Atoms have very little mass. The atomic weight is the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12, which is known as the unified atomic mass unit. Thus, the atomic weight scale indicates an atom’s mass relative to the mass of other atoms. This scale is based upon assigning the carbon atom a mass of 12. On this scale, a hydrogen atom has an atomic weight of approximately 1, indicating that it has one-twelfth the mass of a carbon atom; a magnesium atom, with an atomic weight of 24, has twice the mass of a carbon atom.