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Arsenic (gray) with realgar (red) and orpiment (yellow)—Courtesy of the Joseph and Helen Guetterman collection; photograph, John H. Gerard
Nonmetallic to semimetallic chemical element, chemical symbol As, atomic number 33. It exists uncombined in two stable (and several unstable) allotropes, one gray and one yellow, but is more often found in nature as the sulfide or oxide. The elemental form is used to form alloys of metals (especially lead), and certain semiconductors are made from crystals of gallium arsenide (GaAs). Arsenious oxide (arsenic trioxide or white arsenic, AsO) is used in pesticides, as a pigment, and as a preservative of hides and wood; this is the poisonous arsenic (seearsenic poisoning) in detective stories. Arsenic pentoxide (AsO) is also used in insecticides, herbicides, metal adhesives, and pigments.
This entry comes from Encyclopædia Britannica Concise. For the full entry on arsenic, visit Britannica.com.