Bi209s

Bi-209 (Bi-209) is the only naturally occurring isotope of the element bismuth. With 83 protons and a mass number of 209, it sits at the end of the natural isotopic chain of bismuth and has a neutron number of 126, a magic number that contributes to its historical stability. In nature, bismuth is found almost entirely as Bi-209.

For many years Bi-209 was regarded as the heaviest stable nuclide. In 2003, direct measurements established that Bi-209 undergoes alpha decay with an extremely long half-life on the order of about 10^19 years, making it effectively stable for practical purposes. The decay process converts Bi-209 into thallium-205 and an alpha particle. Because the half-life is so long, Bi-209 remains a negligibly radioactive component of natural bismuth under ordinary conditions, and its activity is extremely low in typical environments.

Bi-209 is of interest in nuclear physics because its neutron number 126 is a closed shell, illustrating nuclear structure effects in very heavy nuclei. The isotope serves as a test case for models of alpha decay and for studies of long-lived radioactivity in the upper reaches of the nuclear chart. In practical terms, Bi-209’s abundance is effectively 100 percent in natural bismuth, and it contributes minimally to radioactivity in most applications.