nuclides of the same element, i.e. with the same number of protons, but having different number of protons, hence having different atomic mass, i.e. 123I, 124I, 125I, 131I. The number of natural elements amounts 97, to which 21 artificial elements must be added.. In most cases only a small number of the isotopes are stable, the other unstable forms are called radioisotopes or radionuclides. Out of the 118 elements currently known, 36 exist only in an unstable, that is to say radioactive form.
    This is the case for uranium, plutonium and radium, but also technetium and astatine. There are so far more than 3,500 identified radionuclides among which about 200 could play a role in nuclear medicine.