(Chemical symbol: K). A silvery-white, lustrous, highly reactive metallic element used in the production of certain types of photoelectric cells; also used instead of mercury in some types of high-temperature thermometers.
Potassium reacts instantly to air or oxygen,, forming a coat of oxide. Potassium is lighter than water and will float in it, reacting violently with it to release hydrogen. Potassium is usually obtained by the electrolysis of caustic potash. Atomic weight: 39; melting point: 65.5″C (149.9″F); specific gravity: 0.859.