A
mononuclidic element is one of the 22
chemical elements that is found naturally on Earth essentially as a single
nuclide (which may, or may not, be a
stable nuclide). This single nuclide will have a characteristic
atomic mass. Thus, the element's
natural isotopic abundance is dominated either by one stable
isotope or by one very long-lived isotope. There are 19 elements in the first category (which are both monoisotopic and mononuclidic), and 3 (
bismuth,
thorium and
protactinium) in the second category (mononuclidic but not monoisotopic, since they have zero, not one, stable nuclides). A list of the 22 mononuclidic elements is given at the end of this article.