Bismuth-209

Bismuth-209 is the "virtually stable" isotope of bismuth with the longest known half-life of any radioisotope that undergoes α-decay (alpha decay). It has 83 protons and 126 neutrons, and an atomic mass of 208.9803987 amu (atomic mass units). All primordial bismuth is of this isotope. It is a β⁻product of lead-209.

Bismuth-209 was long thought to have the heaviest stable nucleus of any element, but in 2003, Noël Coron and his colleagues at the Institut d’Astrophysique Spatiale in Orsay, France, discovered that ²⁰⁹Bi undergoes alpha decay with a half-life of approximately 600 yottaseconds (1.9×10¹⁹ years), over a billion times longer than the current estimated age of the universe. 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 x 10²⁴ years by double β-decay (beta decay).

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