Semi-metallic element ARSENIC is a chemical element with the symbol As and atomic number 33. It is a metalloid, which means it has properties of both metals and nonmetals. ARSENIC occurs in many minerals, usually in combination with sulfur and metals, and also as a pure elemental crystal. ARSENIC is used in alloys, pesticides, and semiconductor devices. It is highly toxic in its pure form and can cause serious health problems if ingested.