Abstract
LONDON. Royal Society, June 16.—F. W. Aston: A new massspectrograph and the whole-number rule (Bakerian Lecture). By means of the first mass-spectrograph, built in 1919, the masses of all atoms, with the exception of hydrogen, were shown to be whole numbers on the oxygen scale, to one or two parts in 1000. In order to measure their divergence a more powerful instrument was necessary. This has been made, with a resolving power of 1 in 600, more than sufficient to separate the mass lines of the isotopes of any known element, and with an accuracy of measurement as high as 1 in 10,000. By means of this instrument the isotopic constitution of mercury has been decided, new isotopes discovered in sulphur and tin, and the two doubtful isotopes of xenon confirmed. 51 types of atom contained in 18 different elements, ranging from hydrogen to mercury, have been examined. Their masses and packing fractions, i.e. their percentage divergence from the whole numbers expressed in parts per 10,000, are tabulated on the oxygen scale; e.g., the atom of phosphorus of mass number 31 has a packing fraction -5.6±1.5 and a mass 30-9825. The relations of tin and xenon have been re-examined and found not to show the striking abnormality previously suggested. The values for Li6 and Li are obtained by a recalculation of Costa's results. When the packing fractions of the atoms are plotted against their mass numbers, for all atoms above mass number 20 these lie roughly on a single curve. From mercury, packing fraction +0.8, the curve descends to -9 in the region of bromine. It then ascends, and in the case of atoms of odd atomic number continues to do so, in a roughly hyperbolic manner, right up to hydrogen + 77.8. The light atoms of even atomic number have packing fractions well below this curve, and approximate to a branch rising much less steeply to helium + 5.4. This suggests that the light atoms of odd atomic number have a common loosely packed, and therefore heavy, outside structure, which is not present in the denser and more stable nuclei of helium, carbon, and oxygen.
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Societies and Academies. Nature 119, 945–948 (1927). https://doi.org/10.1038/119945a0
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DOI: https://doi.org/10.1038/119945a0