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.
The lead from the uranium-rich ore had an average atomic mass of 206.08 compared to 207.69 for the lead from the thorium-rich ore, thus verifying Soddy’s conclusion.
The unambiguous confirmation of isotopes in stable elements not associated directly with either uranium or thorium followed a few years later with the development of the Francis William Aston. Thomson, Aston had learned that the gaseous element neon produced two positive rays.
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.
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.
In fact, it is precisely the variation in the number of neutrons in the nuclei of atoms that gives rise to isotopes. The three share the place in the periodic table assigned to atomic number 1 and hence are called isotopes (from the Greek Sources: G. As chemists used the criterion of chemical indistinguishability as part of the definition of an element, they were forced to conclude that ionium and mesothorium were not new elements after all, but rather new forms of old ones.
With considerable prescience, he extended the scope of his conclusion to include not only radioactive species but stable elements as well.
A bar of pure uranium, for instance, would consist entirely of atoms with atomic number 92. Uranium ores, for example, yielded ionium, and thorium ores gave mesothorium.
The periodic table of the elements assigns one place to every atomic number, and each of these places is labeled with the common name of the element, as, for example, calcium, radon, or uranium. Painstaking work completed soon afterward revealed, however, that ionium, once mixed with ordinary thorium, could no longer be retrieved by chemical means alone.
The ease or difficulty with which these nuclear transformations occur varies considerably and reflects differing degrees of stability in the isotopes.
Accordingly, it is important and useful to measure stability in more quantitative terms.