Collected by Alfred B. Garrett Ohio State University
Columbus
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The Flash of Genius, 7
The Discovery of the Oxygen Isotopes:
W. F. Giauque and H. 1. Johnston
In 1905 oxygen was chosen as the standard for atomic weights. At that time it seemed to he the ideal standard. Less then 25 years later a disoovery- was made that oxygen in nature is not pure as ' 0 0 but has an appreciable concentration1 of 180 and 8. small amountPof "0. ~t once this gave two conflicting standards for atomic weights-the one of the chemists who used 0 = 16.0000 for the oxygen in nature, and the one of the physicists who used 0 = 16.0000 far the "0 isotope. Here is the story of how the discovery of theisotopesof oxygen by a graduate student, Herriok L. Johnston, and his preceptor, Professor William F. Giauque threw this complication into the whole soheme of international atomic weight standards. Johnston was doing his doctoral theais research on the thermodynamics of oxygen. He was measuring the heat capacity of oxygen from liquid helium temperature to room temperature. He nee&d spectroscopic data on oxygen from which to calculate entropy values. Just previous to this time a paper had appeared on the "Absorption Spectra of Oxygen in the Earth's Atmosphere,"= in which the authors described their success in interpreting most of the lines in the spectra hut concluded-"There is still a small number of unclessified faint lines of atmospheric origin, but it is not certain whether they belong to these hands. They may he due to water vapor." This became the clue for Giauque and Johnston to make their discovery--here m e excerpts from their careful analysis of these data of Dieke and Babcock:'
"In connection with our study of the entropies of gases we have recently considered the available spectroscopic data for oxygen. The atmospheric absorp-
' GIAUQUE,w. F.,A N 0 JOHNSTON, H.L., J. Am. Chon. SOC., 51, 1436 (1929).
GIA~QUE, W. F., AND JOHNSTON, H. L., J. Am. Chem. Soe., 51, 3528 (1929). a DIEKE G. H., AND BABCOCH, H. D., PTOC. Nat. Acad. s&.,13, 676 (1927).
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Journal o f Chemical Education
tion bands of oxygen contain the necessary information concerning the rotation levels of the oxygen molecule bnt we found that no completely satisfactory interpretation of these bands has been given. . . . "Our principal problem mas to decide whether the A and the A' bands originated from a common source or not. We tried many ways of combining the lines both within the strong A and the hands, respectively, and also combining weak with strong but could find no scheme that would account for the bands arising from a single molecular form. "It occurred to us that the A' band might result from an isotope of oxygen and we have found that it is fully explained as originating from an oxygen molecule consisting of an atom of mass 16 combined with an atom of mass 18. Such an isotope has not previously been observed but its existence in small amount has certainly not been disproved. Since it is of practically the same mass as water, it might easily be misinterpreted in a mass spectrograph. "We are thus forced to conclude that an isotope of oxygen, mass 18, exists in the earth's atmosphere. From the relative intensities of the bands the amount must be very small and would thus be difficult to detect in a mass spectrograph. It seems possible that the 16-18 molecule mieht be a more efficient absorber of radiation than t h e 16-16 molecule due to the slight polarity introduced by the zero point vibration; thus relative intensities may be taken as an upper limit to the amount of 16-18 oxygen present."
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