词条 | Atomic mass unit |
释义 |
| bgcolour = | name = Unified atomic mass unit (Dalton) | image = | caption = | standard = Physical constant (Accepted for use with the SI) | quantity = mass | symbol = u or Da | namedafter = John Dalton | units1 = kg | inunits1 = {{physconst|mu|unit=no|ref=no}} | units2 = MeV/c2 | inunits2 = {{val|931.4940954|(57)}} | units3 = me | inunits3 = {{val|1822.888486192|(53)}} }} The unified atomic mass unit or dalton (symbol: u, or Da or AMU) is a standard unit of mass that quantifies mass on an atomic or molecular scale (atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1{{nbsp}}g/mol.[1] It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest,[2] and has a value of {{physconst|mu|after=,}} or approximately 1.66 yoctograms.[3] The CIPM has categorised it as a non-SI unit accepted for use with the SI, and whose value in SI units must be obtained experimentally.[2] The atomic mass unit (amu) without the "unified" prefix is technically an obsolete unit based on oxygen, which was replaced in 1961. However, many sources still use the term amu but now define it in the same way as u (i.e., based on carbon-12).[4][5] In this sense, most uses of the terms atomic mass units and amu, today, actually refer to unified atomic mass unit. For standardization, a specific atomic nucleus (carbon-12 vs. oxygen-16) had to be chosen because the 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) 1 u. The atomic mass unit is not the unit of mass in the atomic units system, which is rather the electron rest mass (me). Until the 2019 redefinition of SI base units, the number of daltons in a gram is exactly the Avogadro number by definition, or equivalently, a dalton is exactly equivalent to 1 gram/mol. Thereafter, these relationships will no longer be exact, but they will still be extremely accurate approximations.[7][8] History of the atomic mass unitThe standard atomic weight (or atomic weight) scale has traditionally been a relative value, that is without a unit, with the first relative atomic mass basis suggested by John Dalton in 1803 as 1H.[6] Despite the initial mass of 1H 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 {{sfrac|1|16}} mass of oxygen (1903). This evaluation was made prior to the discovery of the existence of elemental isotopes, which occurred in 1912.[6] 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 16O (oxygen-16) was given the mass value of exactly 16 amu in physics. The divergence of these values could result in errors in computations, and was unwieldy. The chemistry amu, based on the relative atomic mass (atomic weight) of natural oxygen (including the heavy naturally-occurring isotopes 17O and 18O), was about {{val|1.000282}} as massive as the physics amu, based on pure isotopic 16O. For these and other reasons, the reference standard for both physics and chemistry was changed to carbon-12 in 1961.[7] The choice of carbon-12 was made to minimise further divergence with prior literature.[6] The new and current unit was referred to as the unified atomic mass unit, u.[8] and given a new symbol, "u", which replaced the now deprecated "amu" that had been connected to the old oxygen-based system. The dalton (Da) is another name for the unified atomic mass unit.[9] Despite this change, modern sources often still use the old term "amu" but define it as u ({{sfrac|12}} of the mass of a carbon-12 atom), as mentioned in the article's introduction. Therefore, in general, "amu" likely does not refer to the old oxygen standard unit, unless the source material originates from the 1960s or before. TerminologyThe 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:
Relationship to the International System of Units: SIThe former definition of the mole, an SI base unit, was accepted by the CGPM in 1971 as:
However, the first part of this definition will be changed on 20 May 2019 to: {{block indent|1=The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly {{val|6.02214076|e=23}} elementary entities. This number is the fixed numerical value of the Avogadro constant, {{math|NA}}, when expressed in the unit mol−1 and is called the Avogadro number.[18][19]}}One consequence of this change is that the current defined relationship between the mass of the 12C atom, the dalton, the kilogram, and the Avogadro number will no longer be valid. One of the following must change:
The wording of the ninth SI Brochure[20][21] implies that the first statement remains valid, which means that the second is no longer true. The molar mass constant, while still with great accuracy remaining 1 g/mol, is no longer exactly equal to that. 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 only approximately NA atoms of carbon-12 in 0.012 kg of carbon-12. This can be expressed mathematically as {{block indent|1=12 u ⋅ NA ≈ 0.012 kg/mol, or}}{{block indent|1=1 u ⋅ NA ≈ 0.001 kg/mol}}UsageMolecular masses of proteins are often expressed in kilodaltons (kDa or kD). For example, a molecule of a protein with molar mass {{val|64000|u=g⋅mol−1}} has a mass of 64 kDa.[1]In research and commerce, the degree of polymerization of synthetic polymers is conventionally expressed in daltons. The US Supreme Court based a major precedent of appellate law on a disputed case of counting daltons for a molecular distribution.[22] Examples
See also{{Portal|Physics}}
Notes1. ^1 {{cite book |last1=Berg |first1=Jeremy M. |last2=Tymoczko |first2=John L. |last3=Stryer |first3=Lubert |year=2007 |title=Biochemistry |edition=6th |publisher=Freeman |location=New York |isbn=978-0-7167-8724-2 |chapter=2 |page=35}} 2. ^1 2 {{SIbrochure8th|page=126}} 3. ^Unified Atomic mass unit. Fundamental Physical Constants from NIST 4. ^{{cite book |last=Chang |first=Raymond |date=2005 |title=Physical Chemistry for the Biosciences |isbn=978-1-891389-33-7 |page=5 |url=https://books.google.com/books?id=PNH1fHj5Tw0C&pg=PA5}} 5. ^{{cite book |last1=Kelter |first1=Paul B. |last2=Mosher |first2=Michael D. |last3=Scott |first3=Andrew |date=2008 |title=Chemistry: The Practical Science |isbn=978-0-547-05393-6 |volume=10 |page=60 |url=https://books.google.com/books?id=VfcKIManfkUC&pg=PA60}} 6. ^1 2 {{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 | year = 1989}} 7. ^{{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}} 8. ^{{GoldBookRef|title=unified atomic mass unit| file = U06554 | accessdate = 2010-07-16}} 9. ^{{GoldBookRef|title=dalton| file = D01514 | accessdate = 2015-04-28}} 10. ^{{cite book | title=Quantities, Units and Symbols in Physical Chemistry International Union of Pure and Applied Chemistry; Physical Chemistry Division | url = https://ciencias.ulisboa.pt/sites/default/files/fcul/dep/dqb/doc/green_book_2ed%20IUPAC.pdf | publisher=International Union of Pure and Applied Chemistry and published for them by Blackwell Science Ltd | edition=2nd | year=1993 | first=Ian | last=Mills | first2=Tomislav | last2= Cvitaš | first3=Klaus | last3=Homann | first4= Nikola | last4= Kallay | first5= Kozo | last5=Kuchitsu | isbn=978-0-632-03583-0}} 11. ^{{cite web | url=http://www.bipm.org/utils/common/pdf/CC/CCU/CCU15.pdf | title=Consultative Committee for Units (CCU); Report of the 15th meeting (17–18 April 2003) to the International Committee for Weights and Measures | accessdate=14 Aug 2010}} 12. ^{{cite web |url = http://archive.iupap.org/commissions/c2/reports/ga-05.html |title = IUPAP: C2: Report 2005 |accessdate = 2018-07-15}} 13. ^{{citation | title = International Standard ISO 80000-1:2009 – Quantities and Units – Part 1: General | publisher = International Organization for Standardization | year = 2009}} 14. ^{{citation | title = International Standard ISO 80000-10:2009 – Quantities and units – Part 10: Atomic and nuclear physics | publisher = International Organization for Standardization | year = 2009}} 15. ^{{cite web | url = http://www.oxfordjournals.org/our_journals/aobpla/for_authors/ | title = Instructions to Authors | work = AoB Plants | publisher = Oxford journals; Oxford University Press | accessdate = 2010-08-22}} 16. ^{{cite journal | journal = Rapid Communications in Mass Spectrometry | title = Author guidelines | year = 2010 | publisher = Wiley-Blackwell}}{{full citation needed|date=August 2017}} 17. ^{{cite journal | url = http://iopscience.iop.org/0026-1394/49/4/487 | title = Why the dalton should be redefined exactly in terms of the kilogram | first1 = B P | last1 = Leonard | year = 2012 | journal = Metrologia | volume = 49 | issue = 4 | pages = 487–491 | doi = 10.1088/0026-1394/49/4/487 | bibcode = 2012Metro..49..487L}} 18. ^[https://www.bipm.org/utils/en/pdf/CIPM/CIPM2017-EN.pdf?page=23 CIPM Report of 106th Meeting] {{webarchive|url=https://web.archive.org/web/20180127202612/https://www.bipm.org/utils/en/pdf/CIPM/CIPM2017-EN.pdf?page=23 |date=27 January 2018 }} Retrieved 7 April 2018 19. ^{{cite web |title=Redefining the Mole |url=https://www.nist.gov/si-redefinition/redefining-mole |website=NIST |publisher=NIST |accessdate=24 October 2018 |date=23 October 2018}} 20. ^1 2 {{cite web |title=Draft of the ninth SI Brochure |publisher=BIPM |url=https://www.bipm.org/utils/en/pdf/si-revised-brochure/Draft-SI-Brochure-2018.pdf |date=5 February 2018|accessdate=12 November 2018}} 21. ^1 A footnote in Table 8 on non-SI units states: "The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state." 22. ^{{cite web|title=Supreme Court Opinion in 'Teva Pharmaceuticals USA, Inc. v. Sandoz, Inc.'|url=https://www.supremecourt.gov/opinions/14pdf/13-854_o7jp.pdf}} 23. ^{{cite journal |vauthors=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 = |bibcode = 2003PNAS..10012688O }} References{{Reflist|30em}}External links
4 : Metrology|Nuclear chemistry|Units of chemical measurement|Units of mass |
随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识,基本涵盖了大多数领域的百科知识,是一部内容自由、开放的电子版国际百科全书。