Origin of the concept The interpretation of the
law of definite proportions in terms of the
atomic theory of matter implied that the masses of atoms of various elements had definite ratios that depended on the elements. While the actual masses were unknown, the relative masses could be deduced from that law. In 1803
John Dalton proposed to use the (still unknown) atomic mass of the lightest atom, hydrogen, as the natural unit of atomic mass. This was the basis of the
atomic weight scale. For technical reasons, in 1898, chemist
Wilhelm Ostwald and others proposed to redefine the unit of atomic mass as the mass of an oxygen atom. That proposal was formally adopted by the
International Committee on Atomic Weights (ICAW) in 1903. That was approximately the mass of one hydrogen atom, but oxygen was more amenable to experimental determination. This suggestion was made before the discovery of isotopes in 1912. Physicist
Jean Perrin had adopted the same definition in 1909 during his experiments to determine the atomic masses and the
Avogadro constant. This definition remained unchanged until 1961. Perrin also defined the "mole" as an amount of a compound that contained as many molecules as 32 grams of oxygen (). He called that number the
Avogadro number in honor of physicist
Amedeo Avogadro.
Isotopic variation The discovery of isotopes of oxygen in 1929 required a more precise definition of the unit. Two distinct definitions came into use. Chemists choose to define the AMU as of the average mass of an oxygen atom as found in nature; that is, the average of the masses of the known isotopes, weighted by their natural abundance. Physicists, on the other hand, defined it as of the mass of an atom of the isotope oxygen-16 (16O).
Joint definition by IUPAP and IUPAC The existence of two distinct units with the same name was confusing, and the difference (about in relative terms) was large enough to affect high-precision measurements. Moreover, it was discovered that the isotopes of oxygen had different natural abundances in water and in air. In April 1957
Alfred O. C. Nier suggested to
Josef Mattauch that
carbon-12 be adopted as the mass scale because of carbon's use as a secondary standard in
mass spectrometry. Also, carbon-12 implied acceptable relative changes in the atomic weight scale, i.e., 42 parts-per-million (ppm) compared to 275 ppm for
oxygen-16, which would not be acceptable to chemists. Following the approval of the
International Union of Pure and Applied Physics (IUPAP) General Assembly at Ottawa, Canada, in 1960 and the
International Union of Pure and Applied Chemistry (IUPAC) General Assembly at Montreal, Canada, in 1961, the atomic weights were officially given on the carbon-12 scale for the first time. The new unit was named the "unified atomic mass unit" and given a new symbol "u", to replace the old "amu" that had been used for the oxygen-based unit. However, the old symbol "amu" has sometimes been used after 1961 to refer to the new unit, particularly in lay and preparatory contexts. With this new definition, the
standard atomic weight of
carbon is about and that of oxygen is about . These values, generally used in chemistry, are based on averages of many samples from
Earth's crust, its
atmosphere, and
organic materials.
Adoption by BIPM The IUPAC 1961 definition of the unified atomic mass unit, with that name and symbol "u", was adopted by the
International Bureau for Weights and Measures (BIPM) in 1971 as a
non-SI unit accepted for use with the SI.
Unit name In 1993, the IUPAC proposed the shorter name "dalton" (with symbol "Da") for the unified atomic mass unit. As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol, "Da", is capitalized. The name was endorsed by the
International Union of Pure and Applied Physics (IUPAP) in 2005. In 2003 the name was recommended to the BIPM by the
Consultative Committee for Units, part of the
CIPM, as it "is shorter and works better with [SI] prefixes". In 2006, the BIPM included the dalton in its 8th edition of the
SI brochure of formal definitions as a
non-SI unit accepted for use with the SI. The name was also listed as an alternative to "unified atomic mass unit" by the
International Organization for Standardization in 2009. It is now recommended by several scientific publishers, and some of them consider "atomic mass unit" and "amu" deprecated. In 2019, the BIPM retained the dalton in its 9th edition of the
SI brochure, while dropping the unified atomic mass unit from its table of non-SI units accepted for use with the SI, but secondarily notes that the dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit.
2019 revision of the SI The definition of the dalton was not affected by the
2019 revision of the SI, that is, 1 Da in the SI is still of the mass of a
carbon-12 atom, a quantity that must be determined experimentally in terms of SI units. However, the definition of a
mole was changed to be the amount of substance consisting of exactly entities, and the definition of the
kilogram was changed as well. As a consequence, the
molar mass constant () remains close to but no longer exactly 1 g/mol, meaning that the mass in
grams of one mole of any substance remains nearly but no longer exactly numerically equal to the average mass of an entity in daltons, although the relative standard uncertainty of at the time of the redefinition is insignificant for all practical purposes. Letting
ent denote one
entity, the smallest amount of any substance (retaining its chemical properties), one mole is an aggregate of an
Avogadro number of entities, i.e. . This means that the appropriate atomic-scale unit for molar mass is dalton per entity (Da/ent), equal to the constant , very nearly equal to 1 g/mol. For Da/ent to be exactly equal to g/mol, the dalton would need to be redefined exactly in terms of the (fixed-
h) kilogram and the Avogadro number so that 1 Da = (1/) g, satisfying the relationships = g/Da and , thus conforming to the original mole concept—that the Avogadro number is the gram-to-dalton mass unit ratio. == Measurement ==