with stable isotopes, except for helium, beryllium, and carbon, has at least three. Only a single odd-numbered element, potassium, has three primordial isotopes; none have more than three. Tables The following tables give the elements with primordial nuclides, which means that the element may still be identified on Earth from natural sources, having been present since the Earth was formed out of the solar nebula. Thus, none are shorter-lived daughters of longer-lived parental primordials, such as radon. Two nuclides which have half-lives long enough to be primordial, but have not yet been conclusively observed as such (244Pu and 146Sm), have been excluded. The tables of elements are sorted in order of decreasing number of nuclides associated with each element. (For a list sorted entirely in terms of half-lives of nuclides, with mixing of elements, see List of nuclides.) Stable and unstable (marked decays) nuclides are given, with symbols for unstable (radioactive) nuclides in italics. Note that the sorting does not quite give the elements purely in order of stable nuclides, since some elements have a larger number of long-lived unstable nuclides, which place them ahead of elements with a larger number of stable nuclides. By convention, nuclides are counted as "stable" if they have never been observed to decay by experiment or from observation of decay products (extremely long-lived nuclides unstable only in theory, such as tantalum-180m, are counted as stable). The first table is for even-atomic numbered elements, which tend to have far more primordial nuclides, due to the stability conferred by proton-proton pairing. A second separate table is given for odd-atomic numbered elements, which tend to have far fewer stable and long-lived (primordial) unstable nuclides. Elements with no primordial isotopes See also Island of stability List of nuclides List of radioactive nuclides by half-life Primordial nuclide Stable nuclide Stable isotope ratio Table of nuclides Footnotes References Stability of isotopes