Atomic mass constant

In physics and chemistry, the atomic mass constant, "m"_u, is one twelfth of the mass of an unbound atom of the carbon-12 nuclide at rest and in its ground state. [GoldBookRef|title=atomic mass constant|url=http://goldbook.iupac.org/A00497.html] It serves to define the atomic mass unit and is, by definition, equal to 1 u. The 2006 CODATA recommended value [CODATA [http://physics.nist.gov/cgi-bin/cuu/Value?na|search_for=abbr_in! 2006] .] is 1.660 538 782(83)e|–27 kg.

In practice, the atomic mass constant is determined as the ratio of the relative atomic mass of the electron (that is, the mass of the electron on a scale where sup|12C = 12) to the rest mass of the electron (in SI units). [cite journal | first = Peter J. | last = Mohr | coauthors = Taylor, Barry N. | title = CODATA recommended values of the fundamental physical constants: 1998 | journal = J. Phys. Chem. Ref. Data | volume = 28 | issue = 6 | pages = 1713–1852 | doi = 0047-2689/99/28(6)/1713/140/$71.00 | year = 1999] The relative atomic mass of the electron can be measured in cyclotron experiments, while the rest mass of the electron can be derived from other physical constants.:$m\_\{\; m\; u\}\; =\; frac\{A\_\{\; m\; r\}(\{\; m\; e\})\}\{m\_\{\; m\; e\; =\; frac\{A\_\{\; m\; r\}(\{\; m\; e\})calpha^2\}\{2R\_infty\; h\}$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.

See also

*Atomic mass

References

External links

* [http://physics.nist.gov/PhysRefData/Compositions/ Atomic weights and isotopic compositions]