Unified atomic mass unit

The unified atomic mass unit (u), or dalton (Da), is a unit of atomic and molecular mass. By definition it is one twelfth of the mass of an unbound carbon-12 (¹²C) atom, at rest and in its ground state.

The relationship of the unified atomic mass unit to the macroscopic SI base unit of mass, the kilogram, is given by Avogadro's number N_A. By the definition of Avogadro's number, the mass of N_A carbon-12 atoms, at rest and in their ground state, is 12 gram ( = 12×10⁻³ kg). From the latest value of N_A follows the latest value of the unified atomic mass unit:

1 u ≈ 1.660 538 782(83) × 10⁻²⁷ kg

Future refinements in Avogadro's number by future improvements in counting large (on the order of 10²⁷) numbers of atoms, will give better accuracy of u. It is hoped that in the future the experimental accuracy of Avogadro's constant will improve so much that the unified atomic mass unit may replace the kilogram as the SI base unit, see this article.

The unit u is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains p protons and n neutrons will have a mass approximately equal to (p + n) u. The mass of a nucleus is not exactly equal to p + n, because the nuclear binding energy gives rise to a relativistic mass defect.

In the literature one still finds the obsolete unit amu (atomic mass unit). This is deplorable, not only because it is not a unit accepted for use with the SI, but also because two different standard masses are denoted by amu. There is the physicist's amu ( = 1/1.000 317 9 u) and there is the chemist's amu ( = 1/1.000 043 u).

This difference arose from the fact that before 1960, in physics the amu was defined as 1/16 of the mass of one atom of oxygen-16, while in chemistry the amu was defined as 1/16 of the average mass of an oxygen atom (averaged over the natural abundance of the different oxygen isotopes). Both units are slightly smaller than the unified atomic mass unit that was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961. Because chemists and physicists now use the same atomic mass unit, it is referred to as unified. It is a non-SI unit accepted for use with the SI, whose value in SI units is obtained experimentally.

The first standardization of atomic mass was when the chemist John Dalton introduced in the early nineteenth century the mass of one atom of hydrogen as the atomic mass unit. Later Francis Aston, inventor of the mass spectrometer, replaced it by one sixteenth of the mass of one atom of oxygen-16.