Harold Urey and the Discovery of Deuterium
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Harold Urey and the discovery of deuterium Chemistry, nuclear physics, spectroscopy and thermodynamics came together to predict and detect heavy hydrogen before the neutron was known. Ferdinand G. Brickwedde It was on Thanksgiving day in 1931 his family to a homestead in Montana. 1940 he was the founding editor of the that Harold Clayton Urey found defini- After graduating from high school, he American Institute of Physics publica- tive evidence of a heavy isotope of taught for three years in public schools, tion, Journal of Chemical Physics. hydrogen. Urey's discovery of deuter- and then entered Montana State Uni- When the biographical publication ium is a story of the fruitful use of versity as a zoology major and chemis- "American Men of Science" took note primitive nuclear and thermodynamic try minor. Money was tight for him as of scientists selected for recognition by models. But it is also a story of missed a college student. During the academic their peers, Urey was elected in phy- opportunity and errors—errors that year he slept and studied in a tent. sics. In 1934—only three years after are particularly interesting because of During his summers he worked on a the discovery of deuterium—Urey was the crucial positive role that some of road gang laying railroad track in the awarded the Nobel Prize in chemistry. them played in the discovery. A look at Northwest. the nature of the theoretical and ex- Urey graduated with a BS degree in Before the search perimental work that led to the detec- 1917, when there was a need for chem- In 1913, Arthur B. Lamb and Richard tion of hydrogen of mass 2 reveals ists in the war effort. He worked for Edwin Lee, working at New York Uni- much about the way physics and chem- the Barrett Chemical Company in versity, reported5 a very precise mea- istry were done half a century ago. Philadelphia on war materials. After surement of the density of pure water. Although George M. Murphy and I the war, Urey taught chemistry for two Their measurements were sensitive to coauthored with Urey the papers1"3 years at Montana State University, 2xlO~7 g/cm3. Various samples of reporting the discovery, it was Urey and in 1921 entered the University of water, which were carefully prepared who proposed, planned and directed California at Berkeley as a graduate using the best purification techniques the investigation. Appropriately, the student in chemistry, working under and temperature controls, varied in Nobel Prize for finding a heavy isotope the guidance of the renowned chemical density by as much as 8xlO~7 g/cm3. of hydrogen went to Urey. thermodynamicist Gilbert N. Lewis. They concluded that pure water does In this article we will look first at the As a graduate student, Urey was a not possess a unique density. research that led to the discovery, as pioneer in the calculation of thermo- Today we know that water varies in that work was understood at the time. dynamic properties from spectroscopic isotopic composition, and that samples Then we will look at some of the same data. He received a PhD in 1923 and of water with different isotopic compo- activity with the understanding that spent the next academic year as an sitions have different vapor pressures, only hindsight can give. Throughout American-Scandinavian Foundation making distillation a fractionating pro- the discussion I will include fragments Fellow in the Physical Institute of cess. The Lamb-Lee investigation is from my memory—illustrative epi- Niels Bohr in Copenhagen. interesting because it was the first sodes connected with the discovery. After Copenhagen, Urey joined the reported experiment in which an isoto- faculty at Johns Hopkins University. pic difference in properties was clearly Urey's career Although in the chemistry department, in evidence. It is the earliest recogniz- Urey died last year at 87 years of age, he attended the physics department's able experimental evidence for iso- after a remarkably productive and in- regular weekly "journal" meetings for topes. (The existence of isotopes was teresting life. He was a chemist with faculty and graduate students, and he proposed independently by Frederick very broad interests in science, remi- participated in the discussions. It was Soddy, in England, and by Kasimir niscent of the natural philosophers of at these meetings that I, as a graduate Fajans, in Germany, in 1913.) Think the eighteenth and nineteenth centur- student in physics, became acquainted what the result might have been had ies. Murphy4, who went on to become with Urey. While Urey was at Hopkins, Lamb and Lee pursued a progressive professor and head of the department he and Arthur E. Ruark coauthored the fractionation of water by distillation of chemistry at New York University, classic textbook, Atoms, Molecules, and and separated natural water into frac- died in 1968. Quanta, which was the first comprehen- tions with different molecular weights. Urey was born on a farm in Indiana sive text on atomic structure written in Less than two decades later, by the in 1893, and in childhood moved with English. I proofread the entire book in time of the discovery of deuterium, galley for the authors. isotopes were an active field of re- Ferdinand G. Brickwedde is Evan Pugh Re- Urey's work bridged chemistry and search. The rapid development of nu- search Professor of Physics emeritus at Penn- physics. In 1929 he was appointed clear physics after 1930 was initiated sylvania State University, in University Park, associate professor of chemistry at Co- by isotope research. It was a time of Pennsylvania. lumbia University, and from 1933 to search for as-yet undiscovered isotopes, 34 PHYSICS TODAY / SEPTEMBER 1982 0031-9228/ 82/ 0900 34-06/ $01.00 (cl 1982 American Institute Of Physics especially of the light elements, hydro- isotopes exist, and what determines Urey had a copy of this chart hang- gen included, and Urey was very much their number, relative abundances and ing on a wall of his laboratory. The a participant. masses (packing fractions)? isotope helium-5 does not exist, and the I remember a conversation in 1929 Urey, along with others, constructed staggered line does not provide a place with Urey and Joel Hildebrand, a fam- charts of the known isotopes to show for the isotope helium-3, which was ous professor of chemistry at Berkeley. relationships bearing on their exis- discovered later. The diagram is only It took place during a taxi ride between tence. The figure on page 36 is one of of historical interest now, but it was an their hotel and the conference center Urey's charts. At the time, the neutron incentive to Urey to look for a heavy for a scientific meeting we were attend- had not been discovered—it was disco- isotope of hydrogen. ing in Washington. When Urey asked vered in 1932, the year after deuter- Hildebrand what was new in research ium. The chart was based on the the- Prediction and evidence at Berkeley, Hildebrand replied that ory that atomic nuclei were composed In 1931—the year of the discovery of William Giauque and Herrick John- of protons, plotted here as ordinates, deuterium—Raymond T. Birge, a pro- ston had just discovered that oxygen and nuclear electrons, plotted as abscis- fessor of physics at the University of has isotopes with atomic weights 17 sae—the number of protons was the California, Berkeley, and Donald H. and 18, the isotope of weight 18 being nuclear mass number, and the number Menzel, professor of astrophysics at the more abundant. Their paper6 of nuclear electrons was the number of Lick Observatory, published7 a letter to would appear shortly in the Journal of protons minus the atomic number of the editor in Physical Review on the the American Chemical Society. Then the element. In Urey's chart, the filled relative abundances of the oxygen iso- Hildebrand added, "They could not circles represent the nuclei from H' to topes in relation to the two systems of have found isotopes in a more impor- Si30 that were known to exist before atomic weights that were then in use— tant element." Urey responded: "No, 1931. The open circles represent nuclei the physical system and the chemical not unless it was hydrogen." This was unknown before 1931. The chart's pat- system. Atomic weights in the physical two years before the discovery of deu- tern of staggered lines, when extended system were determined with the mass terium. Urey did not remember this down to H1, suggested to Urey that the spectrograph and were based on setting remark, but I did. atoms H2, H3 and He5 might exist the atomic weight of the isotope O16 at At the time, answers were being because they are needed to complete exactly 16. In the chemical system, sought to questions such as: Why do the pattern. atomic weights were determined by Harold Clayton Urey and a country schoolhouse in Indiana where he taught after graduating from high school. Urey taught for three years in public schools in Indiana and Montana before he entered Montana State University. (Schoolhouse photo from the Urey collection, AIP Niels Bohr Library.) PHYSICS TODAY / SEPTEMBER 1982 35 bulk techniques, and the values were ence in the vapor pressures of solid H2 based on setting at 16 the atomic and HD. On this basis Urey expected a weight of the naturally occurring mix- sizeable difference in the vapor pres- 16 17 ture of oxygen isotopes, O , O and sures of liquid H2 and HD at 20.4 K, the 18 O . Thus the atomic weights of a boiling point of H2. single isotope or element on the two Urey approached me at the National scales should differ.