Lithium is the lightest chemical element that is solid at room temperature.
Lithium has atomic number 3 and a relative atomic mass of about 6.94. It is a soft, silvery alkali metal with the lowest density of all metals, about 0.534 grams per cubic centimeter at room temperature.
Lithium reacts readily with water and oxygen, so it is not found naturally as free metal in ordinary geological settings. Instead, it occurs in minerals and brines. Its compounds are used in ceramics, glass, lubricating greases, and rechargeable batteries.
Lithium-ion batteries rely on lithium ions moving between electrodes during charging and discharging. The metal itself is not simply poured into consumer batteries; engineered compounds and electrolytes are used to control reactivity and performance. Hydrogen and helium are lighter elements, but neither is a solid metal at room temperature, which is why lithium is the correct answer.