Unified atomic mass unit: Difference between revisions
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The '''unified atomic mass unit''' ('''u'''), or '''dalton''' ('''Da'''), is a unit of atomic and molecular mass. By definition it is one twelfth of the mass of an unbound [[carbon]]-12 (<sup>12</sup>C) [[atom]], at rest and in its ground state. | The '''unified atomic mass unit''' ('''''u'''''), or '''dalton''' ('''Da'''), is a unit of atomic and molecular mass. By definition it is one twelfth of the mass of an unbound [[carbon]]-12 (<sup>12</sup>C) [[atom]], at rest and in its ground state. | ||
The relationship of the unified atomic mass unit to the macroscopic [[SI]] base unit of mass, the [[kilogram]], is given by [[Avogadro's number]] ''N''<sub>A</sub>. By the definition of Avogadro's number, the mass of ''N''<sub>A</sub> carbon-12 atoms, at rest and in their ground state, is 12 gram ( = 12×10<sup>−3</sup> kg). From the latest value of ''N''<sub>A</sub> follows the latest value<ref> [http://physics.nist.gov/cuu/ | The relationship of the unified atomic mass unit to the macroscopic [[SI]] base unit of mass, the [[kilogram]], is given by [[Avogadro's number]] ''N''<sub>A</sub>. By the definition of Avogadro's number, the mass of ''N''<sub>A</sub> carbon-12 atoms, at rest and in their ground state, is 12 gram ( = 12×10<sup>−3</sup> kg). From the latest value of ''N''<sub>A</sub> follows the latest value of the unified atomic mass unit:<ref name=NISTu> | ||
:1 u ≈ 1.660 | |||
{{cite web |title= Fundamental physical constants: atomic mass unit-kilogram relationship 1 ''u'' |publisher=[[NIST]] |accessdate=2011-09-09 |url=http://physics.nist.gov/cgi-bin/cuu/Value?ukg}} | |||
</ref> | |||
:1 ''u'' ≈ 1.660 538 921(73) × 10<sup>−27</sup> kg | |||
Future refinements in Avogadro's number by future improvements in counting large (on the order of 10<sup>27</sup>) numbers of atoms, will give better accuracy of u. It is hoped that in the future the experimental accuracy of Avogadro's constant will improve so much that the unified atomic mass unit may replace the kilogram as the SI base unit, see [[kilogram|this article]]. | Future refinements in Avogadro's number by future improvements in counting large (on the order of 10<sup>27</sup>) numbers of atoms, will give better accuracy of ''u''. It is hoped that in the future the experimental accuracy of Avogadro's constant will improve so much that the unified atomic mass unit may replace the kilogram as the SI base unit, see [[kilogram|this article]]. | ||
The unit ''u'' is convenient because one [[hydrogen]] atom has a mass of approximately 1 ''u'', and more generally an [[atom]] or [[molecule]] that contains ''p'' [[proton]]s and ''n'' [[neutron]]s will have a mass approximately equal to (''p'' + ''n'') ''u''. The mass of a nucleus is not exactly equal to ''p'' + ''n'', because the [[nuclear binding energy]] gives rise to a relativistic [[mass defect]]. | |||
==Confusions== | |||
In the system of [[atomic units]], the "atomic unit of mass" is the mass of the electron ''m<sub>e</sub>''.<ref name=NISTe> | |||
{{cite web |title= Fundamental physical constants: atomic unit of mass ''m<sub>e</sub>'' |publisher=[[NIST]] |accessdate=2011-09-09 |url=http://physics.nist.gov/cgi-bin/cuu/Value?ttme}} | |||
</ref> But further confusion prevails: | |||
In the literature one still finds the obsolete unit amu (atomic mass unit). This is deplorable, not only because it is not a unit accepted for use with the [[SI]], but also because two ''different'' standard masses are denoted by amu. There is the physicist's amu ( = 1/1.000 317 9 u) and there is the chemist's amu ( = 1/1.000 043 u). | In the literature one still finds the obsolete unit ''amu'' (atomic mass unit). This is deplorable, not only because it is not a unit accepted for use with the [[SI]], but also because two ''different'' standard masses are denoted by ''amu''. There is the physicist's ''amu'' ( = 1/1.000 317 9 ''u'') and there is the chemist's ''amu'' ( = 1/1.000 043 ''u''). | ||
This difference arose from the fact that before 1960, in physics the amu was defined as 1/16 of the mass of one atom of oxygen-16, while in chemistry the amu was defined as 1/16 of the ''average'' mass of an oxygen atom (averaged over the natural abundance of the different oxygen [[isotope]]s). Both units are slightly smaller than the unified atomic mass unit that was adopted by the [[International Union of Pure and Applied Physics]] in 1960 and by the [[International Union of Pure and Applied Chemistry]] in 1961. Because chemists and physicists now use the same atomic mass unit, it is referred to as ''unified''. It is a non-[[SI]] unit accepted for use with the SI, whose value in SI units is obtained experimentally. | This difference arose from the fact that before 1960, in physics the amu was defined as 1/16 of the mass of one atom of oxygen-16, while in chemistry the amu was defined as 1/16 of the ''average'' mass of an oxygen atom (averaged over the natural abundance of the different oxygen [[isotope]]s). Both units are slightly smaller than the unified atomic mass unit that was adopted by the [[International Union of Pure and Applied Physics]] in 1960 and by the [[International Union of Pure and Applied Chemistry]] in 1961. Because chemists and physicists now use the same atomic mass unit, it is referred to as ''unified''. It is a non-[[SI]] unit accepted for use with the SI, whose value in SI units is obtained experimentally. | ||
Revision as of 11:17, 10 September 2011
The unified atomic mass unit (u), or dalton (Da), is a unit of atomic and molecular mass. By definition it is one twelfth of the mass of an unbound carbon-12 (12C) atom, at rest and in its ground state.
The relationship of the unified atomic mass unit to the macroscopic SI base unit of mass, the kilogram, is given by Avogadro's number NA. By the definition of Avogadro's number, the mass of NA carbon-12 atoms, at rest and in their ground state, is 12 gram ( = 12×10−3 kg). From the latest value of NA follows the latest value of the unified atomic mass unit:[1]
- 1 u ≈ 1.660 538 921(73) × 10−27 kg
Future refinements in Avogadro's number by future improvements in counting large (on the order of 1027) numbers of atoms, will give better accuracy of u. It is hoped that in the future the experimental accuracy of Avogadro's constant will improve so much that the unified atomic mass unit may replace the kilogram as the SI base unit, see this article.
The unit u is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains p protons and n neutrons will have a mass approximately equal to (p + n) u. The mass of a nucleus is not exactly equal to p + n, because the nuclear binding energy gives rise to a relativistic mass defect.
Confusions
In the system of atomic units, the "atomic unit of mass" is the mass of the electron me.[2] But further confusion prevails:
In the literature one still finds the obsolete unit amu (atomic mass unit). This is deplorable, not only because it is not a unit accepted for use with the SI, but also because two different standard masses are denoted by amu. There is the physicist's amu ( = 1/1.000 317 9 u) and there is the chemist's amu ( = 1/1.000 043 u).
This difference arose from the fact that before 1960, in physics the amu was defined as 1/16 of the mass of one atom of oxygen-16, while in chemistry the amu was defined as 1/16 of the average mass of an oxygen atom (averaged over the natural abundance of the different oxygen isotopes). Both units are slightly smaller than the unified atomic mass unit that was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961. Because chemists and physicists now use the same atomic mass unit, it is referred to as unified. It is a non-SI unit accepted for use with the SI, whose value in SI units is obtained experimentally.
The first standardization of atomic mass was when the chemist John Dalton introduced in the early nineteenth century the mass of one atom of hydrogen as the atomic mass unit. Later Francis Aston, inventor of the mass spectrometer, replaced it by one sixteenth of the mass of one atom of oxygen-16.
Reference
- ↑ Fundamental physical constants: atomic mass unit-kilogram relationship 1 u. NIST. Retrieved on 2011-09-09.
- ↑ Fundamental physical constants: atomic unit of mass me. NIST. Retrieved on 2011-09-09.