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
Bi-209 is of interest in nuclear physics because its neutron number 126 is a closed shell, illustrating