By 1919 he had done so and convincingly argued for the existence of neon-20 and neon-22.Information from his and other laboratories accumulated rapidly in the ensuing years, and by 1935 the principal isotopes and their relative proportions were known for all but a handful of elements.Not all the atoms of an element need have the same number of neutrons in their nuclei. Three nuclei with one proton are known that contain 0, 1, and 2 neutrons, respectively. Wapstra, "The 1995 Update to Atomic Mass Evaluation," Nuclear Physics A595, 409–480 (1995). Similarly, mesothorium was shown to be chemically indistinguishable from radium.
A few years later, Soddy published a comparison of the atomic masses of the stable element lead as measured in ores rich in uranium and thorium, respectively.
He expected a difference because uranium and thorium decay into different isotopes of lead.
Modeled on an analogy to a liquid drop, the first term represents the favourable contribution to the binding of the nucleus made by short-range, attractive nuclear forces between neutrons and protons.
The second term corrects the first by allowing for the expectation that nucleons at the surface of the nucleus, unlike those in the interior, do not experience forces of nuclear attraction equally from all sides.
The fourth term makes a small correction to the third.
This correction is necessitated by the observation that the nuclear charge distribution becomes somewhat more spread out near the surface of the nucleus.A uniform scale of nuclear stability, one that applies to stable and unstable isotopes alike, is based on a comparison of measured isotope masses with the masses of their constituent electrons, protons, and neutrons.For this purpose, electrons and protons are paired together as hydrogen atoms.The total separate masses of 6 electrons and 6 protons (treated as 6 hydrogen atoms) and of 6 neutrons add up to 12.09894 amu. The numerical values of these terms do not come from theory but from a selection process that ensures the best possible agreement with experimental data.On the other hand, theory helps justify, at least qualitatively, the mathematical form of each term.The ease or difficulty with which these nuclear transformations occur varies considerably and reflects differing degrees of stability in the isotopes.