Aston isotopes

Francis Aston, the inventor of mass spectrometry, discovered that for several elements the atomic mass is not the mass of all atoms of the element. Instead, the atomic mass is a weighted average of two or more isotopes of different masses. The concept of isotope had been introduced earlier, in connection with radioactive elements. Aston's work extended it to stable elements with well established atomic masss.)

1) Aston obtained the first such results with neon, results which held up as he reported them, although he was careful to note that the initial data were equivocal:

... results were obtained which indicated atomic weights roughly 20 and 22 respectively, the accepted atomic weight being 20.2. This naturally led to the expectation that neon might be a mixture of isotopes, but the weight 22 might possibly be due to other causes, and the method of analysis did not give sufficient accuracy to distinguish between 20 and 20.2 with certainty.
If neon is a mixture of two isotopes of mass 20 and 22, what must the proportions of these two isotopes be to produce an atomic mass of 20.2?

2) Aston next turned his attention to chlorine:

The next element investigated was chlorine; this is characterized by four strong lines 35, 36, 37, 38, and fainter ones at 39, 40; there is no trace of a line at 35.46, the accepted atomic weight. From reasoning which cannot be given here in detail it seems certain that chlorine is a complex element, and consists of isotopes of atomic weights 35 and 37, with possibly another at 39. The lines at 36, 38 are due to the corresponding HCl's.
If chlorine is a mixture of two isotopes of mass 35 and 37, what must the proportions of these two isotopes be to produce an atomic mass of 35.46?

3) Aston mentioned only the positions of mass spectrograph peaks in his paper. Is there additional information a more modern mass spectrometer could provide that might be used to test the fractional abundances computed here?


Francis Aston, "Isotopes and atomic weights," Nature 105, 617 (1920).
Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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