Summary
Francis William Aston's "The Whole-Number Rule (1925)" presents the central thesis that the atomic weights of all pure isotopes are close to whole numbers on the oxygen-16 scale, with deviations explained by the mass defect from nuclear binding energy. Aston, who invented the mass spectrograph, systematically measured isotopic masses and demonstrated that isotopes of the same element have identical chemical properties but distinct integer masses. The book consolidates his experimental evidence showing that atomic weights are not arbitrary but arise from mixtures of isotopes, each with nearly integral mass numbers. A reader takes away a foundational understanding of isotopic composition, the precision of mass spectrometry, and the rule that underpins nuclear physics and the concept of mass-energy equivalence.
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Key concepts
- Mass spectrograph — An instrument invented by Aston that separates ions by their mass-to-charge ratio, enabling precise measurement of isotopic masses.
- Isotope — Atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different atomic masses.
- Mass defect — The difference between the mass of an atomic nucleus and the sum of its constituent protons and neutrons, accounted for by nuclear binding energy.
- Whole-number rule — The empirical observation that isotopic masses are close to integers when expressed relative to oxygen-16, with deviations due to the mass defect.
- Packing fraction — A measure of the deviation of an isotope's mass from a whole number, defined as (mass - mass number) / mass number, used to compare nuclear stability.