As a subject matter expert in materials science, I can tell you that the lightest metal on Earth is Lithium. It has a density of only 0.534 grams per cubic centimeter, which is about half the density of water. Lithium is a soft, silvery-white metal that belongs to the alkali metal group in the periodic table. It is highly reactive and is often stored in oil to prevent reactions with air or moisture.

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Lightest Elemental Metal. The lightest or least dense metal that is a pure element is lithium, which has a density of 0.534 g/cm3. This makes lithium nearly half as dense as water, so if lithium was not so reactive, a chunk of the metal would float on water. Two other metallic elements are less dense than water.read more >>