World of Scientific Discovery on Harold Clayton Urey
Urey was born in Walkerton, Indiana, on April 29, 1893. He earned a bachelor's degree in zoology from Montana State University in 1917 and a Ph.D. in chemistry from the University of California in 1923. From 1923 to 1924, he did post-graduate research on the theory of atomic structure under Niels Bohr at the Institute of Theoretical Physics in Copenhagen, Denmark. He held teaching positions at the University of Montana (1919-1921), Johns Hopkins University (1924-1929), Columbia University (1929-1945), the University of Chicago (1945-1952), and the University of California (1952-1981).
Urey received the Nobel Prize in chemistry in 1934 for his discovery of deuterium, the isotope of hydrogen with atomic weight of 2. Frederick Soddy's discovery of isotopes in 1913 had raised the possibility that an isotope of hydrogen other than the familiar hydrogen-1 might exist. That possibility intrigued scientists from all fields. Hydrogen is the simplest of all elements with atoms that contain only a single proton and a single electron. Atoms of a " heavy" hydrogen would also have to contain at least one neutron, giving them a mass twice as great as that of ordinary hydrogen, also known as protium. Scientists were curious to find out how such a relatively massive atom would differ from its smaller twin.
Urey began his research with the assumption that the vapor pressure of heavy hydrogen would be less than that of ordinary hydrogen. He theorized that if he allowed a large volume of liquid hydrogen to evaporate over time, the liquid that remained would be richer in the heavier isotope. With two colleagues, F. G. Brickwedde and G. M. Murphy, Urey carried out this experiment in 1931. Four liters of liquid hydrogen were allowed to evaporate until only a single milliliter was left. Spectroscopic analysis of the remaining liquid revealed two very faint lines that were not part of the spectrum of ordinary hydrogen. Urey and his colleagues announced that these lines confirmed the presence of heavy hydrogen, which they named deuterium.
Based on his work with deuterium, Urey next turned his attention to isotopes of other elements. He found that the chemical properties of isotopes differ slightly from each other. He then used this information to separate isotopes of carbon, oxygen, nitrogen, and sulfur.
This knowledge soon proved to have an important practical application. One of the most challenging tasks in the construction of the first nuclear weapons and nuclear reactors was the separation of two uranium isotopes, uranium-235 and uranium-238. The former will undergo nuclear fission and, thus, can be used in bombs and reactors. The latter cannot. During World War II, Urey's research team at Columbia worked on methods for separating the heavier isotope (U-238) from the lighter isotope (U-235).
After the war, Urey turned his attention to another question involving isotopic differences. He was able to show that the amount of oxygen-18 (a rare isotope of oxygen) incorporated into sea shells was affected by the ocean's temperature. When the oceans are warm, the percentage of oxygen-18, compared to that of ordinary oxygen-16, is larger than it is when the oceans are cool. He used this discovery to develop a method for estimating ocean temperatures as far back as the Jurassic period. This research on paleotemperatures (ancient temperatures) led Urey in yet another direction.
In the early 1950s, he tried to estimate the abundance of the chemical elements in the earth, the Sun, and other stars. His work, in cooperation with that of Hans Suess, produced values that are close to those now accepted. Urey's studies of elemental abundances prompted him to think about the origins of the earth and of the solar system. He came to the conclusion that the Earth's early atmosphere was a reducing environment, much like that of present-day Jupiter, consisting of hydrogen, methane, and ammonia. One of his students, Stanley Miller, was able to demonstrate that electrical discharges in such an atmosphere result in the formation of simple organic compounds, like those needed in the development of simple life forms. Urey also proposed a theory about the formation of the solar system. He suggested that a disk of gaseous matter, rotating around the X-ray's equator, may have broken apart and formed the planets and their satellites. Possibly as a result of these studies, in his later life Urey became enthusiastic advocate of space exploration.
Urey died in La Jolla, California, on January 5, 1981.
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