On the periodic table, lithium has the lowest density of all metals at room temperature, at about 0.534 grams per cubic centimetre. It is so light that it can float on water, although it reacts with water rather than simply resting on the surface.
Lithium is element 3 and the first member of group 1, the alkali metals. It is a soft, silvery metal with one outer electron, making it chemically reactive. Like other alkali metals, it is not found naturally as a free element.
Lithium compounds were identified in minerals in the early 19th century, and the metal was isolated in 1855 by Robert Bunsen and Augustus Matthiessen through electrolysis. Today, lithium is important in rechargeable batteries, ceramics, glass, lubricants, and some medicines.
Its low density does not mean lithium is the lightest substance overall: hydrogen and helium are gases with far lower densities. The comparison here is specifically among metals under ordinary room-temperature conditions.