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The Scientific Rivalry that Created the Nuclear Age
By Amir D. Aczel
Palgrave MacmillanCopyright © 2009 Amir D. Aczel
All rights reserved.
PHYSICS AND URANIUM
Uranium is the heaviest element found in our natural environment. With an atomic weight of 238 (or 235, for the rare form of this metal), it is in fact so heavy that it cannot be produced in the same way as light elements. Unlike many lighter elements, uranium is created in a supernova—a tremendous stellar explosion. Our solar system, including our own planet, was formed from the remnants of stars that lived and died in our cosmic neighborhood. The hydrogen and helium that were formed during the big bang burn inside stars through a nuclear process called fusion, which merges the nuclei of small elements to create larger ones. Thus carbon, nitrogen, oxygen, and all elements up to iron in the periodic table are produced inside stars. When a star with a mass the size of our Sun or even somewhat greater dies, it sheds its atmosphere, and the elements that had been produced by its nuclear flames dissipate into open space. Millions of years later, such resulting clouds of elements may condense, as happened when our solar system came into being about 4.5 billion years ago, and this is how many of the elements on Earth came to be. The clouds of matter from supernovae mix with those of the remnants of stars that died a less violent death, and this is how uranium found its way to our environment on Earth.
Uranium thus exists everywhere on our planet. It makes up a tiny percentage of rocks, as well as of sea water. But what is uranium?
The matter in our universe consists of atoms, which combine with other atoms to form the molecules of the substances we know from everyday life, such as water—two parts hydrogen to one part oxygen—or carbon dioxide—one carbon atom for every two atoms of oxygen. Each atom has a core, called the nucleus. The nucleus itself is much, much smaller than the atom as a whole. If an atom were the size of a bus, then the nucleus would be the dot on the letter "i" in a newspaper story read by a passenger on the bus. The nucleus is dense and contains protons, which carry a positive electrical charge, and also electrically neutral components called neutrons.
Depending on the element, there will be a different number of protons and neutrons inside the nucleus. The rest of the atom is made up of electrically negative components called electrons, which orbit the nucleus. These orbits and the empty space they occupy form most of the volume of the atom.
Hydrogen is the simplest and lightest element in the universe: it has only one proton in its nucleus. Helium is larger and has two protons as well as two neutrons. Hydrogen has one electron in orbit around its nucleus; helium has two. Uranium is very heavy—it has 92 protons in its core together with 146 neutrons—and it has, as is usual for an atom, the same number of electrons as it has protons. Thus there are 92 electrons in orbit around the uranium nucleus.
What is unusual about the uranium atom is the very large number of neutrons, and they add significantly to the uranium atom's weight and make it prone to decay. Because it is so heavy and dense and because of conditions inside it, the uranium nucleus disintegrates slowly, producing radiation, mainly in the form of alpha particles, which are helium nuclei. In the process, uranium gives rise to other, lighter radioactive elements—which also disintegrate by emitting radiation—and eventually becomes (nonradioactive) lead. The typical, standard unit of time that it takes a radioactive element to disintegrate is called a half-life. This is the length of time it takes half the mass present to disintegrate through radiation—and uranium's half-life is very long. Half of any amount of uranium 238 will become lead after 4.47 billion years. The radiation from uranium produces heat energy in rocks deep inside the Earth, and this process helps keep our planet's core warm; uranium is thus responsible for some of our planet's geological activity.
Uranium forms natural compounds that have many beautiful colors: bright yellow, glowing orange, fluorescent green, dark red, and black. These shiny minerals caught the eye of ancient Roman artists, who used uranium compounds to decorate pottery and tint glass. Some attractive Roman glass urns containing uranium minerals for color have been found in Cape Posillipo, near Naples, during archeological excavations.
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THE MODERN STORY of uranium began early in the sixteenth century when a major silver discovery was made in an area with thermal baths in the German principality of Saxony. The silver rush led to the founding of a town called St. Joachim's Valley, or Joachimsthal.
It soon became the largest mining center in Europe, with a population numbering 20,000; Prague, the nearest large city, had only 50,000 inhabitants at the time. Eventually, two million silver coins, called Joachimsthaler after the town, were minted for the Austro-Hun-garian crown, which owned the mines. The Joachimsthaler, later shortened to thaler, became accepted in many countries and gave its name to our familiar monetary unit, the dollar.
In 1570, the emperor Maximilian II gave the order to exploit the Joachimsthal mines and find more silver and—he hoped—other valuable metals. Using an improved mining technology, within a few years bismuth and cobalt deposits were found. Then something strange was discovered. It didn't look like silver, or cobalt, or tin, or any other mined metal. It was a dark compound the miners named pitchblende, from the German words for black and mineral. Nobody understood its properties and it was ignored, pushed aside as mere tailings in the mining operation.
Martin Heinrich Klaproth (1743–1817) was trained as an apothecary, and spent most of his life working in pharmacies in various locations in Germany, finally settling in Berlin. Klaproth had a severe face and an exacting, punctilious nature. He was a successful businessman as well as a curious scientist. His ambition went far beyond mixing and dispensing medicines, and he began to study chemistry on his own. Klaproth devised new methods of analysis of chemical compounds, which led to the founding of the field of analytic chemistry. He had a special talent for treating minerals—dissolving them in hydrochloric and sulfuric acids, then oxidizing them or heating them—so that he could determine their composition. Within a few years of applying his methods, Klaproth was able to discover cerium (a rare-earth, silvery metal), and explain the composition of a number of compounds.
Klaproth heard a rumor that in Joachimsthal miners had found a strange new mineral, and this fired his interest. He traveled there to inspect the mysterious compound and took a sample of the material back to his shop in Berlin. He submitted the compound to various tests, attacking it with acids and oxidizing agents to uncover its nature. After months of hard and often frustrating work, in 1789 he managed to find the right mixture of chemical agents that allowed him to finally extract from the pitchblende something he described as "a strange kind of half metal." Inspecting the odd compound he had just created, he determined that it was an oxide of a metal that had never been seen before.
In 1781 the planet Uranus—named after a Greek god—had been discovered by the German-born English astronomer William Her-schel. To honor Herschel's discovery, Klaproth named his new element uranium. It was a generous tribute since by scientific convention he could have bestowed his own name on the new element, which might then have been named klaprothium.
Klaproth's discovery of uranium and of other metals he isolated and identified established him as the greatest chemist in Germany, and one of the greatest of all time. In 1810 the University of Berlin created a chaired professorship for him.
Following Klaproth's discovery, uranium was identified in minerals mined in many places around the world, but the deposits known then were never as rich as those found at Joachimsthal. By the twenty-first century, regions in Canada, Australia, and the Congo would surpass the Saxony mine. By then, uranium also had been found in Cornwall in Britain, Morvan in France, and locations in Austria and Romania.
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SINCE KLAPROTH HAD only synthesized an oxide of the new metal—uranium combined with oxygen—chemists wanted to see the actual metal in its pure state. They understood that what had been synthesized was a compound and not a pure element, as one would be able to tell the difference between a powder and a solid metal. They realized that the metal was very heavy and dense, but had difficulties in isolating it from its compounds found in nature. In 1841, the French chemist Eugène Péligot used a powerful thermal reaction, heating uranium oxide together with potassium to separate uranium from oxygen. He achieved this complicated task by first turning the uranium oxide into a salt, uranium chloride. Then he reduced the salt chemically using the potassium. As the potassium began to act on the uranium salt (because at the much higher temperature it was reactive with chlorine to a higher degree than uranium), Péligot suddenly saw a shiny metal appear. This was pure uranium. It looked like silver, but it quickly oxidized again in the air.
By the middle of the nineteenth century chemists knew definitively that a very heavy element, a metal, had been discovered. But what was its place among all the other known elements? How did it relate to other elements found in nature?
By the end of the eighteenth century, chemists had known how to distinguish two groups of substances: pure elements, such as the metal sodium, and chemical compounds, such as sodium chloride (common salt). But how should the elements be classified? Nobody had yet answered this question. Surely, there was some method to the elements' chemical reactivities—the way elements combined to form compounds. Then, throughout the nineteenth century, as chemistry advanced as a scientific discipline, many new chemical compounds and the pure elements synthesized from them were discovered with increasing frequency. But there was still a great disorder in our understanding of the elements—how they fit together in the universe, and how they relate to one another. Chemists were discovering some rules of behavior—which elements reacted with which—and were compiling a list of elements, which by 1830 numbered 55. Were these all the elements in the universe, or were there others? How many? Since the rules of behavior of the elements were not well understood, the list was not very meaningful. What was needed was a kind of table that could arrange all the elements in a logical way that reflected and demonstrated their reactions with one another.
The first steps toward a logical classification of the elements in chemists' lists were made starting in 1817 by a German chemist named Johann Wolfgang Döbereiner, who showed that when the elements' atomic weights were arranged in increasing order, there were elements whose weight fit in the middle between the weights of pairs of other elements. For example, strontium (weight about 88) fit between calcium (weight close to 40) and barium (weight about 137). He found a number of such triplets of elements and began to look for groupings of other chemical elements. Several chemists improved on that idea, but the real breakthrough was achieved by a visionary Russ-ian chemist.
Dmitri Mendeleyev (1834–1907) was born in Siberia and became a professor at the University of St. Petersburg in 1865. In 1871, he completed his masterpiece: the periodic table of the elements. Mendeleyev arrived at the idea of the periodic table by trying to arrange all the known elements by their atomic weights and in a way that would somehow capture their shared chemical reactivities and their similar physical properties. His table classified all the known elements of the time by each one's chemical properties and increasing atomic weight, and it showed uranium with the highest weight of all the elements. The structure of the periodic table placed the elements naturally into groups that behaved in similar ways. Thus chlorine, fluorine, bromine, and iodine (called halogens) were placed in one column—they form similar chemical compounds (by taking or sharing a single electron, as we understand it today). Similarly, sodium, potassium, and lithium are metals that behave in common ways (they donate one electron each to form salts). Later it was discovered that it was the atomic number (the number of protons or electrons in an atom) rather than the weight (which incorporates the number of neutrons as well as the protons) that determines chemical activity. Even with these discoveries, the modifications to Mendeleyev's table were minimal.
Years later, laboratory-produced elements exceeding uranium in weight would be added to the table. These included plutonium, ein-steinium, and mendelevium—the last two honoring Einstein and Mendeleyev.
Uranium held a privileged place in the periodic table. Literally, uranium is the element out of this world. It was created during the supernova explosion of a massive star, and it was the last element in the table, being the biggest and heaviest naturally occurring element. With atomic number 92, the valence (the number of electrons it shares or donates in chemical reactions) of uranium is 6 or 4. Thus, when purified in an industrial process to separate its different isotopes, it is made to react with six fluorine atoms to make uranium hexafluoride, which is a gas, and can thus be separated according to weight using a centrifuge. Pure uranium is a silvery white metal, and it is very heavy—it feels like a piece of lead, but it is not as dark and can be polished to a shine. Uranium is radioactive and decays into other elements. All of these elements, except for lead—the final outcome of some radioactive decay chains starting from uranium—are radioactive. But what is radiation? What is radioactivity? And how were they discovered?
No one was looking for radiation. Its discovery was one of the most serendipitous moments in scientific history and took place at dusk on November 8, 1895, in a laboratory at the University of Würzburg in Germany. Wilhelm Conrad Röntgen (1845–1923), a 50-year-old professor of physics, was carrying out a routine experiment with an electric tube he had invented, when he suddenly noticed that a chemically coated sheet of paper on a bench several feet away from him was glowing lightly. He was stunned. He turned off the electrical current in the tube, and the glow disappeared; he turned it on again, and it returned. Röntgen realized that he had chanced on a fascinating discovery—a glow that could be induced from afar. He surmised that unseen rays traveled from the tube to the paper, causing the glow. And, with more experimentation, he realized that the rays that produced the fluorescence were able to penetrate certain materials (paper, wood, and human flesh). Here was a technical application that made the wonders of the human body accessible. Before, you had to cut someone open to peer inside. Now Röntgen realized that with his newly discovered X-ray radiation, the insides of the body could be visible. This held great promise for beneficial use in medicine, and hence the great excitement about this amazing advance.
Röntgen spent many months studying radiation and discovered that lead shielding blocked the rays. He published his results on X rays (which in some countries are still called Röntgen rays in his honor) in a paper he read to the Würzburg Physical and Medical Society in De-cember 1895 (translated and published in Nature in 1896). In 1901, he would receive the first Nobel Prize awarded in physics. Scientists the world over set out to investigate the new phenomenon. Two related questions that occupied many were: Does radiation occur naturally? Do any natural compounds give off similar radiation?
The French mathematician Jules Henri Poincaré (1854–1912) read the new scientific paper by Röntgen describing his discovery and experiments with X rays, and championed the findings at a meeting at the French Academy of Sciences in 1896. Eminent French scientists were fascinated by Röntgen's work. Among them was the physicist Antoine Henri Becquerel (1852–1908), who had been studying the way objects give off internal light or phosphorescence, such as the glow of a firefly or certain algae. Becquerel was then studying uranium salts in his lab. Poincaré suggested to him that if X rays could cause fluorescence, perhaps the glowing salts in his lab also emitted some kind of rays.
Becquerel took up Poincaré's suggestion and spent several weeks experimenting with the uranium salts. He could not detect any luminescence in the compounds. He wanted to take some photographs outside, but since the weather was inclement, he placed—by chance—his photographic plates in a drawer containing the uranium salts. A few days later, he took pictures with these plates and in developing them noticed something very odd: the plates were cloudy. Pondering this mystery, he concluded that the streaks had to have been caused by the uranium salts. Perhaps here was proof that the uranium salt was creating a radiation similar to X rays. (To this day, film is often used to detect radiation.) Becquerel presented his results—which by then he had confirmed using controlled experiments—to his colleagues at the Academy of Sciences, and in 1903 he would share a Nobel Prize with a married couple, who lived and worked across Paris from his lab, for their co-discovery of radioactivity.
It had then been established that Earth contained a strange element, uranium, which possessed the property of radioactivity: It emitted radiation that could be detected, but whose full nature was not understood. Scientists were set to uncover its mysteries.
Excerpted from Uranium Wars by Amir D. Aczel. Copyright © 2009 Amir D. Aczel. Excerpted by permission of Palgrave Macmillan.
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