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Summary Article: Urey, Harold Clayton (1893-1981) from The Hutchinson Dictionary of Scientific Biography

Place: Ulugh Beg

Subject: biography, chemistry

US chemist who in 1931 discovered heavy water and deuterium, the isotope of hydrogen of mass 2. For this extremely significant discovery, which was to have a profound effect on future research in chemistry, physics, biology, and medicine, he was awarded the 1934 Nobel Prize for Chemistry.

Urey was born in Walkerton, Indiana, on 29 April 1893 and educated at schools in De Kelb County, Kendallville, and Walkerton. He graduated from high school in 1911 and worked as a schoolteacher for three years in Indiana and Montana. He went to Montana State University, Missoula, in 1917, gaining his BS degree three years later. During 1918 and 1919 he worked as a research chemist at the Barrett Chemical Company in Philadelphia, where he helped to produce war materials. From 1919 to 1921 he was an instructor in chemistry at Montana State University, and then went to the University of California in Berkeley, where he developed his interest in physical and mathematical chemistry. He received his PhD in 1923 and during the following year was a fellow of the American Scandinavian Foundation and attended the Institute of Theoretical Physics at the University of Copenhagen. He studied there under Niels Bohr, who was engaged on his pioneering work on the theory of atomic structure.

After Urey returned to the USA he worked for five years as an associate in chemistry at Johns Hopkins University in Baltimore, Maryland. Then from 1929 to 1934 he was associate professor of chemistry at Columbia University, New York City, and was Ernest Kempton Adams Fellow there 1933-36. He was appointed full professor of chemistry in 1934, and was the executive officer of the chemistry department 1939-42. During World War II he was director of research of the Substitute Alloy Materials Laboratories at Columbia, which became part of the Manhattan Project for the development of the atomic bomb. In 1945 Urey became professor of chemistry at the Institute of Nuclear Studies at the University of Chicago, and from 1952 to 1958 was Martin A Ryerson Professor of Chemistry there. In 1958 he was named professor-at-large of chemistry at the University of California in La Jolla; he was also a member of the Space Science Board of the Academy of Sciences. He died in La Jolla on 5 January 1981.

Urey discovered deuterium (heavy hydrogen, symbol D) in 1931 with F G Brickwedde and G M Murphy. He predicted that it would be possible to separate hydrogen from HD (whose molecules contain one hydrogen atom and one deuterium atom) by the distillation of liquid hydrogen, taking advantage of the difference in their vapour pressures. In heavy water, D2O, both hydrogen atoms of normal water (H2O) are replaced by deuterium atoms. Two years after its discovery by Urey, the US chemist Gilbert Newton obtained nearly pure heavy water by fractional electrolysis of water. Today it is manufactured by a process that involves isotopic chemical exchange between hydrogen sulphide and water. Its chief use is as a moderator to slow down fast neutrons in a nuclear reactor.

Urey was one of the first to calculate thermodynamic properties from spectroscopic data, particularly the equilibrium constants for exchange reactions between isotopes. He went on to isolate heavy isotopes of carbon, nitrogen, oxygen, and sulphur. Urey's group provided the basic information for the separation of the fissionable isotope uranium-235 from the much more common uranium-238 by gaseous diffusion of their fluorides. After World War II he worked on tritium for use in the hydrogen bomb.

The evolution of the Earth was another topic that exercised Urey's mind. It was traditionally believed a molten Earth had formed by processes similar to those that occur in oil-smelting furnaces. Today there is evidence to suggest that the Earth and other planets were formed by condensation and accumulation from a dust cloud at low temperatures. Urey theorized that the final accumulation of the Earth had occurred at 0°C/32°F from small planetary particles containing metallic iron, carbon, iron carbide, titanium nitride, and some ferrous (iron(II)) sulphide. He considered that most gases had been lost during the previous high-temperature phase, leaving a primitive atmosphere consisting of hydrogen, ammonia, methane, water vapour, nitrogen, and hydrogen sulphide. In 1952 he suggested that some of these molecules could have united spontaneously to form the basic ‘building blocks’ of life. The iron core of the Earth would have accumulated slowly through geological history from a mixture of metallic iron and silicates, meaning that the Earth was not molten at the time when its materials had accumulated. Urey also contributed to theories about the origin of the Moon, subscribing to the view that it had not formed from the Earth but had a separate origin.

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