Difference between revisions of "Atomic mass unit"
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| − | The '''atomic mass unit''' (symbol: '''u'''), also called the '''dalton''' (symbol: '''Da'''), is a [[Unit of measurement|unit]] of [[mass]] used with the [[International System of Units|SI]] | + | The '''atomic mass unit''' (symbol: '''u'''), also called the '''dalton''' (symbol: '''Da'''), is a [[Unit of measurement|unit]] of [[mass]] used with the [[International System of Units|SI]]<ref name="SI">{{SIBrochure8th|page=126}}.</ref> 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: '''''m''<sub>u</sub>'''): the 2006 [[CODATA]] recommended value is {{nowrap|1.660 538 782(83){{e|−27}} kg}}.<ref>{{CODATA 2006|url=http://physics.nist.gov/cgi-bin/cuu/Value?u}}.</ref> |
==Definition and measurement== | ==Definition and measurement== | ||
| − | The atomic mass constant is defined as one twelfth of the rest mass of an | + | 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.<ref name="SI"/><ref>{{GoldBookRef|title=atomic mass constant|file=A00497|accessdate=2010-07-16}}.</ref> An equivalent definition used to determine to value of the atomic mass constant is the [[molar mass constant]] divided by the [[Avogadro constant]]. |
:''m''<sub>u</sub> = ''M''<sub>u</sub>/''N''<sub>A</sub> | :''m''<sub>u</sub> = ''M''<sub>u</sub>/''N''<sub>A</sub> | ||
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. | 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. | ||
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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 [[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 [[isotope]]s 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 1960, and hence the current unit is often referred to as the "unified" atomic mass unit.<ref>{{GoldBookRef|title=unified atomic mass unit| file = U06554 | accessdate = 2010-07-16}}.</ref> | + | The discovery of [[isotope]]s 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 1960, and hence the current unit is often referred to as the "unified" atomic mass unit.<ref>{{GoldBookRef|title=unified atomic mass unit| file = U06554 | accessdate = 2010-07-16}}.</ref> |
==Terminology== | ==Terminology== | ||
| − | Both "dalton" and "atomic mass unit" are acceptable and alternative names to refer to the unit.<ref name="SI"/> [[SI prefix]]es can be used with the name "dalton", and the kilodalton is a convenient unit for the weight of [[protein]]s in [[biochemistry]] and [[molecular biology]]. | + | Both "dalton" and "atomic mass unit" are acceptable and alternative names to refer to the unit.<ref name="SI"/> [[SI prefix]]es can be used with the name "dalton", and the kilodalton is a convenient unit for the weight of [[protein]]s in [[biochemistry]] and [[molecular biology]]. SI prefixes are also occaisionally used with the symbol u.<ref>{{AME 2003}}.</ref> |
==References== | ==References== | ||
Revision as of 09:15, 25 August 2010
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 1960, and hence the current unit is often referred to as the "unified" atomic mass unit.[4]
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", and the kilodalton is a convenient unit for the weight of proteins in biochemistry and molecular biology. SI prefixes are also occaisionally used with the symbol u.[5]
References
- ↑ 1.0 1.1 1.2 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).
- ↑ 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).
- ↑ 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..
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