Alkali metals. Freshly cut lithium (Li), sodium (Na) and potassium (K), clockwise from top left. These metallic elements are relatively soft and react rapidly with the water and oxygen in the air. They are not found in elemental form (as seen) in nature due to their highly reactive nature. Lithium is very light, 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.