Alkali metals. From left to right: Lithium (Li), sodium (Na) & potassium (K). These elemental metals are relatively soft and react rapidly with the water and oxygen in the air. They are not found in elemental form in nature due to their highly reactive nature. Lithium is very light and floats in oil, it is used in alloys and to treat manic-depressive disorders. Sodium is most commonly found as table salt (sodium chloride, NaCl). Potassium is mainly used to manufacture fertilizers and liquid detergents.