A chemical element, symbol As, atomic number 33. Arsenic is found widely distributed in nature (approximately 5 x 10⁻4 of the Earth's crust). It is one of the 22 known elements composed of only one stable nuclide, ⁷⁵As; the atomic weight is 74.92158. There are 17 other radioactive arsenic nuclides known.
There are three polymorphic modifications of arsenic. The yellow cubic a-form is made by condensing the vapor at very low temperatures. The black ,B-polymorph is isostructural with black phosphorus. Both these modifications revert to the stable form, gray or metallic, rhombohedral arsenic, on heating or exposure to light. The metallic form is a moderately good thermal and electric conductor and is brittle, easily fractured, and of low ductility.
Arsenic is found native as the mineral scherbenkobalt, but generally occurs among surface rocks combined with sulfur or metals such as Mn, Fe, Co, Ni, Ag, or Sn. The principal arsenic mineral is FeAsS (arsenopyrite, mispickel); other metal arsenide ores are FeAs2 (lollinqite), NiAs (nicolite), CoAsS (cobalt glance), NiAsS (gersdorffite), and COAs₂ (smaltite). Naturally occurring arsenates and thioarsenates are common, and most sulfide ores contain arsenic. As4S4 (realgar) and As4S6 (orpiment) are the most important sulfur-containing minerals. The oxide, arsenolite, As406 is found as the prod­uct of the weathering of other arsenical minerals, and is also recovered from flue dusts collected during the extraction of Ni, Cu, and Sn from their ores; it also results when the arsenides of Fe, Co, or Ni are roasted in air or oxygen. The element may be obtained by roasting FeAsS or FeAs2 in the absence of air or by reduction of As4O6 carbon, when As₄ may be sublimed away.
Elemental arsenic has few uses. It is one of the few minerals available in 99. 9999+ % purity, which is largely used in the laser material GaAs and as a doping agent in the manufacture of various solid-state devices. Arsenic oxide is used in glass manufacture. The arsenic sulfides are used as pigments and in pyrotechnics. Dihydrogen arsenate is used in medicine, as are several other arsenic compounds. Most of the medicinal uses of arsenic compounds depend on their toxic nature.