Lithium is the least dense metallic element under standard conditions.
Its density is about 0.534 grams per cubic centimetre, making it light enough to float on water. Sodium and potassium can also float on water, but both are denser than lithium. Aluminium and magnesium, the other options, are substantially heavier by volume.
Lithium is a soft, silvery-white alkali metal with the chemical symbol Li and atomic number 3. Like other alkali metals, it has one valence electron and reacts readily with air and water. Because it is highly reactive and flammable, lithium metal is stored under mineral oil or an inert atmosphere.
Its low density is useful in specialised alloys and batteries, although modern rechargeable batteries usually use lithium ions or lithium-containing compounds rather than large amounts of metallic lithium. Lithium should not be confused with the lightest element overall: hydrogen is far lighter, but it is a nonmetal, so it does not qualify for this question.