Why don some elements have a standard atomic weight?
More than 25 elements have no stable nuclides and do not have a characteristic terrestrial isotopic composition, including Tc, Pm, Po, At, Rn, Fr, Ra, Ac, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, and Rg. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements an atomic weight is tabulated in the Commission’s Table of Standard Atomic Weights.
More than 25 elements have no stable isotopes and do not have a characteristic terrestrial isotopic composition, including Tc, Pm, Po, At, Rn, Fr, Ra, Ac, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, and Rg. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these elements an atomic weight is tabulated in the Commission’s Table of Standard Atomic Weights.
