Harold Clayton Urey
1893-1981
American Chemist
Harold Clayton Urey is best known for his work with isotopes. He received the Nobel Prize for Chemistry in 1934 for the discovery and isolation of deuterium, a heavy isotope of hydrogen. He also played a major role in the development of the atomic bomb and made important contributions to the study of isotopes of a number of elements and to the theory of the origin of the planets.
After receiving his doctorate at the University of California at Berkeley, Urey worked with Niels Bohr (1855-1962) in Denmark. Returning to the United States, he held positions at Johns Hopkins University, Columbia University, the Institute of Nuclear Study, the University of Chicago, and the University of California.
Isotopes of an element have the same number of protons (i.e., atomic number) but a different number of neutrons—and, therefore, different mass numbers. Much of Urey's research concerned the heavy isotope of hydrogen known as deuterium. The most common form of hydrogen is the simplest atom that can exist, having one proton in its nucleus and one associated electron. Its atomic number is 1, and its mass number is 1. The deuterium atom contains a neutron in its nucleus in addition to a proton. Its atomic number is 1, but its mass number is 2. Since both hydrogen isotopes have one electron and one proton, they are virtually identical in their chemical reactivity, but properties that are dependent on mass are significantly different. Urey noticed that the mass of hydrogen gas isolated from naturally occurring substances is minutely larger than would be expected if only hydrogen with mass number 1 was present. He proposed the existence of deuterium to explain this observation. In 1931 he announced the discovery of heavy water, which contains two atoms of deuterium instead of two "normal" hydrogen atoms. In 1931 Urey announced the production and isolation of pure deuterium, which he obtained by successive distillations of liquid hydrogen.
Urey was awarded the Nobel Prize for Chemistry in 1934 for his work on deuterium. He subsequently applied the methods that he had developed for the study of the isotopes of hydrogen to the separation and study of the isotopes of carbon, nitrogen, oxygen, and sulfur.
During World War II, Urey's expertise in the separation of isotopes resulted in his playing an important role in the Manhattan Project, the successful American effort to develop an atomic bomb. He directed the program at Columbia University that developed a method, employing the gaseous diffusion of uranium hexafluoride, for separating the radioactive isotope of uranium (U-235), which could be used in the bomb's fission reaction, from its more abundant isotope (U-238). After the war, he spoke out against thedanger inherent in the use of nuclear power, especially nuclear war.
His work with isotopes led to a study of the abundance of naturally occurring isotopes on earth, resulting in a theory of the origin of the elements and their relative abundance in the sun and on other planets in the solar system. His research led him to propose that the earth's atmosphere was once made up of ammonia, methane, and hydrogen, and that reactions among these molecules could lead to the production of living entities. One of his students, Stanley Miller (1930- ), successfully demonstrated that when energy is supplied to a mixture of these gases, biological molecules are indeed produced. Theoretically, at least, these molecules could then interact to build living systems. Urey proposed further theories that explain the origin of the solar system as a condensation of gasses around the sun. He published the results of this theoretical work in The Planets: Their Origin and Development (1952).
Electrophoresis Cracks the Case
Every person (except for identical twins) has a unique set of genetic material in the form of DNA. A technique called DNA fingerprinting takes advantage of this individuality. To create a DNA fingerprint, scientists must first acquire a sample of a person's cells, such as from blood, hair, or skin. After the DNA is removed from the cells, special proteins are added to it that cut the extremely long DNA molecules into fragments. The next step is to separate the fragments from one another based on their size. This is done by using elec trophoresis—the technique Arne Tiselius helped to develop. Elec trophoresis separates the fragments into bands. The pattern formed by the bands is different for each person.
The technique of DNA fingerprinting is sometimes used in criminal investigations. If a blood or skin sample is found at a crime scene, for example, a DNA fingerprint can be made from these cells. This fingerprint can then be compared to that of a suspect. If the patterns of the bands are identical, the results indicate that the suspect was present at the crime scene. If the patterns of the bands differ, the results prove that someone other than the suspect was the source of the crime-scene sample.
STACEY R. MURRAY
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