Isotopy, A General Chemistry Experiment Finding an interesting and informative laboratory experiment suitable for eedy in the first semester of general chemistry is, to say the least, difficult because of t.he students' lack of hackground in chemistly. I would like to describe an inexpensive paper-and-pencil experiment which has been used successfully as the first experiment in our introductory course for nonmtljors. First, the concept of isotopy is described in s. brief 1a.bborstory-lecture. Included is 1111 illustration of the spectral evidence for belief in the existence of isotopes, i.e., x slide of the Hg mtLss spectrum. Then each student is presented with a. recording of the mass spectrum of the isotopes of an unknown element and asked to identify the element. Only the direction of increasing mass is indicated on the spectrum. The identification consists of measuring with a ruler the heights of each of the spectral peaks, summing these and then dividing each height by the sum so as to obtain the relative shundance of each species. In addition, the pattern of missing peaks is noted. From this information the student is able to identify the element by comparison with isotope abundance and distribution data from the literature. A particularly useful source of such data is the Chart of the Nuclides, available free from Education Relations, General Electric Co., Sehenectady, N.Y. 12305. We have recorded the isotope spectra of approximately 25 elements using our Consolidated Electrodynamics Corporation Model 21-103 mass spectrometer. By recording at four known sensitivities simultaneously the speclra include all species more than 0.1% in abundance approximately. Students seeking a bit more challenge are given simple molecular trpectrs, e.g., CIS, the interpretation of which requires an understanding of elementary probability theory (see MARGRAVE, J. L. AND POLANSKY, R. B., J. CHEM.EDUC.,39, 335 (1962)). The experiment also illustrates the lack of stability of certain nuclides, the meaning of significant figures, and the detection-limit problem encountered by experimentalists. Two to three hours are required for the experiment, including the 30-min lecture. Persons interested in obtaining sets of these spectra, each an a. S1/l X ll-in. paper, are asked to contact the author. +
MARQUETTE UNIVERSITY MILWAUKEE, WISCONSIN 53233
THOMAS C. EHLERT
Volume 48, Number 4, April 1971
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