Bismuth-209 was long thought to have the heaviest stable nucleus of any element, but in 2003, a research team at the Institut d’Astrophysique Spatiale in Orsay, France, discovered that 209Bi undergoes alpha decay with a half-life of approximately 19 exayears (1.9×1019, approximately 19 quintillion years), over a billion times longer than the current estimated age of the universe. The heaviest nucleus considered to be stable is now lead-208. Theory had previously predicted a half-life of 4.6×1019 years. The decay event produces a 3.14 MeV alpha particle and converts the atom to thallium-205.
Bismuth-209 will eventually form 205Tl:
Due to its extraordinarily long half-life, for nearly all applications 209Bi can still be treated as if it were non-radioactive. Although 209Bi holds the half-life record for alpha decay, bismuth does not have the longest half-life of any radionuclide to be found experimentally—this distinction belongs to tellurium-128 (128Te) with a half-life estimated at 7.7 × 1024 years by double β-decay (double beta decay).
The half-life value of bismuth-209 was confirmed in 2012 by an Italian team in Gran Sasso who reported (2.01±8)×1019 years, and an even longer half-life, for bismuth-209 alpha decay to the first excited state of thalium-205 at 204 keV, was estimated to be 1.66×1021 years. Even though this value is shorter than the measured half-life of tellurium-128, both alpha decays of bismuth-209 hold the record of the thinnest natural line widths of any measurable physical excitation, estimated respectively at ΔΕ~5.5×10−43 eV and ΔΕ~1.3×10−44 eV in application of the uncertainty principle of Heisenberg (double beta decay would produce energy lines only in neutrinoless transitions, which has not been observed yet).