Astatine has 23 nuclear isomers (nuclei with one or more nucleons – protons or neutrons – in an excited state).
[10] Lighter astatine isotopes have quite high energies of alpha decay, which become lower as the nuclei become heavier.
[10] This decay mode is especially important for astatine: as early as 1950, it was postulated that the element has no beta-stable isotopes (i.e. ones that do not undergo beta decay at all),[11] though nuclear mass measurements reveal that 215At is in fact beta-stable, as it has the lowest mass of all isobars with A = 215.
This isotope's primary decay mode is positron emission to the relatively long-lived alpha emitter, polonium-210.
It undergoes alpha decay to the extremely long-lived (in practice, stable) isotope bismuth-209.