Nonmetallic to semimetallic chemical element , chemical symbol As, atomic number 33.
It exists uncombined in two stable (and several unstable) allotrope s, one gray and one yellow, but is more often found in nature as the sulfide or oxide . The elemental form is used to form alloy s of metal s (especially lead), and certain 2 O 3 ) is used in pesticides, as a pigment, and as a preservative of hides and wood; this is the poisonous "arsenic" (see {{link=arsenic poisoning">arsenic poisoning ) in detective stories. Arsenic pentoxide (As 2 O 5 ) is also used in insecticides, herbicides, metal adhesives, and pigments.
Arsenic (gray) with realgar (red) and orpiment (yellow)
By courtesy of the Joseph and Helen Guetterman collection; photograph, John H. Gerard