Atomic mass constant
Oxford Dictionary of Biochemistry:
Top
atomic mass constant
symbol: mu; a fundamental constant equal to the mass of a carbon-12 atom divided by 12; it is equal to the unified atomic mass unit.
| atomic absorption spectroscopy, atomic absorption spectrophotometry, atom percent excess | |
| atomic mass unit, atomic number, atomic orbital |
Wikipedia on Answers.com:
Top
Atomic mass constant
|
In physics and chemistry, the atomic mass constant, mu, is one twelfth of the mass of an unbound atom of carbon-12 at rest and in its ground state.[1] It serves to define the atomic mass unit and is, by definition, equal to 1 u. The 2010 CODATA recommended value is 1.660538921(73)×10−27 kg [1][2]
In practice, the atomic mass constant is determined as the ratio of the electron rest mass me to the electron relative atomic mass Ar(e) (that is, the mass of the electron on a scale where 12C = 12).[3] The relative atomic mass of the electron can be measured in cyclotron experiments, while the rest mass of the electron can be derived from other physical constants.
The current uncertainty in the value of the atomic mass constant – one part in 20 million – is almost entirely due to the uncertainty in the value of the Planck constant.
Energy equivalents
The atomic mass constant can also be expressed as its energy equivalent, that is muc2. The 2010 CODATA recommended values are:
The megaelectronvolt (MeV) is commonly used as a unit of mass in particle physics, and these values are also important for the practical determination of relative atomic masses. Although relative atomic masses are defined for neutral atoms, they are measured (by mass spectrometry) for ions: hence, the measured values must be correct for the mass of the electrons that were removed to form the ions, and also for the mass equivalent of the electron binding energy, Eb/muc2. The total binding energy of the six electrons in a carbon-12 atom is 1030.1089 eV = 1.650 4163×10−16 J: Eb/muc2 = 1.105 8674×10−6, or about one part in 10 million of the mass of the atom.[4]
See also
References
- ^ Nic, M.; Jirat, J.; Kosata, B., eds. (2006–). "atomic mass constant". IUPAC Compendium of Chemical Terminology (Online ed.). doi:10.1351/goldbook.{{{file}}}. ISBN 0-9678550-9-8. http://goldbook.iupac.org/A00497.html.
- ^ a b c P.J. Mohr, B.N. Taylor, and D.B. Newell (2011), "The 2010 CODATA Recommended Values of the Fundamental Physical Constants" (Web Version 6.0). This database was developed by J. Baker, M. Douma, and S. Kotochigova. Available: http://physics.nist.gov/constants. National Institute of Standards and Technology, Gaithersburg, MD 20899.
- ^ Mohr, Peter J.; Taylor, Barry N. (1999). "CODATA recommended values of the fundamental physical constants: 1998". J. Phys. Chem. Ref. Data 28 (6): 1713–1852. doi:10.1103/RevModPhys.72.351.
- ^ Mohr, Peter J.; Taylor, Barry N. (2005). "CODATA recommended values of the fundamental physical constants: 2002". Rev. Mod. Phys. 77 (1): 1–107. Bibcode 2005RvMP...77....1M. doi:10.1103/RevModPhys.77.1. .
External links
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Post a question - any question - to the WikiAnswers community:
Copyrights:
Oxford Dictionary of Biochemistry
Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.
Read more
