Thallium (Tl). Diagram of the nuclear composition and electron configuration of an atom of thallium-205 (atomic number: 81), the most common isotope of this element. The nucleus consists of 81 protons (red) and 124 neutrons (yellow). 81 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. Thallium is a post-transition metal in group 13, period 6, and the p-block of the periodic table. It has a melting point of 304 degrees Celsius. Highly toxic, it is used in the electronics, pharmaceutical and glass industries.