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| {{Infobox unit
| | Flower Grower Gonsales from Prince Albert, has pastimes which includes reading to the, health and fitness and urban exploration. Has enrolled in a global contiki tour. Is extremely ecstatic specifically about visiting Biodiversity and Culture.<br><br>Review my blog post - [https://trac.cc.jyu.fi/projects/grain/ticket/93274 best kosher Weight gainer] |
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| | name = unified atomic mass unit<br>or dalton
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| | image =
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| | caption =
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| | standard = [[Physical constant]]<br>([[Non-SI units mentioned in the SI|Accepted for use with the SI]])<!-- not a member of any system of units – see article Natural units for more info-->
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| | quantity = [[mass]]
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| | symbol = u or Da
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| | namedafter = [[John Dalton]]
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| | units1 = kg
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| | inunits1 = {{val|fmt=commas|1.660538921|(73)|e=-27}}
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| | units2 = eV/''c''<sup>2</sup>
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| | inunits2 ={{val|fmt=commas|931.494061|(21)|e=6}}
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| | units3 = ''m''<sub>e</sub>
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| | inunits3 = 1822.88839
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| | units4 = [[Planck mass|''m''<sub>P</sub>]]
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| | inunits4 = {{val|fmt=commas|7.62936|e=-20}}
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| }}
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| The '''unified atomic mass unit''' (symbol: '''u''') or '''dalton''' (symbol: '''Da''') is the standard [[Unit of measurement|unit]] that is used for indicating [[mass]] on an atomic or molecular scale ([[atomic mass]]). One unified atomic mass unit is approximately the mass of a [[nucleon]] and is equivalent to 1{{nbsp}}[[Molar mass|g/mol]].<ref name="Stryer 2007 35">{{cite book|last=Stryer|first=Jeremy M. Berg; John L. Tymoczko; Lubert|title=Biochemistry|year=2007|publisher=Freeman|location=New York|isbn=978-0-7167-8724-2|edition=6. ed., 3. print.|accessdate=27 April 2012|page=35|chapter=2}}</ref> It is defined as one twelfth of the mass of an unbound neutral atom of [[carbon-12]] in its nuclear and electronic [[ground state]],<ref name="SI">{{SIbrochure8th|page=126}}</ref> and has a value of {{val|fmt=commas|1.660538921|(73)|e=-27|ul=kg}}.<ref name="CODATA">[http://physics.nist.gov/constants Fundamental Physical Constants from NIST<!-- Bot generated title -->]</ref> The [[International Committee for Weights and Measures|CIPM]] has categorised it as a non-[[International System of Units|SI]] unit accepted for use with the SI, and whose value in SI units must be obtained experimentally.<ref name="SI"/>
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| The '''amu''' without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, some sources still use the amu but now define it in the same way as u (based on carbon-12). In this sense, most uses of atomic mass units or amu today actually refer to unified atomic mass units or u. For standardization a specific atomic nucleus (carbon-12 vs. oxygen-16) had to be chosen because average mass of a nucleon depends on the count of the nucleons in the atomic nucleus due to [[mass defect]]. This is also why the mass of a proton (or neutron) by itself is more than (and not equal to) 1u.
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| Atomic mass unit does ''not'' stand for the unit of mass in the [[atomic units]] system, which is rather ''m''<sub>e</sub>.
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| ==History==
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| The [[relative atomic mass]] (atomic weight) scale has traditionally been a relative scale, that is without an explicit unit, with the first relative atomic mass basis suggested by [[John Dalton]] in 1803 as <sup>1</sup>H.<ref name="Petley89">{{citation | last = Petley | first = B. W. | title = The atomic mass unit | journal = IEEE Trans. Instrum. Meas. | volume = 38 | issue = 2 | pages = 175–79 | doi = 10.1109/19.192268}}</ref> Despite the initial mass of <sup>1</sup>H being used as the natural unit for relative atomic mass, it was suggested by [[Wilhelm Ostwald]] that relative atomic mass would be best expressed in terms of units of 1/16 mass of oxygen. This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.<ref name="Petley89"/>
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| The discovery of isotopic oxygen in 1929 led to a divergence in relative atomic mass representation, with isotopically weighted oxygen (i.e., naturally occurring oxygen [[relative atomic mass]]) given a value of exactly 16 atomic mass units (amu) in chemistry, while pure <sup>16</sup>O (oxygen-16) was given the mass value of exactly 16 amu in physics.
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| The divergence of these values could result in errors in computations, and was unwieldy. The chemistry amu, based on the [[relative atomic mass]] of natural oxygen (including the heavy naturally-occurring isotopes <sup>17</sup>O and <sup>18</sup>O), was about 1.000282 more massive than the physics amu, based on pure isotopic <sup>16</sup>O.
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| For these and other reasons, the reference standard for both physics and chemistry was changed to [[carbon-12]] in 1961.<ref>{{citation | last = Holden | first = Norman E. | year = 2004 | title = Atomic Weights and the International Committee—A Historical Review | journal = Chem. Int. | volume = 26 | issue = 1 | pages = 4–7 | url = http://www.iupac.org/publications/ci/2004/2601/1_holden.html}}</ref> The choice of carbon-12 was made to minimise further divergence with prior literature.<ref name="Petley89"/> The new and current unit was referred to as the "unified atomic mass unit" u.<ref>{{GoldBookRef|title=unified atomic mass unit| file = U06554 | accessdate = 2010-07-16}}</ref> and given a new symbol "u," which replaced the now deprecated "amu" that had been connected to the old oxygen-based system.
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| However, modern sources often still use "amu" in place of "u" (as a synonym) and define "amu" in terms of carbon-12. In general, "amu" probably does not refer to the old oxygen standard amu, unless the material is older than the 1960s.
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| The "unified atomic mass unit" u was defined as:
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| : <math> 1 \mathrm{u} = m_{\mathrm{u}} = \frac{1}{12}m\left({}^{12}\mathrm{C}\right)</math>
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| ==Terminology==
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| The unified atomic mass unit and the dalton are different names for the same unit of measure. As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol Da is capitalized. With the introduction of the name "dalton", there has been a gradual change towards using that name in preference to the name "unified atomic mass unit":
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| *In 1993, the International Union of Pure and Applied Chemistry approved the use of the dalton with the qualification that the GCPM had not given its approval.<ref>{{cite book
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| |title=Quantities, Units and Symbols in Physical Chemistry International Union of Pure and Applied Chemistry; Physical Chemistry Division
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| |url = http://www.iupac.org/publications/books/gbook/green_book_2ed.pdf
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| |publisher=International Union of Pure and Applied Chemistry and published for them by Blackwell Science Ltd
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| |edition=2nd
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| |year=1993n
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| |first=Ian|last=Mills
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| |first2=Tomislav|last2= Cvitaš
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| |first3=Klaus|last3=Homann
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| |first4= Nikola|last4= Kallay
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| |first5= Kozo |last5=Kuchitsu
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| |ISBN=0-632-03583-8}}</ref>
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| *In 2003 the Consultative Committee for Units, part of the [[CIPM]], recommended a preference for the usage of the "''dalton''" over the "''unified atomic mass unit''" as it "''is shorter and works better with prefixes''".<ref name="bipm-ccu115">{{cite web
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| |url=http://www.bipm.org/utils/common/pdf/CCU15.pdf
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| |title=Consultative Committee for Units (CCU); Report of the 15th meeting (17–18 April 2003) to the International Committee for Weights and Measures
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| |accessdate=14 Aug 2010}}</ref>
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| *In 2005, the International Union of Pure and Applied Physics endorsed the use of the dalton as an alternative to the unified atomic mass unit.<ref>{{cite web
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| |url=http://www.iupap.org/commissions/interunion/iu14/ga-05.html|title=IU14. IUPAC Interdivisional Committee on Nomenclature and Symbols (ICTNS)
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| |accessdate = 2010-08-14}}</ref>
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| *In 2006, in the 8th edition of the formal definition of [[SI]], the [[CIPM]] cataloged the dalton alongside the unified atomic mass unit as a "Non-SI units whose values in SI units must be obtained experimentally: Units accepted for use with the SI".<ref name="SI"/> The definition also noted that "''The dalton is often combined with SI prefixes ...''"
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| *In 2009, when the International Organization for Standardization published updated versions of ISO 80000, it gave mixed messages as to whether or not the unified atomic mass unit had been deprecated: ISO ISO 80000-1:2009 (General), identified the dalton as having "''earlier [been] called the unified atomic mass unit u''",<ref name = ISO1>{{citation
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| |title = International Standard ISO 80000-1:2009 – Quantities and Units – Part 1: General
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| |publisher = International Organization for Standardization
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| |year = 2009}}</ref> but ISO 80000-10:2009 (atomic and nuclear physics) catalogued both as being alternatives for each other.<ref name = ISO10>{{citation
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| |title = International Standard ISO 80000-10:2009 – Quantities and units – Part 10: Atomic and nuclear physics
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| |publisher = International Organization for Standardization
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| |year = 2009}}</ref>
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| *The 2010 version of the Oxford University Press style guide for authors in life sciences gave the following guidance "''Use the Système international d'unités (SI) wherever possible ... The dalton (Da) or more conveniently the kDa is a permitted non-SI unit for molecular mass or mass of a particular band in a separating gel.''"<ref>{{cite web
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| |url = http://www.oxfordjournals.org/our_journals/aobpla/for_authors/
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| |title = Instructions to Authors
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| |work = AoB Plants
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| |publisher = Oxford journals; Oxford University Press
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| |accessdate = 2010-08-22}}</ref> At the same time, the author guidelines for the journal "''Rapid Communications in Mass Spectrometry''" stated "''The dalton (Da) is a unit of mass normally used for the molecular weight ... use of the Da in place of the u has become commonplace in the mass spectrometry literature ... The "atomic mass unit", abbreviated "amu", is an archaic unit''".<ref>{{cite journal
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| |journal = Rapid Communications in Mass Spectrometry
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| |url = http://en.wikipedia.org/w/index.php?title=Atomic_mass_unit&action=edit§ion=2
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| |title = Author guidelines
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| |year = 2010
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| |publisher = Wiley-Blackwell
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| |accessdate = 2011-05-08}}</ref>
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| *In 2012, in response to the proposed [[New SI definitions|redefinition of the kilogram]], it was proposed that the dalton be redefined as being 0.001/N<sub>A</sub> kg, thereby breaking the link with <sup>12</sup>C. This would result in the dalton and the atomic mass unit having different definitions and differing from each other by a factor of the order of 10<sup>−9</sup>.<ref>{{cite journal
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| |url = http://iopscience.iop.org/0026-1394/49/4/487;jsessionid=986AE228F53922C2D638EDD0E0814D97.c3
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| |title = Why the dalton should be redefined exactly in terms of the kilogram
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| |first1 = B P
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| |last1 = Leonard
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| |year = 2012
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| |journal = Metrologia
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| |number = 49
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| |page = 487
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| |doi = 10.1088/0026-1394/49/4/487|bibcode = 2012Metro..49..487L }}</ref>
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| ==Relationship to SI==
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| The definition of the [[mole (unit)|mole]], an [[SI base unit]], was accepted by the [[CGPM]] in 1971 as:
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| # The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12; its symbol is "mol".
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| # When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
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| The definition of the mole also determines the value of the universal constant that relates the number of entities to amount of substance for any sample. This constant is called the [[Avogadro constant]], symbol ''N''<sub>A</sub> or ''L'', and is equal to {{val|fmt=commas|6.02214129|(27)|e=23}} entities per mole.<ref>{{SIbrochure8th|pages=114,115}}</ref>
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| Given that the unified atomic mass unit is one twelfth the mass of one atom of carbon-12, meaning the mass of such an atom is 12 u, it follows that there are ''N''<sub>A</sub> atoms of carbon-12 in 0.012 kg of carbon-12. This can be expressed mathematically as
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| :''N''<sub>A</sub> (12 u) = 0.012 kg/mol, or
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| :''N''<sub>A</sub> u = 0.001 kg/mol
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| Masses of proteins are often expressed in daltons. For example, a protein with a molecular weight of 64,000 g mol<sup>−1</sup> has a mass of 64,000 daltons or 64 kDa in short.<ref name="Stryer 2007 35"/>
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| ==Examples==
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| * A [[Hydrogen|hydrogen-1]] atom has a mass of 1.007 825 0 u (1.007 825 0 Da).
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| * By definition, a [[carbon-12]] atom has a mass of 12 u (12 Da).
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| * A molecule of [[Aspirin|acetylsalicylic acid (Aspirin)]] has a mass of 180.16 u (180.16 Da).
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| * [[Titin]], the largest known [[protein]], has an atomic mass of 3-3.7 megadaltons (3,000,000 Da).<ref name="pmid14563922">{{cite journal | author = Opitz CA, Kulke M, Leake MC, Neagoe C, Hinssen H, Hajjar RJ, Linke WA | title = Damped elastic recoil of the titin spring in myofibrils of human myocardium | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 100 | issue = 22 | pages = 12688–93 |date=October 2003 | pmid = 14563922 | pmc = 240679 | doi = 10.1073/pnas.2133733100 | url = | issn = }}</ref>
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| == See also ==
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| * [[Kendrick mass]]
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| * [[Mass-to-charge ratio]]
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| * [[Atomic mass constant]]
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| == Notes and references ==
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| {{Reflist|2}}
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| ==External links==
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| *[http://www.sizes.com/units/atomic_mass_unit.htm atomic mass unit] at sizes.com
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| {{SI units}}
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| [[Category:Metrology]]
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| [[Category:Nuclear chemistry]]
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| [[Category:Units of chemical measurement]]
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| [[Category:Units of mass]]
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