Harold Clayton Urey

Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in the development of the atom bomb, as well as contributing to theories on the development of organic life from non-living matter.

Born in Walkerton, Indiana, Urey studied thermodynamics under Gilbert N. Lewis at the University of California, Berkeley. After he received his PhD in 1923, he was awarded a fellowship by the American-Scandinavian Foundation to study at the Niels Bohr Institute in Copenhagen. He was a research associate at Johns Hopkins University before becoming an associate professor of Chemistry at Columbia University. In 1931, he began work with the separation of isotopes that resulted in the discovery of deuterium.

During World War II, Urey turned his knowledge of isotope separation to the problem of uranium enrichment. He headed the group located at Columbia University that developed isotope separation using gaseous diffusion. The method was successfully developed, becoming the sole method used in the early post-war period. After the war, Urey became professor of chemistry at the Institute for Nuclear Studies, and later Ryerson professor of chemistry at the University of Chicago.

Urey speculated that the early terrestrial atmosphere was composed of ammonia, methane, and hydrogen. One of his Chicago graduate students was Stanley L. Miller, who showed in the Miller–Urey experiment that, if such a mixture were exposed to electric sparks and water, it can interact to produce amino acids, commonly considered the building blocks of life. Work with isotopes of oxygen led to pioneering the new field of paleoclimatic research. In 1958, he accepted a post as a professor at large at the new University of California, San Diego (UCSD), where he helped create the science faculty. He was one of the founding members of UCSD's school of chemistry, which was created in 1960. He became increasingly interested in space science, and when Apollo 11 returned moon rock samples from the moon, Urey examined them at the Lunar Receiving Laboratory. Lunar astronaut Harrison Schmitt said that Urey approached him as a volunteer for a one-way mission to the Moon, stating "I will go, and I don't care if I don't come back."

Early life

Harold Clayton Urey was born on April 29, 1893, in Walkerton, Indiana, the son of Samuel Clayton Urey, a school teacher and a minister in the Church of the Brethren, and his wife, Cora Rebecca née Reinoehl. He had a younger brother, Clarence, and a younger sister, Martha. The family moved to Glendora, California, after Samuel became seriously ill with tuberculosis, in hopes that the climate would improve his health. When it became clear that he would die, the family moved back to Indiana to live with Cora's widowed mother. Samuel died when Harold was six years old.

Urey was educated in an Amish grade school, from which he graduated at the age of 14. He then attended high school in Kendallville, Indiana. After graduating in 1911, he obtained a teacher's certificate from Earlham College, and taught in a small school house in Indiana. He later moved to Montana, where his mother was then living, and continued to teach there.

Urey entered the University of Montana in Missoula in the autumn of 1914. Unlike Eastern universities of the time, the University of Montana was co-educational in both students and teachers. Urey earned a Bachelor of Science (BS) degree in zoology there in 1917.

As a result of the United States entry into World War I that same year, there was strong pressure to support the war effort. Urey had been raised in a religious sect that opposed war. One of his professors suggested that he support the wartime effort by working as a chemist. Urey took a job with the Barrett Chemical Company in Philadelphia, making TNT, rather than joining the army as a soldier. After the war, he returned to the University of Montana as an instructor in chemistry.

An academic career required a doctorate, so in 1921 Urey enrolled in a PhD program at the University of California, Berkeley, where he studied thermodynamics under Gilbert N. Lewis. His initial attempt at a thesis was on the ionization of cesium vapor. He ran into difficulties, and Meghnad Saha published a better paper on the same subject. Urey then wrote his thesis on the ionization states of an ideal gas, which was subsequently published in the ''Astrophysical Journal''. After he received his PhD in 1923, Urey was awarded a fellowship by the American-Scandinavian Foundation to study at the Niels Bohr Institute in Copenhagen, where he met Werner Heisenberg, Hans Kramers, Wolfgang Pauli, Georg von Hevesy, and John Slater. At the conclusion of his stay, he traveled to Germany, where he met Albert Einstein and James Franck.

On returning to the United States, Urey received an offer of a National Research Council fellowship to Harvard University, and also received an offer to be a research associate at Johns Hopkins University. He chose the latter. Before taking up the job, he traveled to Seattle, Washington, to visit his mother. On the way, he stopped by Everett, Washington, where he knew Dr. Kate Daum, a colleague from the University of Montana. Dr. Daum introduced Urey to her sister, Frieda. Urey and Frieda soon became engaged. They were married at her father's house in Lawrence, Kansas, in 1926. The couple had four children: Gertrude Bessie (Elizabeth), born in 1927; Frieda Rebecca, born in 1929; Mary Alice, born in 1934; and John Clayton Urey, born in 1939.

At Johns Hopkins, Urey and Arthur Ruark wrote ''Atoms, Quanta and Molecules'' (1930), one of the first English texts on quantum mechanics and its applications to atomic and molecular systems. In 1929, Urey became an associate professor of Chemistry at Columbia University, where his colleagues included Rudolph Schoenheimer, David Rittenberg, and T. I. Taylor.

Deuterium

In the 1920s, William Giauque and Herrick L. Johnston discovered the stable isotopes of oxygen. Isotopes were not well understood at the time; James Chadwick would not discover the neutron until 1932. Two systems were in use for classifying them, based on chemical and physical properties. The latter was determined using the mass spectrograph. Since it was known that the atomic weight of oxygen was almost exactly 16 times as heavy as hydrogen, Raymond Birge, and Donald Menzel hypothesized that hydrogen had more than one isotope as well. Based upon the difference between the results of the two methods, they predicted that only one hydrogen atom in 4,500 was of the heavy isotope.

In 1931, Urey set out to find it. Urey and George M. Murphy (1903–1968) calculated from the Balmer series that the heavy isotope should have lines blueshifted (correspondingly the light isotope redshifted) by . Urey had access to a grating spectrograph, a sensitive device that had been recently installed at Columbia and was capable of resolving the Balmer series. With a resolution of 1 Å per millimetre, the machine should have produced a difference of about 1 millimetre. However, since only one atom in 4,500 was heavy, the line on the spectrograph was very faint. Urey therefore decided to delay publishing their results until he had more conclusive evidence that it was heavy hydrogen.

Urey and Murphy calculated from the Debye model that the heavy isotope would have a slightly higher boiling point than the light one. By carefully warming liquid hydrogen, 5 litres of liquid hydrogen could be distilled to 1 millilitre, which would be enriched in the heavy isotope by 100 to 200 times. To obtain five litres of liquid hydrogen, they traveled to the cryogenics laboratory at the National Bureau of Standards in Washington, D.C., where they obtained the help of Ferdinand Brickwedde, whom Urey had known at Johns Hopkins.

The first sample that Brickwedde sent was evaporated at at a pressure of . To their surprise, this showed no evidence of enrichment. Brickwedde then prepared a second sample evaporated at at a pressure of . On this sample, the Balmer lines for heavy hydrogen were seven times as intense. The paper announcing the discovery of heavy hydrogen, later named deuterium, was jointly published by Urey, Murphy, and Brickwedde in 1932. Urey was awarded the Nobel Prize in Chemistry in 1934 "for his discovery of heavy hydrogen". He declined to attend the ceremony in Stockholm, so that he could be present at the birth of his daughter Mary Alice.

Working with Edward W. Washburn from the Bureau of Standards, Urey subsequently discovered the cause of the anomalous sample. Brickwedde's hydrogen had been separated from water by electrolysis, resulting in a depleted sample. Moreover, Francis William Aston had reported that his calculated value for the atomic weight of hydrogen was wrong, thereby invalidating Birge and Menzel's original reasoning. The discovery of deuterium stood, however.

Urey and Washburn attempted to use electrolysis to create pure heavy water. Their technique was sound, but they were beaten to it in 1933 by Lewis, who had the resources of the University of California at his disposal. Using the Born–Oppenheimer approximation, Urey and David Rittenberg calculated the properties of gases containing hydrogen and deuterium. They extended this to enriching compounds of carbon, nitrogen, and oxygen. These could be used as tracers in biochemistry, resulting in a whole new way of examining chemical reactions. He founded the Journal of Chemical Physics

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