these new and dramatically penetrating radiations. One of them was the French physicist Antoine-Henri Becquerel. His father, Alexandre Edmond (the physicist who had first photographed the solar spectrum), had been particularly interested in fluorescence, which is visible radiation given oil by substances after exposure to the ultraviolet rays in sunlight.
The elder Becquerel had, in particular, studied a fluorescent substance called potassium uranyl sulfate (a compound made up of molecules each containing an atom of uranium). Henri wondered whether the fluorescent radiations of the potassium uranyl sulfate contained X rays. The way to find out was to expose the sulfate to sunlight (whose ultraviolet light would excite the fluorescence), while the compound lay on a photographic plate wrapped in black paper. Since the sunlight could not penetrate the black paper, it would not itself affect the plate, but, if the fluorescence it excited contained X rays, they would penetrate the paper and darken the plate. Becquerel tried the experiment in 1896, and it worked. Apparently there were X rays in the fluorescence. Becquerel even got the supposed X rays to pass through thin sheets of aluminum and copper, and thus seemed to clinch the matter, for no radiation except X rays was known to do this.
But then, by a great stroke of good fortune, although Becquerel undoubtedly did not view it as such at the time, a siege of cloudy weather intervened. Waiting for the return of sunlight, Becquerel put away his photographic plates, with pinches of sulfate lying on them, in a drawer. After several days, he grew impatient and decided to develop his plates anyway, with the thought that even without direct sunlight some trace of X rays might have been produced. When he saw the developed pictures, Becquerel experienced one of those moments of deep astonishment and delight that are the dream of all scientists. The photographic plate was deeply darkened by strong radiation! Something other than fluorescence or sunlight was responsible for it. Becquerel decided (and experiments quickly proved) that this “something” was the uranium in the potassium uranyl sulfate.
This discovery further electrified scientists, already greatly excited by the recent discovery of the X rays. One of the scientists who at once set out to investigate the strange radiation from uranium was a young Polish-born chemist named Marie Sklodowska, who just the year before had married Pierre Curie, the discoverer of the Curie temperature (see chapter 5).
Pierre Curie, in collaboration with his brother Jacques, had discovered that certain crystals, when put under pressure, develop a positive electric charge on one side and a negative charge on the other. This phenomenon is called piezoelectricity (from a Greek word meaning “to press”). Marie Curie decided to measure the radiation given off from uranium by means of piezoelectricity. She set up an arrangement whereby this radiation would ionize the air between two electrodes, a current would then flow, and the strength of this small current would be measured by the amount of pressure that had to be placed on a crystal to produce a balancing countercurrent. This method worked so well that Pierre Curie dropped his own work at once and, for the rest of his life, joined Marie as an eager second.
It was Marie Curie who suggested the term radioactivity to describe the ability of uranium to give off radiations, and who went on to demonstrate the phenomenon in a second radioactive substance—thorium. In fast succession, enormously important discoveries were made by other scientists as well. The penetrating radiations from radioactive substances proved to be even more penetrating and more energetic than X rays; they are now called gamma rays. Radioactive elements were found to give off other types o
f radiation also, which led to discoveries about the internal structure of the atom, but this is a story for another chapter (see chapter 7). What has the greatest bearing on this discussion of the elements is the discovery that the radioactive elements, in giving off the radiation, changed to other elements—a modern version of transmutation.
Marie Curie was the first to come on the implications of this phenomenon, and she did so accidentally. In testing pitchblende for its uranium content, to see if samples of the ore had enough uranium to be worth the refining effort, she and her husband found, to their surprise, that some of the pieces had more radioactivity than they ought to have even if they had been made of pure uranium. The implication was, of course, that there had to be other radioactive elements in the pitchblende. These unknown elements could only be present in small quantities, because ordinary chemical analysis did not detect them, so they must be very radioactive indeed.
In great excitement, the Curies obtained tons of pitchblende, set up shop in a small shack, and—under primitive conditions and with only their unbeatable enthusiasm to drive them on—they proceeded to struggle through the heavy, black ore for the trace quantities of new elements. By July of 1898, they had isolated a trace of black powder 400 times as intensely radioactive as the same quantity of uranium.
This contained a new element with chemical properties like those of tellurium, and it therefore probably belonged beneath it in the periodic table. (It was later given the atomic number 84.) The Curies named it polonium, after Marie’s native land.
But polonium accounted for only part of the radioactivity. More work followed; and, by December of 1898, the Curies had a preparation that was even more intensely radioactive than polonium. It contained still another element, which had properties like those of barium (and was eventually placed beneath barium and was found to have the atomic number 88). The Curies called it radium, because of its intense radioactivity.
They worked on for four more years to collect enough pure radium so that they could see it. Then Marie Curie presented a summary of her work as her Ph.D. dissertation in 1903. It was probably the greatest doctoral dissertation in scientific history. It earned her not one but two Nobel Prizes. Marie and her husband, along with Becquerel, received the Nobel Prize for physics in 1903 for their studies of radioactivity; and, in 1911, Marie alone (her husband having died in a traffic accident in 1906) was awarded the Nobel Prize for chemistry for the discovery of polonium and radium.
Polonium and radium are far more unstable than uranium or thorium, which is another way of saying that they are far more radioactive. More of their atoms break down each second. Their lifetimes are so short that practically all the polonium and radium in the universe should have disappeared within a matter of a million years or so. Why do we still find them in the billions-of-years-old earth? The answer is that radium and polonium are continually being formed in the course of the breakdown of uranium and thorium to lead. Wherever uranium and thorium are found, small traces of polonium and radium are likewise to be found. They are intermediate products on the way to lead as the end product.
Three other unstable elements on the path from uranium and thorium to lead were discovered by means of the careful analysis of pitchblende or by researches into radioactive substances. In 1899, Andre Louis Debierne, on the advice of the Curies, searched pitchblende for other elements and came up with one he called actinium (from the Greek word for “ray”), which eventually received the atomic number 89. The following year, the German physicist Friedrich Ernst Dorn demonstrated that radium, when it broke down, formed a gaseous element. A radioactive gas was something new! Eventually the element was named radon (from radium and from argon, its chemical cousin) and given the atomic number 86. Finally, in 1917, two different groups=—Ottu Hahn and Lise Meitner in Germany, and Frederick Soddy and John Arnold Cranston in England—isolated from pitchblende element 91, named protactinium.
FINDING THE MISSING ELEMENTS
By 1925, then, the score stood at eighty-eight identified e1ements—eighty-one stable and seven unstable. The search for the missing four—numbers 43, 61, 85, 87—became avid indeed.
Since all the known elements from number 84 to 92 were radioactive, it was confidently expected that 85 and 87 would be radioactive as well. On the other hand, 43 and 61 were surrounded by stable elements, and there seemed no reason to suspect that they were not themselves stable; consequently, they should be found in nature.
Element 43, lying just above rhenium in the periodic table, was expected to have similar properties and to be found in the same ores. In fact, the team of Noddack, Tacke, and Berg, who had discovered rhenium, felt certain that they had also detected X rays of a wavelength that went along with element 43. So they announced its discovery, too, and named it masurium, after a region in East Prussia. However, their identification was not confirmed: and science, a discovery is not a discovery unless and until it has been confirmed at least one independent researcher.
In 1926, two University of Illinois chemists announced that they had found element 61 in ores containing its neighboring elements (60 and 62), and they their discovery illinium. The same year, a pair of Italian chemists at University of Florence thought that they had isolated the same element and named it florentium. But other chemists could not confirm the work of either group.
A few years later, an Alabama Polytechnic Institute physicist, using a new analytical method of his own devising, reported that he had found small traces of element 87 and of element 85; he called them virginium and alabamium, after his native and adopted states, respectively. But these discoveries could not be confirmed either.
Events were to show that the “discoveries” of elements 43, 61, 85, and 87 been mistaken.
The first of the four to be identified beyond doubt was element 43. The physicist Ernest Orlando Lawrence, who was to receive the Nobel in physics for his invention of the cyclotron (see chapter 7), made the in his accelerator by bombarding molybdenum (element 42) with high-speed particles. His bombarded material developed radioactivity, and Lawrence sent it for analysis to the Italian chemist Emilio Gino Segrè, who interested in the element-43 problem. Segrè and his colleague Carlo Perrier, after separating the radioactive part from the molybdenum, found that it resembled rhenium in its properties but was not rhenium. They decided that it could only be element 43, and that element 43, unlike its neighbors in the periodic table, was radioactive. Because it is not being produced as a breakdown product of a higher element, virtually none of it is left in the earth’s crust, and so Noddack and company were undoubtedly mistaken in thinking they had found it. Segrè and Perrier eventually were given the privilege of naming element 43; they called it technetium, from a Greek word meaning “artificial,” because it was the first laboratory-made element. By 1960, enough technetium had been accumulated to determine its melting point—close to 2200° C. (Segrè was later to receive a Nobel Prize for quite another discovery, having to do with another laboratory-made bit of matter—see chapter 7.)
In 1939, element 87 was finally discovered in nature. The French chemist Marguerite Perey isolated it from among the breakdown products of uranium. Element 87 was present in extremely small amounts, and only improvements in technique enabled it to be found where earlier it had been missed. She later named the new element francium, after her native land.
Element 85, like technetium, was produced in the cyclotron, by bombardment of bismuth (element 83). In 1940, Segrè, Dale Raymond Corson, and Kenneth Ross MacKenzie isolated element 85 at the University of California, Segrè having by then emigrated from Italy to the United States. The Second World War interrupted their work on the element, but after the war they returned to it and, in 1947, proposed the name astatine for the element, from a Greek word meaning “unstable.” (By that time, tiny traces of astatine had, like francium, been found in nature among the breakdown products of uranium.)
Meanwhile, the fourth and final missing element, number 61, had been discovered among the produc
ts of the fission of uranium, a process that is explained in chapter 10. (Technetium, too, turned up among these products.) Three chemists at the Oak Ridge National Laboratory—J. A. Marinsky, L. E. Clendenin, and Charles DuBois Coryell—isolated element 61 in 1945. They named it promethium, after the Greek demigod Prometheus, who had stolen fire for mankind from the sun. Element 61, after all, had been stolen from sunlike fires of the atomic furnace.
So the list of elements, from 1 to 92, was at last complete. And yet, in a sense, the strangest part of the adventure had only begun. For scientists had broken through the bounds of the periodic table; uranium was not the end.
TRANSURANIUM ELEMENTS
A search for elements beyond uranium—transuranium elements—had actually begun as early as 1934. Enrico Fermi in Italy had found that when he bombarded an element with a newly discovered subatomic particle called the neutron (see chapter 7), this often transformed the element into the one of the next higher atomic number. Could uranium be built up to element 93—a totally synthetic element that, as far as was then known, did not exist in nature? Fermi’s group proceeded to attack uranium with neutrons and got a product that they thought was indeed element 93. They called it uranium X.
In 1938, Fermi received the Nobel Prize in physics for his studies in neutron bombardment. At the time, the real nature of his discovery, or its consequences for humanity, was not even suspected. Like that other Italian, Columbus, he had found, not what he was looking for, but something far more important of which he was not aware.
Suffice it to say here that, after a series of chases up a number of false trails, it was finally discovered that what Fermi had done was, not to create a new element, but to split the uranium atom into two nearly equal parts. When physicists turned in 1940 to studies of this process, element 93 cropped up as an almost casual result of their experiments. In the melange of elements that came out of the bombardment of uranium by neutrons, there was one that at first defied identification. Then it dawned on Edwin McMillan of the University of California that perhaps the neutrons released by fission had converted some of the uranium atoms to a higher element, as Fermi had hoped would happen. McMillan and Philip Abelson, a physical chemist, were able to prove that the unidentified element was in fact element 93. The proof of its existence lay in the nature of its radioactivity, as was to be the case in all subsequent discoveries.
