Monoisotopic Elements

An element is considered to be monoisotopic by CIAAW if it has one and only one isotope that is either stable or has a half-life greater than 1 × 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 be monoisotopic because ²³²Th has a half-life of 1.4 × 10¹⁰ a and the half-lives of the other isotopes of Th are all less than 1 × 10¹⁰ a.  In contrast, Pa is not classified as a monoisotopic element because the half-life of the most abundant isotope ²³¹Pa is only 3.25(1) x 10⁴ a.  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 be monoisotopic 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.

    CIAAW Monoisotopic Elements