Arsenic (As). Diagram of the nuclear composition and electron configuration of an atom of arsenic-75 (atomic number: 33), the most common isotope of this element. The nucleus consists of 33 protons (red) and 42 neutrons (yellow). 33 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. Arsenic, which is extremely toxic, is used as a dopant in semiconductors. It is a metalloid in group 15, period 4, and the p-block of the periodic table. It sublimes (goes from solid to gas) at 615 degrees Celsius.