<unk>  is a chemical element with the symbol As and atomic number 33.<unk>  occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal.<unk>  is a metalloid. It has various allotropes, but only the gray form, which has a metallic appearance, is important to industry. The primary use of<unk>  is in alloys of lead (for example, in car batteries and ammunition).<unk>  is a common n-type dopant in semiconductor electronic devices. It is also a component of the III-V compound semiconductor gallium<unk> e.<unk>  and its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. These applications are declining with the increasing recognition of the toxicity of<unk>  and its compounds. A few species of bacteria are able to use<unk>  compounds as respiratory metabolites. Trace quantities of<unk>  are an essential dietary element in rats, hamsters, goats, chickens, and presumably other species. A role in human metabolism is not known. However,<unk>  poisoning occurs in multicellular life if quantities are larger than needed.<unk>  contamination of groundwater is a problem that affects millions of people across the world. The United States' Environmental Protection Agency states that all forms of<unk>  are a serious risk to human health. The United States' Agency for Toxic Substances and Disease Registry ranked<unk>  as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites.<unk>  is classified as a Group-A carcinogen.
Arsenic
Vinyl chloride
Sybil 
Metaldehyde
Basilicon
Lewis lead
Mercury
Lead