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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.

Generalizing from these and other data, English chemist Frederick Soddy in 1910 observed that “elements of different atomic weights [now called atomic masses] may possess identical (chemical) properties” and so belong in the same place in the periodic table.

With considerable prescience, he extended the scope of his conclusion to include not only radioactive species but stable elements as well.

The last term, the so-called pairing energy, takes on any one of three values depending on whether ).

More-detailed treatments sometimes give other values for δ as well.

Accordingly, it is important and useful to measure stability in more quantitative terms.

His work grew out of the study of positive rays (sometimes called canal rays), discovered in 1886 by Eugen Goldstein and soon thereafter recognized as beams of positive ions. The ions in the heavier ray had masses about two units, or 10 percent, greater than the ions in the lighter ray.

To prove that the lighter neon had a mass very close to 20 and that the heavier ray was indeed neon and not a spurious signal of some kind, Aston had to construct an instrument that was considerably more precise than any other of the time.

(Authors who do not wish to use symbols sometimes write out the element name and mass number—hydrogen-1 and uranium-235 in the examples above.)The term is used to describe particular isotopes, notably in cases where the nuclear rather than the chemical properties of an atom are to be emphasized.

U to an isotope of uranium widely used for nuclear power generation and nuclear weapons fabrication.

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