Radioactive elements do not have standard atomic weights but many versions of the Periodic Tables include the mass number of the most stable isotopes, usually in square brackets. Below is the compilation of such isotopes from the most recent authoritative work The 2020 Nubase Evaluation.

Most stable known isotopes of radioactive elements

Notes      Back to Top Uncertainties associated with the quoted half-live values are given as standard uncertainties (1s) applicable to the last digits. For example, a value of 1.4(7) ms refers to 1.4 ms with a standard uncertainty of 0.7 ms whereas a value of 3.1(1.6) min refers to 3.1 min with a standard uncertainty of 1.6 min. Half-lives are given in standard units of time (a=year, d=day, min=minute, s=second).

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F.G. Kondev et alThe NUBASE2020 evaluation of nuclear physics properties Chinese Physics C, 2021 (45) 030001

For some elements, there is no single isotope which could be regarded as the most stable (longest lived). This is because the half-lives are measured with asymmetric uncertainties whereas the recommended values are reported with calculated symmetric uncertainties. Thus, for example, technetium-97 and technetium-98 could both be considered as equally stable. Likewise, one cannot readily distinguish between the longevity of meitnerium-277 (t1/2 = 9(6) s) and meitnerium-278 (t1/2 = 6(3) s).

Last updated: 29 Mar 2022