Arsenic is a metalloid - a natural element that is not actually a metal but which has some of the properties of a metal. It is a natural component of the Earth’s crust, generally found in trace quantities in all rock, soil, water and air. However, concentrations may be higher in certain areas due to either natural conditions or human activities.
Arsenic can exist in many different chemical forms in combination with other elements. Some forms are inorganic, which do not contain carbon, and others are organic, which always contain carbon. Inorganic arsenic exists in four main chemical forms known as valency or oxidation states. Valency is a measure of the ability of a compound to combine with other elements, such as hydrogen. The dominant forms are arsenite, with a valency of 3, and arsenate, with a valency of 5.