The number of electrons associated with an element dictates its oxidation state. Elements can exist in several oxidation states. For example, iron commonly exists in the +2 or +3 state, arsenic as +3 or +5, and chromium as +3 or +6. Oxidation-reduction (redox) reactions involve a transfer of electrons and, therefore, a change in the oxidation state of elements. Redox reactions are important to studies of subsurface contamination because the chemical properties for the elements can change substantially with changes in the oxidation state