Bismuth (Bi). Diagram of the nuclear composition and electron configuration of an atom of bismuth-209 (atomic number: 83), the most common isotope of this element. The nucleus consists of 83 protons (red) and 126 neutrons (yellow). 83 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Bismuth is a post-transition metal in group 15, period 6, and the p-block of the periodic table. It has a melting point of 271 degrees Celsius. It is used in pigments and cosmetics, and also in alloys to replace the more toxic lead.