The set of monoisotopic elements overlaps with, but is not the same as, the set of 21
mononuclidic elements, which are characterized as having essentially only one isotope (nuclide) found in nature. The reason for this is the occurrence of certain long-lived radioactive
primordial nuclides in nature, which may be of a monoisotopic element, and thus prevent them from being naturally mononuclidic. This happens in the cases of seven of the monoisotopic elements. These elements are monoisotopic, but due to the presence of a long-lived radioactive primordial nuclide, not mononuclidic. These cases are
vanadium,
rubidium,
indium,
lanthanum,
europium,
lutetium, and
rhenium, and for indium and rhenium, the long-lived radionuclide is even the more abundant isotope in nature, and the stable isotope the less. In two additional cases (
bismuth and
protactinium), elements that are mononuclidic by definition are not monoisotopic because the naturally occurring nuclide is radioactive, and thus the element has no stable isotopes at all. For an element to be monoisotopic, it must have one stable nuclide. == List of stable isotopes of monoisotopic elements ==