Atomic mass unit

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The unified atomic mass unit (symbol: u) or Dalton (symbol: Da) is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound atom of carbon-12 in its nuclear and electronic ground state,[1] and has a value of 1.660538782(83)×10−27
.[2] 1 Da is approximately equal to the mass of 1 proton or 1 neutron. The CIPM have categorised it as a "non-SI unit whose values in SI units must be obtained experimentally".[1]



The atomic weight scale has traditionally been a relative scale, that is without an explicit unit, with the first atomic weight basis suggested by John Dalton in 1803 as 1H.[3] Despite the initial mass of 1H being used as the natural unit for atomic weight, it was suggested by Wilhelm Ostwald that atomic weights would be best expressed in terms in units of 1/16 weight of oxygen. This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.[3]

The discovery of isotopic oxygen in 1929 lead to a divergence in atomic weight representation, with isotopically weighted oxygen (chemistry) and pure 16O (physics) bases both used as the basis for the atomic mass unit (amu). The inevitable divergence could result in errors in computations, and was thus unwieldy. The reference was changed to carbon-12 in 1961[4] and a new symbol "u" replaced the now deprecated "amu".

The current unit is referred to as the "unified atomic mass unit" u. [5] The choice of carbon-12 was used to minimise further divergence with prior literature.[3]


The unified atomic mass unit and the dalton are different names for the same unit of measure. Since the dalton was first introduced, there has been a gradual change towards using it in preference to the unified atomic mass unit.

  • In 1993, the International Union of Pure and Applied Chemistry approved the use of the dalton with the qualification that the GCPM had not given its approval.[6]
  • In 2003 the Consultative Committee for Units, part of the CIPM, recommended a preference for the usage of the "dalton" over the "unified atomic mass unit" as it "is shorter and works better with prefixes".[7]
  • In 2005, the International Union of Pure and Applied Physics endorsed the use of the dalton as an alternative to the unified atomic mass unit.[8]
  • In 2006, in the 8th edition of the formal definition of SI, the CIPM cataloged the dalton alongside the unified atomic mass unit as a "Non-SI units whose values in SI units must be obtained experimentally: Units accepted for use with the SI".[1] The definition also noted that "The dalton is often combined with SI prefixes ..."
  • In 2009, when the International Organization for Standardization published updated versions of ISO 80000, it gave mixed messages as to whether or not the unified atomic mass unit had been deprecated: ISO ISO 80000-1:2009 (General), identified the dalton as having "earlier [been] called the unified atomic mass unit u",[9] but ISO 80000-10:2009 (atomic and nuclear physics) catalogued both as being alternatives for each other.[10]
  • The 2010 version of the Oxford University Press style guide for authors in life sciences gave the following guidance "Use the Système international d'unités (SI) wherever possible ... The Dalton (Da) or more conveniently the kDa is a permitted non-SI unit for molecular mass or mass of a particular band in a separating gel."[11]

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