Oxygen Isotopes in Nature and in the Laboratory Author(s): Harold C. Urey Source: Science, New Series, Vol. 108, No. 2810 (Nov. 5, 1948), pp. 489-496 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1677444 Accessed: 22/10/2008 09:24 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=aaas. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org Oxygen Isotopes in Nature and in the Laboratory Harold C. Urey Institute for Nuclear Studies, University of Chicago T HAT THE ELEMENTS CONSIST OF The interest in these varieties of atomic species is ATOMS having different atomic weights was of several kinds. In the first place, there are the discovered in 1911 by Fayans and Soddy as a purely physical properties which physicists are study- result of the study of radioactive elements which oc- ing today. During the last 25 years particularly it cur in nature. It was found that two varieties of has been possible to unravel the laws governing the elements could be secured having different radioactive behavior of the electrons in the outer shells of the properties but otherwise chemically identical in every atom, and at the present time the center of the stage respect. The theory of radioactive disintegration of uranium and thorium showed that these varieties of TABLE 1 atoms must have different atomic weights. In par- ISOTOPES OF THE FIRST 10 ELEMENTS ticular it was as a result of the careful found, very Symbol Atomic No. Masses atomic weight determinations of Theodore Richards, that samples of leads from different parts of the H(D) 1 1, 2, 3 earth's crust had different atomic in He 2 3, 4, 6 weights and, par- Li 3 6, 7, 8 ticular, that samples of lead isolated from thorium Be 4 7, 9, 10 minerals had atomic weights approaching 208, whereas B 5 10,11, 12 C 6 14 those from uranium minerals had atomic 10,11, 12, 13, isolated N 7 13, 14, 15, 16 weights approaching 206. Sir J. J. Thompson showed 0 8 15, 16, 17, 18, 19 F that charged atoms moving through cross electrical 9 17, 18, 19, 20 Ne 10 19, 20, 21, 22, 23 and magnetic fields should be separated if they have different ratios of charge to mass, and hence the iso- of is held the with the topes of the elements should be separated by such a physics by questions dealing structure of the nucleus of atoms. How are the fun- process. He determined that this was the case for damental held and what are the the element neon, which is nonradioactive, and hence parts together, energy relations The which are in showed that ordinary elements also consist of mixtures properties important connection with this are the lifetimes of the radioac- of atoms of different atomic weights. tive the It is now known that there are more than 600 varie- nuclei, energy of binding of the elementary particles, the nuclear or nuclear mo- ties of atoms which make up our 96 known elements. spins angular and the moments. are NMostof these are radioactive and have been produced menta, magnetic Physicists interested in the relative abundances of those atoms by artificial means. We have found that a few ele- which are how these varieties of atoms were ments exist only in radioactive form and thus are con- stable, and what was the origin of the universe. staiitly disappearing and have been doing so since the produced, .earth's crust was created. Some have completely dis- USES OF ISOTOPICSPECIES appeared from the earth's crust and therefore must be But there are uses of these made in our laboratories today. In this discussion I many isotopic species for other sciences and such uses have been discussed :shall largely limit myself to one of the ordinary ele- times in recent The use as tracers or :ments, namely, oxygen. Table 1 shows a very limited many years. indicators was discovered Prof. over 30 number of these varieties of atoms, those for the first by Hevesy and the 10 elements of the present periodic system. The stable years ago, general outlines of the possible uses of such elements were made him isotopes are printed in heavy type and the radioactive by using naturally radioactive elements. It of .ones in light type. These elements are very important is, course, natural that the discovered artificial indeed, since they contain several of the elements nec- many recently radioactive ele- ments should be useful in this .essary for living processes. Also, oxygen is the most connection, especially in view of the fact that abundant element in the surface layer of the earth, effective radioactive tracers for of the common :namely, the layer with which we come most in contact. many elements have now been discov- ered. There are few for which there are no satisfac- on the of This lecture was presented evening September tory tracers of this but it has been to 15 in the Presidential Ballroom of the Statler Hotel, Wash- kind, possible ington, D. C., during the AAAS Centennial Celebration. separate the stable isotopes in the case of those ele- ;SCIENCE, November 5, 1948, Vol. 108 489 ments for which no radioactive tracers are known, and uranium. If all the lead 207 in the crust of the earth hence tracers for these elements, hydrogen, nitrogen, was produced from uranium 235, and none was pres- oxygen, and carbon, are also available. The use of ent at the time the elements of the earth were formed, these tracers is particularly valuable in the case of the then, knowing the amount of uranium 235 and lead study of living processes. We have in living organ- 207 in the crust of the earth now and the rate of trans- isms an enormous complexity of chemical substances formation, we can calculate the maximum time which and chemical reactions, and every possible method that has elapsed since the creation of our elements as can be devised is of value if we are to understand these somewhat greater than 5,000,000,000 years. processes. During recent years we have heard par- Prof. A. 0. Nier, of the University of Minnesota, has ticularly of the use of such tracers of hydrogen, car- shown that the carbon in nature varies in isotopic bon, nitrogen, and oxygen, since these are especially composition. Living organisms, plants, and animals of their important elements in living organisms, as well as together with all probable geological remains, natural and contain less phosphorus, sulfur, iodine, sodium, potassium, and namely, petroleum, gas, coal, The of this iron, which are present only in comparatively small carbon 13 than does limestone. origin amounts but which are nevertheless very important fractionation of carbon isotopes is not exactly known. Differences in of the were for the vital processes taking place. chemical properties isotope in neon the There are other uses to which isotopes have been first detected the case of and hydrogen isotopes. It was shown during the 1930s that differ- put besides those which we have indicated. The radio- ences in the of active isotopes occurring in nature have been useful thermodynamic properties isotopic should exist because of the differences in in establishing the geological time scale on a quanti- compounds tative basis. Uranium and thorium decompose into vibrational frequency of molecules, depending upon whether contain one or another. The lead and helium, and it is possible to measure the rate they isotope the chemical of iso- of this transformation. Uranium produces lead of early observation that properties of elements were identical is an atomic weight of 206 and thorium of lead of 208. topes only approximately true. careful measurements have shown that Thus, if we measure the amount of thorium and the Very and amount of lead 208 in a mineral containing thorium, there are slight differences in chemical properties that these are to be the basis of our it is possible to calculate the time required for the expected upon modern of the of such amount of thorium present to produce the amount of knowledge physical properties molecules. It is probable that the fractionation of the lead present, and the age of the mineral could not be in nature as obtained Nier has its longer than the time so calculated. On the other carbon isotope by in effects. It also be that this hand, if we measure the amount of uranium and lead origin partly such may of chemical reaction.