Which element is the lightest metal?

The lightest metal is lithium. With a density of about 0.534 grams per cubic centimetre under standard conditions, lithium is the least dense metal and the least dense solid element. It is also the first metal in the periodic table, with atomic number 3.

Lithium is a soft, silvery-white alkali metal. Like the other alkali metals, it has one valence electron and reacts readily with other substances. It can react with water to form lithium hydroxide and hydrogen, although the reaction is less vigorous than those of heavier alkali metals such as sodium and potassium.

Lithium is sometimes described as a metal that can float: it is less dense than water and can float briefly, though its reaction with water makes that demonstration unsafe. Its low mass and density also make lithium useful in lightweight aluminium alloys. Modern lithium-ion batteries rely on lithium compounds and ions, not exposed metallic lithium.