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]
Contents
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][4] 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 in 1909.
The name "dalton" was first used for the unit about 1928; the name "atomic mass unit" was first used about 1942.[5]
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,[6] and hence the current unit is often referred to as the "unified" atomic mass unit.[7]
Terminology
Both "dalton" and "atomic mass unit" are acceptable and alternative names to refer to the unit.[1][8][9] SI prefixes can be used with the name "dalton",[1][10] and the kilodalton is a convenient unit for the weight of proteins in biochemistry and molecular biology.[11] SI prefixes are also occasionally used with the symbol u.[12]
New SI
The proposals to redefine the kilogram and other SI base units in terms of fundamental physical constants in the so-called "New SI" would have consequences for the atomic mass unit, either in fixing its value exactly or in reducing the uncertainty in the value. The changes will not take place until the 24th General Conference on Weights and Measures (CGPM) in 2011 at the earliest.
| Particle | ur[Ar(X)] |
|---|---|
| Electron | 4.2 × 10−10 |
| Carbon-12 atom | exact |
| Gold-197 atom | 3.3 × 10−9 |
| Bismuth-209 atom | 7.4 × 10−9 |
| ur(mu) = 5.0 × 10−7 | |
One proposal, which has the support of the technical committee of the International Organization for Standardization (ISO), is to define the kilogram in terms of a fixed number of atoms of carbon-12: using the 2006 CODATA recommended values, the number would be 6.022 141 79 × 1026⁄12 ≈ 5.018 451 491 66 × 1025. Other numbers have been proposed, such as 2250 × 28 148 9633 = 50 184 513 538 686 668 007 780 750. In either case, the dalton would then become an exact submultiple of the kilogram.
Defining the kilogram in terms of another particle mass, rather than the mass of the carbon-12 atom, would reduce the uncertainty in the value of the dalton but not fix it exactly: the uncertainty in the dalton would be equal to the uncertainty in the relative atomic mass of the particle. The most commonly proposed particles apart from the carbon-12 atom are the electron (as a fundamental particle) or atoms of gold-197 or bismuth-209 (gold and bismuth are naturally monoisotopic).
Defining the kilogram in terms of a fixed value of the Planck constant h would also reduce the uncertainty in the value of the dalton, to about ur = 1.4 × 10−9 based on 2006 values.
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).
- ↑ International Standard ISO 80000-1:2009 – Quantities and Units – Part 1: General; International Organization for Standardization: Geneva, 2009.
- ↑ dalton. In Merriam–Webster's Collegiate Dictionary, online ed., <http://www.merriam-webster.com/dictionary/dalton>. (accessed 26 August 2010). atomic mass unit. In Merriam–Webster's Collegiate Dictionary, online ed., <http://www.merriam-webster.com/dictionary/atomic+mass+unit>. (accessed 26 August 2010).
- ↑ 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).
- ↑ International Standard ISO 80000-10:2009 – Quantities and units – Part 10: Atomic and nuclear physics; International Organization for Standardization: Geneva, 2009.
- ↑ IU14. IUPAP Interdivisional Committee on Nomenclature and Symbols (ICTNS); International Union for Pure and Applied Physics, <http://www.iupap.org/commissions/interunion/iu14/ga-05.html>. (accessed 14 August 2010).
- ↑ Report of the 15th meeting of the Consultative Committee for Units (CCU), 2003, <http://www.bipm.org/utils/common/pdf/CCU15.pdf>.
- ↑ 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
 |
See also the corresponding article on Wikipedia. |
| Error creating thumbnail: Unable to save thumbnail to destination |
This page is currently licensed under the Creative Commons Attribution 3.0 Unported license and any later versions of that license. |
