Isotopes of astatine
has 41 known isotopes, all of which are radioactive, whose mass numbers range from 188 to 229 except 189; they are accompanied by almost as many metastable excited states. The longest-lived isotope is 210At, which has a half-life of 8.1 hours, followed by the medically useful 211At, with a half-life of 7.214 hours. The longest-lived isomer is 202m1At with a half-life of just over 3 minutes. However, the longest-lived isotope existing in naturally occurring decay chains is 219At with a half-life of only 56 seconds.Alpha decay
energy follows the same trend as for other heavy elements. The lighter astatine isotopes have quite high decay energies, which become lower as more neutrons are added, reaching a minimum at 125 neutrons, even though 126 is the magic number. The decay energies increase much more steeply, though, on the next two steps, reaching a high at 128 neutrons where the alpha-decay product would have the magic number of 126. Here this is astatine-213, releasing the highest energy and having the shortest life of all the isotopes. The energy then declines again, and alpha lifetimes increase quickly, no long-lived astatine isotope exists; this happens due to the increasing role of beta decay. This decay mode is especially important for astatine: as early as 1950, it was postulated that the element has no beta-stable isotopes, though nuclear mass measurements reveal that 215At is in fact beta-stable, as it has the lowest mass of all isobars with A = 215. A beta decay mode has been found for all other astatine isotopes except for 212-216At and their isomers. Among other isotopes, if they do not undergo alpha decay: astatine-210 and the lighter isotopes decay by electron capture or positron emission, 211 by electron capture only, and astatine-217 and heavier isotopes undergo β- decay. Astatine-212, 214, and 216 should be able to decay either way.