Arsenic has diverse chemical behavior in the natural environment. It has the ability to readily change oxidation state and bonding configuration, which creates rich inorganic and organic chemistry. This behavior is a consequence of the electronic configuration of its valence orbitals, with partially filled states capable of both electron donation and overlap in covalent bonds. In natural compounds, arsenic bonds primarily to oxygen and sulfur, generating a variety of aqueous species and minerals. The affinity of arsenic for these two elements, along with its stable bonding to methyl groups, constitutes the structural basis for most organic and biosynthetic compounds. The agile chemistry of arsenic helps to explain its contradictory action as both a toxin and a curative, and its sometimes-elusive behavior in the environment.