Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals usually in combination with sulfur and metals but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes but only the gray form, which has a metallic appearance is important to industry. The primary use of arsenic is in alloys of lead. Arsenic is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the second most commonly used semiconductor after doped silicon.