单一同位素元素是指只有一个稳定同位素的元素。

另外，铋209，钍232及镤231是有一个自然界的同位素的，但其为放射性，故并不符合资格。

A monoisotopic element is one of 26 chemical elements which have only a single stable isotope (nuclide). A list is given in a following section.

Stability is experimentally defined for chemical elements, as there are a number of stable nuclides with atomic numbers over ~ 40 which are theoretically unstable, but apparently have half-lives so long that they have not been observed directly or indirectly (from measurement of products) to decay.