As is a chemical element with the symbol As and atomic number 33. It is a metalloid that has interesting properties, including semiconductor behavior, making it useful in electronics. Although it is not found free in nature, it occurs in many minerals, usually in small amounts. Arsenic and its compounds have been known since antiquity and used by humans for various purposes. These uses have caused arsenic to enter the environment where it is a health hazard to both people and animals.
Elemental arsenic is a steel-gray semi-metal that looks similar to lead. When heated in air, arsenic oxidizes to form a grayish-white powder of arsenite (As2O3), which quickly tarnishes upon exposure to moisture or air to give arsenic trioxide (As2O3), the most common ore of arsenic. The main commercial use for Arsine gas (AsH3) is as an insecticide fumigant; other uses include production of semiconductor gallium arsenide (GaAs) single crystals and glasses containing metallic copper particles for applications such as far infrared optics and nonlinear optics respectively. Arsenic compounds are also used in herbicides, wood preservatives, rat poisons, and as pigments (especially realgar) in paints.