Arsenic is a metaliod widely distributed in the earths crust and present at an average concentration of 2 mg/kg. Arsenic can exist in four valency states: 3, 0, +3 and +5. Under reducing conditions, arsenite (As (III)) is the dominant form; arsenate (As (V)) is generally the stable form in oxygenated environments. Elemental arsenic is not soluble in water. Arsenic salts exhibit a wide range of solubilities depending on pH and the ionic environment.
Arsenic and its compounds occur in crystalline, powder, amorphous or vitreous forms. They usually occur in trace quantities in all rock, soil, water and air. However, concentrations may be higher in certain areas as a result of weathering and anthropogenic activities including metal mining and smelting, fossil fuel combustion and pesticide use.
There are many arsenic compounds of environmental importance. Inorganic (not carbon based) compounds include the trivalent arsenic trioxide, arsenic trichloride, arsenic trisulphide and sodium arsenite. Pentavalent ones include arsenic pentoxide, arsenic acid and sodium arsenate. Representative organic (carbon based) compounds are monomethyl-, dimethyl- and trimethylarsine, and arsenobetaine.
http://www.dartmouth.edu/~toxmetal/metals/questions/arsenic.html
