The modern atomic unit is based on the mass of carbon-12. Specifically, it is defined as one twelfth of the mass of a carbon-12 atom in its ground state. This unit of mass is known as the unified atomic mass unit (u) or dalton (Da). Thus, the atomic mass of carbon-12 is exactly 12 u by definition. This standard provides a convenient way for scientists to compare the masses of different atoms and molecules, as it is based on a specific, widely available atom.