Dalton (unit)
dalton (unified atomic mass unit) | |
---|---|
Unit of | mass |
Symbol | Da or u |
Named after | John Dalton |
Conversions | |
1 Da or u in ... | ... is equal to ... |
kg | 1.66053906660(50)×10−27 |
mu | 1 |
me | 1822.888486209(53) |
MeV/c2 | 931.49410242(28) |
The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest.[1][2] The atomic mass constant, denoted mu, is defined identically, giving mu = 1/12 m(12C) = 1 Da.[3]
This unit is commonly used in physics and chemistry to express the mass of atomic-scale objects, such as atoms, molecules, and elementary particles, both for discrete instances and multiple types of ensemble averages. For example, an atom of helium-4 has a mass of 4.0026 Da. This is an intrinsic property of the isotope and all helium-4 atoms have the same mass. Acetylsalicylic acid (aspirin), C
9H
8O
4, has an average mass of about 180.157 Da. However, there are no acetylsalicylic acid molecules with this mass. The two most common masses of individual acetylsalicylic acid molecules are 180.0423 Da, having the most common isotopes, and 181.0456 Da, in which one carbon is carbon-13.
The molecular masses of proteins, nucleic acids, and other large polymers are often expressed with the units kilodalton (kDa) and megadalton (MDa).[4] Titin, one of the largest known proteins, has a molecular mass of between 3 and 3.7 megadaltons.[5] The DNA of chromosome 1 in the human genome has about 249 million base pairs, each with an average mass of about 650 Da, or 156 GDa total.[6]
The mole is a unit of amount of substance used in chemistry and physics, which defines the mass of one mole of a substance in grams as numerically equal to the average mass of one of its particles in daltons. That is, the molar mass of a chemical compound is meant to be numerically equal to its average molecular mass. For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water is about 18.0153 grams. A protein whose molecule has an average mass of 64 kDa would have a molar mass of 64 kg/mol. However, while this equality can be assumed for practical purposes, it is only approximate, because of the 2019 redefinition of the mole.[4][1]
In general, the mass in daltons of an atom is numerically close but not exactly equal to the
The dalton differs from the unit of mass in the
Energy equivalents
The atomic mass constant can also be expressed as its energy-equivalent, muc2. The 2018 CODATA recommended values are:
The megaelectronvolt mass-equivalent (MeV/c2) 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.
History
Origin of the concept
The interpretation of the
For technical reasons, in 1898, chemist
Isotopic variation
The discovery of isotopes of oxygen in 1929 required a more precise definition of the unit. Two distinct definitions came into use. Chemists choose to define the AMU as 1/16 of the average mass of an oxygen atom as found in nature; that is, the average of the masses of the known isotopes, weighted by their natural abundance. Physicists, on the other hand, defined it as 1/16 of the mass of an atom of the isotope oxygen-16 (16O).[13]
Definition by IUPAC
The existence of two distinct units with the same name was confusing, and the difference (about 1.000282 in relative terms) was large enough to affect high-precision measurements. Moreover, it was discovered that the isotopes of oxygen had different natural abundances in water and in air. For these and other reasons, in 1961 the International Union of Pure and Applied Chemistry (IUPAC), which had absorbed the ICAW, adopted a new definition of the atomic mass unit for use in both physics and chemistry; namely, 1/12 of the mass of a carbon-12 atom. This new value was intermediate between the two earlier definitions, but closer to the one used by chemists (who would be affected the most by the change).[12][13]
The new unit was named the "unified atomic mass unit" and given a new symbol "u", to replace the old "amu" that had been used for the oxygen-based units.[17] However, the old symbol "amu" has sometimes been used, after 1961, to refer to the new unit, particularly in lay and preparatory contexts.
With this new definition, the
Adoption by BIPM
The IUPAC 1961 definition of the unified atomic mass unit, with that name and symbol "u", was adopted by the
Unit name
In 1993, the IUPAC proposed the shorter name "dalton" (with symbol "Da") for the unified atomic mass unit.[19][20] As with other unit names such as watt and newton, "dalton" is not capitalized in English, but its symbol, "Da", is capitalized. The name was endorsed by the International Union of Pure and Applied Physics (IUPAP) in 2005.[21]
In 2003 the name was recommended to the BIPM by the
2019 redefinition of the SI base units
The definition of the dalton was not affected by the
Measurement
Though relative atomic masses are defined for neutral atoms, they are measured (by
Before the 2019 redefinition of SI units, experiments were aimed to determine the value of the Avogadro constant for finding the value of the unified atomic mass unit.
Josef Loschmidt
A reasonably accurate value of the atomic mass unit was first obtained indirectly by Josef Loschmidt in 1865, by estimating the number of particles in a given volume of gas.[32]
Jean Perrin
Perrin estimated the Avogadro number by a variety of methods, at the turn of the 20th century. He was awarded the 1926 Nobel Prize in Physics, largely for this work.[33]
Coulometry
The electric charge per
The classic experiment is that of Bower and Davis at
The NIST scientists devised a method to compensate for silver lost from the anode by mechanical causes, and conducted an isotope analysis of the silver used to determine its atomic weight. Their value for the conventional Faraday constant was F90 = 96485.39(13) C/mol, which corresponds to a value for the Avogadro constant of 6.0221449(78)×1023 mol−1: both values have a relative standard uncertainty of 1.3×10−6.
Electron mass measurement
In practice, the atomic mass constant is determined from the
where c is the speed of light, h is the Planck constant, α is the fine-structure constant, and R∞ is the Rydberg constant.
As may be observed from the old values (2014 CODATA) in the table below, the main limiting factor in the precision of the Avogadro constant was the uncertainty in the value of the Planck constant, as all the other constants that contribute to the calculation were known more precisely.
Constant | Symbol | 2014 CODATA values
|
Relative standard uncertainty | Correlation coefficient with NA |
---|---|---|---|---|
Proton–electron mass ratio
|
mp/me | 1836.15267389(17) | 9.5×10−11 | −0.0003 |
Molar mass constant | Mu | 0.001 kg/mol = 1 g/mol | 0 (defined) | — |
Rydberg constant | R∞ | 10973731.568508(65) m−1 | 5.9×10−12 | −0.0002 |
Planck constant | h | 6.626070040(81)×10−34 J⋅s | 1.2×10−8 | −0.9993 |
Speed of light | c | 299792458 m/s | 0 (defined) | — |
Fine structure constant
|
α | 7.2973525664(17)×10−3 | 2.3×10−10 | 0.0193 |
Avogadro constant | NA | 6.022140857(74)×1023 mol−1 | 1.2×10−8 | 1 |
The power of the
Constant | Symbol | 2018 CODATA values[37]
|
Relative standard uncertainty | Correlation coefficient with NA |
---|---|---|---|---|
Proton–electron mass ratio
|
mp/me | 1836.15267343(11) | 6.0×10−11 | — |
Molar mass constant | Mu | 0.99999999965(30)×10−3 kg/mol | 3.0×10−10 | — |
Rydberg constant | R∞ | 10973731.568160(21) m−1 | 1.9×10−12 | — |
Planck constant | h | 6.62607015×10−34 J⋅s | 0 (defined) | — |
Speed of light | c | 299792458 m/s | 0 (defined) | — |
Fine structure constant
|
α | 7.2973525693(11)×10−3 | 1.5×10−10 | — |
Avogadro constant | NA | 6.02214076×1023 mol−1 | 0 (defined) | — |
X-ray crystal density methods
Silicon single crystals may be produced today in commercial facilities with extremely high purity and with few lattice defects. This method defined the Avogadro constant as the ratio of the molar volume, Vm, to the atomic volume Vatom:
The unit cell of silicon has a cubic packing arrangement of 8 atoms, and the unit cell volume may be measured by determining a single unit cell parameter, the length a of one of the sides of the cube.[38] The 2018 CODATA value of a for silicon is 5.431020511(89)×10−10 m.[39]
In practice, measurements are carried out on a distance known as d220(Si), which is the distance between the planes denoted by the Miller indices {220}, and is equal to a/√8.
The
See also
Notes
- Uncertainty notation.
References
- ^ a b c d e Bureau International des Poids et Mesures (2019): The International System of Units (SI), 9th edition, English version, page 146. Available at the BIPM website.
- S2CID 115540416.
- ^ ISBN 978-0-7167-8724-2.
- PMID 14563922.
- ^ Integrated DNA Technologies (2011): "Molecular Facts and Figures Archived 2020-04-18 at the Wayback Machine". Article on the IDT website, Support & Education section Archived 2021-01-19 at the Wayback Machine, accessed on 2019-07-08.
- ^ "2018 CODATA Value: proton mass in u". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2022-09-11.
- ^ "2018 CODATA Value: neutron mass in u". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2020-06-24.
- ^ "2018 CODATA Value: atomic mass constant energy equivalent". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-07-21.
- ^ "2018 CODATA Value: atomic mass constant energy equivalent in MeV". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-07-21.
- ^ .
- ^ a b c Holden, Norman E. (2004). "Atomic Weights and the International Committee—A Historical Review". Chemistry International. 26 (1): 4–7.
- Annales de Chimie et de Physique. 8e Série. 18: 1–114. Extract in English, translation by Frederick Soddy.
- ISBN 978-1-891389-33-7.
- ISBN 978-0-547-05393-6.
- ^ Bureau International des Poids et Mesures (1971): 14th Conference Générale des Poids et Mesures Archived 2020-09-23 at the Wayback Machine Available at the BIPM website.
- ISBN 978-0-632-03583-0.
- ^ "IUPAP: C2: Report 2005". Retrieved 2018-07-15.
- ^ "Consultative Committee for Units (CCU); Report of the 15th meeting (17–18 April 2003) to the International Committee for Weights and Measures" (PDF). Retrieved 14 Aug 2010.
- ISBN 92-822-2213-6, archived(PDF) from the original on 2021-06-04, retrieved 2021-12-16
- ^ International Standard ISO 80000-1:2009 – Quantities and Units – Part 1: General. International Organization for Standardization. 2009.
- ^ International Standard ISO 80000-10:2009 – Quantities and units – Part 10: Atomic and nuclear physics, International Organization for Standardization, 2009
- ^ "Instructions to Authors". AoB Plants. Oxford journals; Oxford University Press. Archived from the original on 2011-11-03. Retrieved 2010-08-22.
- ^ "Author guidelines". Rapid Communications in Mass Spectrometry. Wiley-Blackwell. 2010.
- ^ International Bureau for Weights and Measures (2017): Proceedings of the 106th meeting of the International Committee for Weights and Measures (CIPM), 16-17 and 20 October 2017, page 23. Available at the BIPM website Archived 2021-02-21 at the Wayback Machine.
- ^ International Bureau for Weights and Measures (2018): Resolutions Adopted - 26th Conference Générale des Poids et Mesures Archived 2018-11-19 at the Wayback Machine. Available at the BIPM website.
- .
- doi:10.1103/RevModPhys.77.1. Archived from the original(PDF) on 2017-10-01.
- ^ Loschmidt, J. (1865). "Zur Grösse der Luftmoleküle". Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften Wien. 52 (2): 395–413. English translation.
- ^ Oseen, C.W. (December 10, 1926). Presentation Speech for the 1926 Nobel Prize in Physics.
- NIST. Accessed on 2019-07-03.
- doi:10.1063/1.556049. Archived from the original(PDF) on 2017-10-01.
- doi:10.1063/1.556049. Archived from the original(PDF) on 2017-10-01.
- ^ "Constants bibliography, source of the CODATA internationally recommended values". The NIST Reference on Constants, Units, and Uncertainty. Retrieved 4 August 2021.
- ^ "Unit Cell Formula". Mineralogy Database. 2000–2005. Retrieved 2007-12-09.
- ^ "2018 CODATA Value: Lattice parameter of silicon". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-08-23.
- ^ "2018 CODATA Value: molar volume of silicon". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 2019-08-23.
External links
- "Atomic weights and isotopic compositions". physics.nist.gov. Physical Reference Data. National Institute for Standards and Technology. 23 August 2009.
- "Atomic mass unit". sizes.com. Archived from the original on 2008-01-15.