Forms of ordinary chemical elements that have gained or lost electrons from their orbit around the atomic nucleus and, thus, have become electrically charged. Positive ions (those that have lost electrons) are called cations because when charged electrodes are placed in a solution containing ions the positive ions migrate to the cathode (negative electrode). Negative ions (those that have gained extra electrons) are called anions because they migrate toward the anode (positive electrode). Environmentally important cations include the hydrogen ion (H⁺) and dissolved metals. Important anions include the hydroxyl ion (OH^-) as well as many of the dissolved ions of nonmetallic elements.

See Also

Ion Exchange; Ionizing Radiation