Atomic mass unit
The atomic mass unit (symbol: u), also called the dalton (symbol: Da), is a unit of mass used with the SI[1] for measuring masses at the atomic or molecular scale. The value of the atomic mass unit is a physical constant called the atomic mass constant (symbol: mu): the 2006 CODATA recommended value is 1.660 538 782(83) × 10−27 kg.[2]
Definition and measurement
The atomic mass constant is defined as one twelfth of the rest mass of an unbound atom of carbon-12 in its nuclear and electronic ground state.[1][3] An equivalent definition used to determine to value of the atomic mass constant is the molar mass constant divided by the Avogadro constant.
- mu = Mu/NA
As the molar mass constant has a defined value in the International System of Units, the atomic mass constant is determined by the Avogadro constant.
History
The atomic weight scale has traditionally been a relative scale, that is without an explicit unit. The possibility of linking the atomic weight scale with other measurements in mass came with the first accurate determinations of the Avogadro constant by Jean Perrin.
The discovery of isotopes confused matters somewhat, as physicists used a scale based on the mass of an oxygen-16 atom as 16 units, while the chemists used a scale where the atomic weight of natural oxygen was 16. The reference was changed to carbon-12 in 1961,[4] and hence the current unit is often referred to as the "unified" atomic mass unit.[5]
Terminology
Both "dalton" and "atomic mass unit" are acceptable and alternative names to refer to the unit.[1] SI prefixes can be used with the name "dalton",[1] and the kilodalton is a convenient unit for the weight of proteins in biochemistry and molecular biology.[6] SI prefixes are also occasionally used with the symbol u.[7]
References
- ↑ 1.0 1.1 1.2 1.3 The International System of Units (SI), 8th ed.; International Bureau of Weights and Measures: Sèvres, France, 2006; p 126. ISBN 92-822-2213-6, <http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf>.
- ↑ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>. Direct link to value.
- ↑ atomic mass constant, <http://goldbook.iupac.org/A00497.html> (accessed 16 July 2010), Compendium of Chemical Terminology Internet edition; International Union of Pure and Applied Chemistry (IUPAC).
- ↑ Holden, Norman E. Atomic Weights and the International Committee—A Historical Review. Chem. Int. 2004, 26 (1), 4–7, <http://www.iupac.org/publications/ci/2004/2601/1_holden.html>.
- ↑ unified atomic mass unit, <http://goldbook.iupac.org/U06554.html> (accessed 16 July 2010), Compendium of Chemical Terminology Internet edition; International Union of Pure and Applied Chemistry (IUPAC).
- ↑ Mills, Ian; Milton, Martin Amount of Substance and the Mole. Chem. Int. 2009, 31 (2), 3–7, <http://www.iupac.org/publications/ci/2009/3102/1_mills.html>.
- ↑ Wapstra, A. H.; Audi, G.; Thibault, C. The AME2003 atomic mass evaluation (I). Evaluation of input data, adjustment procedures. Nucl. Phys. A 2003, 729, 129–336. DOI: 10.1016/j.nuclphysa.2003.11.002. Wapstra, A. H.; Audi, G.; Thibault, C. The AME2003 atomic mass evaluation (II). Tables, graphs, and references. Nucl. Phys. A 2003, 729, 337–676. DOI: 10.1016/j.nuclphysa.2003.11.003. Data tables.
External links
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