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Under the proper conditions, however, say in a nuclear reactor or particle accelerator or in the interior of a star, even stable isotopes may be transformed, one into another.The ease or difficulty with which these nuclear transformations occur varies considerably and reflects differing degrees of stability in the isotopes. (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.

A bar of pure uranium, for instance, would consist entirely of atoms with atomic number 92. These substances were thought to be elements and accordingly received special names.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. Uranium ores, for example, yielded ionium, and thorium ores gave mesothorium.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, A59–480 (1995); K. Painstaking work completed soon afterward revealed, however, that ionium, once mixed with ordinary thorium, could no longer be retrieved by chemical means alone.With considerable prescience, he extended the scope of his conclusion to include not only radioactive species but stable elements as well.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.

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