Potassium is the nineteenth element in the periodic table. Its symbol is K. Potassium is an alkali metal, forming a single ion, K+, the form it is almost always found in. Pure potassium is rarely found outside a laboratory as it reacts violently with water. Potassium is an important nutrient and serves many biological functions. The nerves, including the brain, transmit information using a sodium-potassium pump.