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 standard of mass, the kilogram, is given by Avogadro's number NA. By the definition of Avogadro's number, the mass of NA carbon-12 atoms, at rest and in their ground state, is 12 gram ( = 12×10−3 kg). From the latest value of NA follows the latest value[1] of one unified mass unit:
- 1 u ≈ 1.660538782(83) × 10−27 kg ≈ 931.494028(23) MeV/c2
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.
The unit 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 this value, 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 so much because it is not a supported SI unit, but because historically their are two different standard masses denoted by amu. There is the physics amu ( = 1/1.000 317 9 u) and there is the chemistry amu ( = 1/1.000 043 u).
This difference arose from the fact that before 1960, the physical atomic mass unit (amu) was defined as 1/16 of the mass of one atom of oxygen-16, while the chemical atomic mass unit (amu) was defined as 1/16 of the average mass of an oxygen atom (taking the natural abundance of the different oxygen isotopes into account). Both units are slightly smaller than the unified atomic mass unit, which 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 since 1961 chemists and physicists used the same unit, it is referred to as unified. 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, introduced 1/16 of the mass of one atom of oxygen-16 as a mass unit.
External links
- ↑ CODATA