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Chemical element with the symbol Bi and the atomic number 83; Bismuth was considered the heaviest stable chemical element until the radioactivity of the isotope 209Bi was discovered in 2003. The naturally occurring Bismuth consists entirely of 209Bi, which has a very long half-life (about 19 quintillion years). Therefore, its specific activity is very low and cannot be detected with normal measuring devices. It decays via alpha decay to the stable Thallium isotope 205Tl. The Bismuth isotopes 210Bi, 211Bi, 212Bi and 214Bi are members of the three natural decay chains (so-called radiogenic radionuclides) and therefore always exist in traces in the environment.

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