Arsenic, a chemical element. It can exist in several forms; these include metallic (or gray) arsenic and amorphous (or black) arsenic. The most stable form at ordinary temperatures and pressures is metallic arsenic. It is a crystalline, brittle substance and only a fair electrical conductor.

Arsenic and its compounds are poisonous. Acute arsenic poisoning results from exposure to a large amount of the poison. It produces acute distress, vomiting, and severe diarrhea and can be fatal if not treated. Chronic arsenic poisoning, from smaller, repeated exposures, damages the major organ systems and can also cause death. Emergency treatment includes hemodialysis; emptying the stomach; and giving activated charcoal (to adsorb the poison), fluids, and chelators (drugs that speed the excretion of the poison).

Arsenic is widely distributed in the earth's crust in small quantities. It is usually found chemically combined with other elements. Major arsenic-bearing minerals include realgar, orpiment, and arsenopyrite (mispickel). Copper and lead ores frequently contain small amounts of arsenic.

Most arsenic is used in compounds. Arsenic trioxide (white arsenic), copper acetoarsenite (Paris green), and other arsenic compounds are used as weed killers and insecticides. Arsenic acid is applied to cotton plants to cause them to dry up as an aid in harvesting. Arsenic compounds also are used in some kinds of wood preservatives and in making some kinds of glass. Metallic arsenic is used mainly in various alloys. An alloy of lead and small amounts of arsenic produces round, hard shot. Some types of brass contain small amounts of arsenic. Arsenic combined with gallium or indium is used in some light-emitting diodes, lasers, and other solid-state devices. Arsenic is sometimes added to silicon as a doping agent—that is, as an impurity that alters silicon's electrical properties.

Arsenic is usually obtained commercially from arsenic trioxide, which is recovered as a by-product of the refining of copper and lead ores. China is the world's leading producer of arsenic trioxide.

A substance the ancient Romans called arsenic was probably not pure arsenic but the mineral orpiment. Pure arsenic may have been discovered in the 13th century by Albertus Magnus.

Symbol: As. Atomic number: 33. Atomic weight: 74.9216. Specific gravity (metallic arsenic): 5.72. Sublimation point (metallic arsenic): 1,135 F. (613 C.). (When heated at atmospheric pressure, metallic arsenic sublimes—that is, it changes directly from a solid to a vapor. It can exist in a liquid state only at a much higher pressure.) Arsenic has one natural isotope: As-75. It belongs to Group VA of the Periodic Table and can have a valence of -3, +3, or +5.