Arsenic, << AHR suh nihk or AHRS nihk, >> is a chemical element classified as a metalloid. It is a deadly poison, and prolonged low-dose exposure to arsenic causes cancer in human beings. Many rat poisons, insecticides, and weedkillers contain arsenic. It is also used to manufacture lead gun shot and certain types of electrical equipment, and to increase the strength of certain alloys.
There are three chief allotropes (solid forms) of arsenic: (1) gray arsenic, (2) yellow arsenic, and (3) black arsenic. Gray arsenic is the ordinary, stable form of the element. It has a shiny appearance and is a moderately good conductor of heat and electricity. But gray arsenic is brittle and breaks easily. When heated to 1135 °F (613 °C), gray arsenic sublimes–that is, it passes directly into a vapor without melting (see Sublimation ).
Arsenic occasionally occurs in its pure form in nature. But it is most commonly found in chemical combination with sulfur or oxygen, or with such metals as cobalt, copper, iron, nickel, silver, and tin. The principal arsenic-containing mineral is arsenopyrite. The most widely used arsenic compound is white arsenic, also called arsenic trioxide. In 2000, arsenic trioxide was approved as a treatment for acute promyelocytic leukemia, a rare cancer that primarily strikes young adults. The arsenic compound is usually produced as a by-product of the smelting (melting) of copper or lead.
Compounds of arsenic have been used since ancient times for many purposes, including medicines and poisons. The German scholar Albertus Magnus is often credited with first isolating the element about 1250 (see Albertus Magnus, Saint ). Arsenic has the chemical symbol As. Its atomic number (number of protons in its nucleus) is 33. Its relative atomic mass is 74.92160. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of an atom of carbon 12, the most abundant form of carbon. For information on the position of arsenic on the periodic table, see the article Periodic table .
See also Arsenical ; Insecticide .
