Arsenic

Arsenic is a chemical element with the symbol As and atomic number 33. It is a metalloid in group 15 of the periodic table. In nature it occurs in minerals and can be found in the Earth's crust at about 1.8 mg/kg. Arsenic has several allotropes, of which gray arsenic is the stable form at room temperature and yellow arsenic is an unstable, lighter allotrope produced under specific conditions.

In minerals, arsenic commonly occurs as arsenic sulfide minerals such as orpiment (As2S3) and realgar (As4S4), or together with iron in arsenopyrite (FeAsS). In the environment, arsenic can enter groundwater as arsenite (As3+) and arsenate (As5+), with mobility influenced by redox conditions. Arsenic forms a variety of compounds, including oxides such as arsenic trioxide (As2O3) and other inorganic and organic species. Its chemistry spans oxidation states from -3 to +5.

Historically, arsenic has been used in pigments (for example, Scheele’s green and Paris green) and in wood preservation (chromated copper arsenate). In modern technology, arsenic is used in electronics and optoelectronics, notably in gallium arsenide and other III-V semiconductors, as well as in phosphors for lighting and displays. It also appears in some glass, ceramics, and alloy formulations.

Health and safety aspects are central to arsenic: inorganic arsenic compounds are highly toxic and are classified as carcinogens. Chronic exposure, often from contaminated drinking water or occupational sources, is associated with skin changes, vascular and nerve effects, and cancers of the skin, lung, and bladder. Metabolism involves methylation to mono- and dimethylarsinic acids and excretion primarily in urine. International guidelines typically aim to keep drinking-water arsenic well below 100 micrograms per liter, with many countries setting a limit near 10 micrograms per liter.