Description

Arsenic is a metalloid of the nitrogen group. Two allotrope forms of elemental arsenic have been reported: yellow arsenic and grey arsenic, the latter being usually the more stable form. Arsenic readily oxidises in air to arsenic trioxide (As2O3). Arsenic is mostly found either in its native state or as arsenic sulfide in the form of realgar (As4S4) or orpiment (As2S3). Arsenic can exist in three different valence states (zerovalent, trivalent and pentavalent). Arsenic forms covalent bonds with carbon, oxygen and hydrogen. The toxicity varies widely and depends on the physical state of the compound and its absorption/elimination rate. Trivalent arsenics (As(III)) are derivatives of the arsenous acid (H2AsO3-arsenite) and arsenic trioxide (AsO3). Examples of pentavalent arsenic (As(V)) include derivatives of the arsenic acid (H3AsO4 -arsenate). Organic arsenic-based compounds, that is, compounds containing arsenic-carbon bonds, are usually less toxic than their inorganic counterparts. This is mainly due to their quicker excretion from the human body. Arsenic is known to be one of the most toxic heavy metals. Compounds containing arsenic have a long history of use as poisons, but they also have a long historical medicinal use.