Lithium is a metallic chemical element that is both the least dense metal and the least dense of all elements that are solids at room temperature. It is soft enough to be cut easily with a knife. As with all alkali metals, it is highly reactive and flammable, and thus must be stored in a vacuum, in an inert gas or in an inert liquid. When cut, it exhibits a metallic luster, but corrodes quickly in moist air to a dull silvery gray and then to a black tarnish. Lithium is comparatively rare in the earth's crust, and it never occurs freely in nature but only in compounds because of its reactivity.
The largest industrial applications for lithium and its compounds include heat-resistant glass and ceramics, lubricants, flux additives for iron, steel and aluminum production, and batteries. Lithium is also present in living organisms in trace amounts, although its functions are uncertain. Lithium compounds are useful as a mood stabilizer and antidepressant in the treatment of some mental conditions.