Because of its unique properties, the distribution of deuterium in the universe constitutes a powerful clue to the history of the development of matter.
—David N. Schramm and Robert V. Wagoner
Nearly all the chemical elements that make up our material world occur in different isotopic forms. Every breath we inhale brings into the lungs oxygen in three isotopic forms: O16, O17, and O18. Each of these oxygen atoms has eight electrons around a nucleus with eight protons. In addition, O16 has eight neutrons, O17 has nine, and O18 has ten neutrons in their nuclei. The eight electrons and protons give oxygen its place in the Periodic Table of the Elements, and the eight electrons also determine oxygen's chemical behavior. Thus, the chemical behavior of each isotope of oxygen is essentially the same. However, the masses of the oxygen isotopes differ because of the different number of neutrons. This mass inequity, as we shall see, influences the physical behaviors of the three isotopic forms of oxygen.
The discovery of isotopes was made by Frederick Soddy in 1910. Earlier, Soddy had worked with Ernest Rutherford at McGill University, where they laid out the basis for understanding radioactivity in their paper "The Cause and Nature of Radioactivity."1 From Montreal, Soddy went first to London for one year and then on to Glasgow University, where he discovered isotopes—a term he coined in 1913.2 Soddy and Rutherford showed that in the process of radioactive transformations, the atoms of one chemical element could be transmuted into the atoms of another chemical element. In addition, it became apparent to Soddy that a radioactive transformation could give rise to atoms that differed in their weights but were chemically identical. Such atoms are isotopes of the same chemical element.
Soddy's discovery of isotopes put Prout's provocative idea that hydrogen was the basic building block of the chemical elements into the inactive archives of science history. The observed weights of the chemical elements are determined by their isotopic composition, which failed to be multiples of hydrogen, as Prout believed it would be. The atomic weight of oxygen, for example, is 15.9994, and that of chlorine is 35.453.
Deuterium is an isotope of hydrogen, often called heavy hydrogen. The most abundant isotope of hydrogen is H1, with a single electron in orbit around a single proton. In deuterium, H2, a neutron joins the proton in the nucleus. A naturally occurring sample of hydrogen consists of 99.985 percent H1 and 0.015 percent H2; thus, ordinary hydrogen (H1) is abundant whereas heavy hydrogen (H2) is scarce.3
The discovery of deuterium is, according to one of the physicists who participated in the experiments that led to its discovery, a "story of missed opportunities and errors."4 In 1913, two scientists at New York University held the discovery of deuterium in their hands when they measured the density of water to great accuracy. They found that the density of water varied from sample to sample and concluded that pure water does not possess a unique density. Their results varied according to the varying presence of deuterium in their samples. This was the first experimental evidence that provided a hint of the existence of deuterium. Had these scientists responded to their results with an experiment to distill water into fractions with different molecular weights, they may well have discovered deuterium twenty years early, but the discovery slipped through their fingers.
In 1929, Harold Urey and Berkeley chemist Joel Hildebrand left their hotel, hopped a taxi, and headed to their conference. Ferdinand G. Brickwedde was with them and listened to their conversation. Hildebrand informed Urey that chemists at Berkeley had just discovered the isotopes of oxygen, O17 and O18. Hildebrand said, "They could not have found isotopes in a more important element." Urey responded, "No, not unless it was hydrogen."5 In 1931, just before Urey started his own experiment to determine whether hydrogen had an isotope, two physicists at Berkeley were examining the physical and chemical bases for establishing atomic weights. The two approaches led to slightly different results. From this work they concluded that hydrogen was a mixture of isotopes—mostly H1 and a small amount of a heavier form. This work was reported in the July 1, 1931 issue of Physical Review. When Urey received this journal, he immediately began planning his investigation.
Urey's experiment had two parts. First, since a sample of hydrogen contains approximately one atom of deuterium, H2, for every 7,000 atoms of hydrogen, H1, a method to increase the concentration of the heavier isotope was necessary. A sample somehow enriched with deuterium would make it easier to detect deuterium's feeble presence and the results would be more definitive. Second, an experimental method to detect the presence of deuterium was needed. The experimental plan was: concentrate and detect.
Urey first tried to detect deuterium directly by using a sample of bottled hydrogen. Urey, a professor of chemistry at Columbia University, brought his colleague George Murphy on board and together they set up the apparatus for a careful spectroscopic study of hydrogen. Specifically, they designed their experiment to produce the spectral lines of the Balmer series. The two atoms of hydrogen, light and heavy, would give rise to spectra that were essentially the same, except the wavelengths of the spectral lines associated with the heavy isotope would be slightly shifted relative to the wavelengths of the lighter isotope. They did see very faint lines at the wavelength positions their calculations suggested. Thus, they believed they were observing evidence for the heavy form of hydrogen. They wanted stronger evidence, however. They did not want to be misled by possible impurities or some instrumental error. Urey then decided that a method must be found to increase the concentration of deuterium in the sample to be analyzed.
For this challenge, Urey went to the National Bureau of Standards, where he talked to Brickwedde. Urey's idea was to distill liquid hydrogen and concentrate the heavier form of hydrogen. This method of concentrating deuterium relies on a physical behavioral difference arising from the disparity between the masses of the two isotopes. At temperatures below 20.4 degrees kelvin (K), or — 252.6°C, hydrogen is a liquid in the form of molecular hydrogen, H-H or H2. Most of the molecules consist of two ordinary hydrogen atoms: H-H. A small fraction of the molecules bring together the ordinary and heavy forms of hydrogen: H-D. In the liquid, the H-H molecular form moves around a little faster than H-D because it is less massive. As the liquid is slowly evaporated, the faster-moving H-H is more likely to leave the surface of the liquid; H-D molecules are more likely to stay behind, hence the liquid becomes slightly more concentrated with H-D. Urey's plan was to evaporate some 6,000 cm3 (six liters) of liquid hydrogen down to a volume of about 2 cm3, which, according to plan, would be greatly enriched in the heavier isotope of hydrogen.
This was Urey's proposal to Brickwedde. Brickwedde agreed to help. In his first attempt, Brickwedde evaporated liquid hydrogen at a temperature of 20K. But some procedural errors negated the intended outcome and Urey detected no enhanced presence of deuterium in the Balmer spectral lines. In his next try, Brickwedde evaporated liquid hydrogen at a lower temperature of 14K. The sample of hydrogen resulting from this distillation was indeed richer in the heavy isotope, and the Balmer spectral lines corresponding to deuterium were more intense by a factor of six or seven. On the basis of these results, Urey concluded that the isotope of hydrogen, deuterium, really existed.
Papers reporting the discovery were published in early 1932. In 1934, Urey won the Nobel Prize in chemistry for the discovery of deuterium.
Harold Urey was an unusual scientist. He grew up in Montana, the son of poor, homesteading parents. After graduating from high school, Urey taught for three years in a small country public school. He attended Montana State University, studied zoology and chemistry, and graduated in 1917. To pay for his education, Urey worked summers on a railroad road gang laying track in the Northwest. During the academic year, Urey lived in a tent.
Soon after his discovery of deuterium, Urey's exceptional character revealed itself in a very selfless way. In honor of his discovery, Urey received a prize from the Carnegie Foundation. The prize was for $7,600—a large amount of money in the early 1930s. In an act of rare generosity, Urey gave half the money to a young and, in his judgment, promising physicist colleague, I. I. Rabi. As Rabi later recalled: "Urey did one of the most extraordinary things imaginable. He gave me half of it [the money]. I had nothing to do with the discovery. What a greatness in Harold Urey—what a tremendous magnanimity to do something like that."6 The money from Urey allowed Rabi to improve and expand his molecular beam experiments. As we shall see in Chapters 11, 12, and 13, Urey's judgment about Rabi's potential was well founded.
It is unusual for a Nobel Prize to be awarded so quickly after a discovery. But because of its simplicity, hydrogen attracts attention. Urey's discovery stimulated an outburst of research activity. Within two years of its discovery, over one hundred papers were published that involved the new isotope of hydrogen. In 1934, another one hundred deuterium-related papers were published. Although both chemists and physicists were intrigued by this new isotope of hydrogen, it was the nuclear physicists who began a long affair with deuterium.
The nucleus of deuterium is called the deuteron. In a basic sense, the deuteron is to physicists interested in the nucleus what the hydrogen atom is to those interested in the atom. The hydrogen atom is the simplest atom; the deuteron is the simplest compound nucleus. Of course, the nucleus of the hydrogen atom is the simplest nucleus of all, but it is a lone proton and does not bring into play any of the forces that hold larger nuclei together. The deuteron, one proton and one neutron bound together, is the nucleus of the deuterium atom. The question that fascinates nuclear physicists is this: What is the nature of the force that binds the proton and neutron together?
The parallels between the hydrogen atom and the deuteron are provocative. Both are two-particle systems: the hydrogen atom, an electron and a proton; the deuteron, a proton and a neutron. Both can be treated in similar ways. There is, however, one significant difference. Whereas the force between the electron and the proton in the hydrogen atom is the well-known Coulomb force, the force that acts between the proton and neutron inside the deuteron was not known in the early decades of the twentieth century. The need to understand the nature of the force between nuclear particles has made the simple deuteron the playground of nuclear physicists. In fact, the deuteron has been studied more than any other nucleus, and the insights gained from this simple nucleus have guided physicists as they grapple with more and more complicated nuclei. As we shall see in Chapter 14, the deu-
teron eventually forced a complete overhaul in the thinking about nuclear forces.
Once again, the hydrogen atom was a source of inspiration— this time for a chemist. The discovery of deuterium by Harold Urey has guided the thinking and experiments of physicists as they have sought to expose the forces at play inside the atomic nucleus.
Naming the newly discovered isotope of hydrogen was far more time-consuming, as it turned out, than Urey's experiments themselves. Normally, the honor of naming the newly discovered isotope would go to the discoverer, Harold Urey. But many leading physicists, including Earnest Lawrence, G. N. Lewis, Ernest Rutherford, R. A. Millikan, and others (including professors of Greek, one from Columbia and the other from Berkeley) joined in the controversy. With all their intelligence, it took these individuals and the larger community two years to agree on the name deuterium—which was Urey's choice.7
The deuterium story is revealing. Physics, based on hard data, is typically easy for scientists to agree on. But deciding on the name of something brings out emotions and vested interests that can spark disagreements and challenge friendships. Discovering deuterium was relatively straightforward; naming it was another matter.
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