Level: introductory
Reference: Francis Aston, "Isotopes and atomic weights," Nature 105, 617 (1920).
Notes: Francis Aston (1877-1945) worked in the laboratory of J. J. Thomson (best known for characterizing cathode rays, identifying the particle we now call the electron) at Cambridge University. Aston's work on cathode ray tubes focused on "positive rays", that is on the cations formed when electrons were stripped from the gases in the tube. Unlike the "negative rays" (also known as cathode rays, that is, the electrons), which were the same extremely light particles no matter what the tube contained, the positive rays were much more massive and were characteristic of the contents of the tube. Separating these particles on the basis of their mass-to-charge ratio is essentially what a mass spectrometer does. As exercise 3 attempts to suggest, the intensities of peaks in a mass spectrograph can also provide information about isotopic abundances; however, quantitative intensities were not available to Aston.
Historical footnote: Aston's first preliminary results on neon date back to 1913, the same year as Frederick Soddy proposed the term "isotope." The concept of isotopes really came out of the study of radioactivity; however, the extension of the concept to stable elements came about almost immediately. (See an annotated version of Soddy's 1913 review on radioactivity for more on isotopes.)
Pedagogical footnote: Instructors concerned about significant figures should feel free to make the peak positions 20.0 and 22.0 in exercise 1 and 35.00 and 37.00 in exercise 2. I infer that Aston's instruments were capable of this resolution by the statement that he did not find peaks at 20.2 and 35.46.
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/aston.doc
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