Lithium's atomic number is 3 and its symbol is Li. Lithium is the lightest metal as well as the lightest solid element. It is so light that the density is comparable to that of pine wood. It is a bright silvery-white color that tarnishes almost instantly if exposed to air. Being an alkali metal, lithium is highly reactive. Upon contact with water, this metal will readily produce hydrogen gas. Lithium is often stored in oil to prevent it from reacting with the oxygen and moisture in the air. In medicine, lithium is used to treat bipolar disorder. Lithium also has many other uses such as its role in lithium grease, lithium-ion batteries, and as an additive in the production of steel and aluminum.