Isotope …, "m" are the atomic masses of each individual isotope, and "x", …, "x" are the relative abundances of these isotopes. Several applications exist that capitalize on properties of the various isotopes of a given element. Isotope separation is a significant technological challenge, particularly with heavy elements such as uranium or plutonium. Lighter elements such as lithium, carbon, nitrogen, and oxygen are commonly separated by gas diffusion of their compounds such as CO and NO. The separation of hydrogen and deuterium is unusual because it is based on chemical rather than physical properties, for example in the Girdler sulfide process.
Mononuclidic element Mononuclidic element A mononuclidic element or monotopic 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