Arsenic

Arsenic is a metalloid, with properties that combine features of metals and nonmetals. Its most common form is a brittle, gray solid with a metallic appearance. In nature, it is usually combined with sulfur and metals in minerals such as arsenopyrite, which contains iron, arsenic, and sulfur. Industrial supplies are recovered mainly from dust collected during copper smelting. Although arsenic is well known for its toxicity, it also has useful roles in technology. Gallium arsenide, a compound of gallium and arsenic, is a semiconductor used in electronic circuits, LEDs, and laser diodes. Small additions of arsenic also strengthen lead alloys used in car batteries. Arsenic compounds released naturally from rocks can contaminate groundwater, creating a serious drinking-water problem in some regions.