One twelfth of the mass of a carbon-12 atom.The atomic mass constant, denoted u (or amu), is defined as exactly one twelfth of the mass of a neutral carbon-12 atom. It is the fundamental unit for expressing atomic and molecular masses on the unified atomic mass scale. Because carbon-12 was chosen as the reference, the constant provides a common baseline that makes relative atomic masses (atomic weights) dimensionless and directly comparable across all elements and isotopes.
The concept was formalized in 1961 when the International Union of Pure and Applied Chemistry (IUPAC) adopted carbon-12 as the standard, replacing the earlier oxygen-16 based scale. The value of the constant was later refined through precise measurements of Avogadro's number and the molar mass constant.
The constant sets the scale: 1 u ≈ 1.66053906660 × 10⁻²⁷ kg. In practice, masses of atoms and molecules are measured relative to this unit using mass spectrometry. The choice of carbon-12 (with 6 protons and 6 neutrons) means that the mass of a nucleon (proton or neutron) is approximately 1 u, though slightly less due to binding energy. This unit simplifies calculations in chemistry and physics, where masses of particles and molecules are often expressed in u rather than kilograms.
It is used universally in chemistry and physics for expressing atomic and molecular masses, and in mass spectrometry for identifying compounds and isotopes.
The atomic mass constant is exactly 1/12 of the mass of a carbon-12 atom, but because of nuclear binding energy, a carbon-12 atom is slightly lighter than the sum of its 12 individual nucleons.
