Dalton (unit)
The dalton, or unified atomic mass unit, is a unit of mass defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. It is a non-SI unit accepted for use with SI. The word "unified" emphasizes that the definition was accepted by both IUPAP and IUPAC. The atomic mass constant, denoted, is an atomic-scale reference mass, defined identically, but it is not a unit of mass. Expressed in terms of, the atomic mass of carbon-12:. The dalton's numerical value in terms of the fixed-h kilogram is an experimentally determined quantity that, along with its inherent uncertainty, is updated periodically. The 2022 CODATA recommended value of the atomic mass constant expressed in the SI base unit kilogram is: The previous 2018 CODATA value was used in the traditional definition of the Avogadro number to obtain the value, which was then rounded to 9 significant figures and used to define it at exactly that value for the 2019 redefinition of the mole.
The value serves as a conversion factor of mass from daltons to kilograms, which can easily be converted to grams and other metric units of mass. The 2019 revision of the SI redefined the kilogram by fixing the value of the Planck constant, improving the precision of the atomic mass constant expressed in SI units by anchoring it to fixed physical constants. Although the dalton remains defined via carbon-12, the revision enhances traceability and accuracy in atomic mass measurements.
The mole is a unit of amount of substance used in chemistry and physics, such that the mass of one mole of a substance expressed in grams is numerically equal to the average mass of an elementary entity of the substance expressed in daltons. For example, the average mass of one molecule of water is about 18.0153 Da, and the mass of one mole of water is about 18.0153 g. A protein whose molecule has an average mass of would have a molar mass of. However, while this equality can be assumed for practical purposes, it is only approximate, because of the 2019 redefinition of the mole.
Usage
The dalton 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. This is an intrinsic property of the isotope and all helium-4 atoms have the same mass. Acetylsalicylic acid,, has an average mass of about. However, there are no acetylsalicylic acid molecules with this mass. The two most common masses of individual acetylsalicylic acid molecules are, having the most common isotopes, and, in which one carbon is carbon-13.The molecular masses of proteins, nucleic acids, and other large polymers are often expressed with the unit kilo
In general, the mass in daltons of an atom is numerically close but not exactly equal to the number of nucleons in its nucleus. It follows that the molar mass of a compound is numerically close to the average number of nucleons contained in each molecule. By definition, the mass of an atom of carbon-12 is 12 daltons, which corresponds with the number of nucleons that it has. However, the mass of an atomic-scale object is affected by the binding energy of the nucleons in its atomic nuclei, as well as the mass and binding energy of its electrons. Therefore, this equality holds only for the carbon-12 atom in the stated conditions, and will vary for other substances. For example, the mass of an unbound atom of the common hydrogen isotope is, the mass of a proton is the mass of a free neutron is and the mass of a hydrogen-2 atom is. In general, the difference is less than 0.1%; exceptions include hydrogen-1, helium-3, lithium-6 and beryllium.
The dalton differs from the unit of mass in the system of atomic units, which is the electron rest mass.
Energy equivalents
The atomic mass constant can also be expressed as its energy-equivalent,. The CODATA recommended values are:The mass-equivalent is commonly used in place of 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 law of definite proportions in terms of the atomic theory of matter implied that the masses of atoms of various elements had definite ratios that depended on the elements. While the actual masses were unknown, the relative masses could be deduced from that law. In 1803 John Dalton proposed to use the atomic mass of the lightest atom, hydrogen, as the natural unit of atomic mass. This was the basis of the atomic weight scale.For technical reasons, in 1898, chemist Wilhelm Ostwald and others proposed to redefine the unit of atomic mass as the mass of an oxygen atom. That proposal was formally adopted by the International Committee on Atomic Weights in 1903. That was approximately the mass of one hydrogen atom, but oxygen was more amenable to experimental determination. This suggestion was made before the discovery of isotopes in 1912. Physicist Jean Perrin had adopted the same definition in 1909 during his experiments to determine the atomic masses and the Avogadro constant. This definition remained unchanged until 1961. Perrin also defined the "mole" as an amount of a compound that contained as many molecules as 32 grams of oxygen. He called that number the Avogadro number in honor of physicist Amedeo Avogadro.
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 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 of the mass of an atom of the isotope oxygen-16.Joint definition by IUPAP and IUPAC
The existence of two distinct units with the same name was confusing, and the difference 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.In April 1957 Alfred O. C. Nier suggested to Josef Mattauch that carbon-12 be adopted as the mass scale because of carbon's use as a secondary standard in mass spectrometry. Also, carbon-12 implied acceptable relative changes in the atomic weight scale, i.e., 42 parts-per-million compared to 275 ppm for oxygen-16, which would not be acceptable to chemists.
Following the approval of the International Union of Pure and Applied Physics General Assembly at Ottawa, Canada, in 1960 and the International Union of Pure and Applied Chemistry General Assembly at Montreal, Canada, in 1961, the atomic weights were officially given on the carbon-12 scale for the first time.
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 unit. 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 standard atomic weight of carbon is about and that of oxygen is about. These values, generally used in chemistry, are based on averages of many samples from Earth's crust, its atmosphere, and organic materials.
Adoption by BIPM
The IUPAC 1961 definition of the unified atomic mass unit, with that name and symbol "u", was adopted by the International Bureau for Weights and Measures in 1971 as a non-SI unit accepted for use with the SI.Unit name
In 1993, the IUPAC proposed the shorter name "dalton" for the unified atomic mass unit. 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 in 2005.In 2003 the name was recommended to the BIPM by the Consultative Committee for Units, part of the CIPM, as it "is shorter and works better with prefixes". In 2006, the BIPM included the dalton in its 8th edition of the SI brochure of formal definitions as a non-SI unit accepted for use with the SI. The name was also listed as an alternative to "unified atomic mass unit" by the International Organization for Standardization in 2009. It is now recommended by several scientific publishers, and some of them consider "atomic mass unit" and "amu" deprecated. In 2019, the BIPM retained the dalton in its 9th edition of the SI brochure, while dropping the unified atomic mass unit from its table of non-SI units accepted for use with the SI, but secondarily notes that the dalton and the unified atomic mass unit are alternative names for the same unit.