What is an element with one stable isitope?
An element is considered to have one stable isotope by CIAAW if it has one and only one isotope that is either stable or has a half-life greater than 1 x 1010 a. At various times, the term “mononuclidic” has been used synonymously with “monoisotopic”; similarly, radioactive isotopes have been referred to as “radionuclides” or “radioisotopes”. Thus, CIAAW considers Th to have one stable isotope because 232Th has a half-life of 1.4 x 1010 a and the half-lives of the other isotopes of Th are all less than 1 x 1010 a. In contrast, Pa is not classified as an element with one stable isotope because the half-life of the most abundant isotope 231Pa is only 3.25(1) x 104a. All isotopes of elements of atomic number greater than 83 have half-lives less than 1010 a, except for 232Th. The following 21 elements are considered to have one stable isotope in the evaluation of the atomic weights: Be, F, Na, Al, P, Sc, Mn, Co, As, Y, Nb, Rh, I, Cs, Pr, Tb, Ho, Tm, Au, Bi and Th.
