Search results
The dalton or unified atomic mass unit (symbols: Da or u) is a 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.
Oct 1, 2024 · The mass of an atom consists of the mass of the nucleus plus that of the electrons, so the atomic mass unit is not exactly the same as the mass of the proton or neutron. Atomic mass units are also called daltons (Da), for chemist John Dalton.
- The Editors of Encyclopaedia Britannica
- Energy Equivalents
- History
- Measurement
- See Also
- External Links
- Notes and References
The atomic mass constant can also be expressed as its energy-equivalent, mc. 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.
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 (still unknown) 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...
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 (16O).
Definition by IUPAC
The existence of two distinct units with the same name was confusing, and the difference (about 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;...
Though relative atomic masses are defined for neutral atoms, they are measured (by mass spectrometry) for ions: hence, the measured values must be corrected for the mass of the electrons that were removed to form the ions, and also for the mass equivalent of the electron binding energy, E/mc. The total binding energy of the six electrons in a carbo...
Web site: Atomic weights and isotopic compositions . Physical Reference Data . physics.nist.gov . 23 August 2009 . .Web site: Atomic mass unit . sizes.com . dead . https://web.archive.org/web/20080115211624/https://www.sizes.com/units/atomic_mass_unit.htm . 2008-01-15 .Barry N. . Taylor . 2009 . Molar mass and related quantities in the new SI . Metrologia . 46 . 3 . L16–L19 . 10.1088/0026-1394/46/3/L01 . 115540416 .Peter J. . Mohr . Barry N. . Taylor . CODATA recommended values of the fundamental physical constants: 2002 . . 77 . 1 . 1–107 . 2005RvMP...77....1M . 10.1103/RevModPhys.77.1 . 2005 . https://web.a...This account is based on the review in Peter J. . Mohr . Barry N. . Taylor . CODATA recommended values of the fundamental physical constants: 1998 . . 28 . 6 . 1713–1852 . 1999JPCRD..28.1713M . 10....Peter J. . Mohr . Barry N. . Taylor . CODATA recommended values of the fundamental physical constants: 1998 . . 28 . 6 . 1713–1852 . 1999JPCRD..28.1713M . 10.1063/1.556049 . 1999 . https://web.arch...The dalton or unified atomic mass unit (abbreviated: amu, u, or Da) is a unit of measurement that is used to measure the mass of atoms. The atomic mass unit is equal to one twelfth of the mass of the carbon-12. The unified atomic mass unit and the dalton are different names for the same thing.
The atomic mass (ma or m) is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1 Da is defined as 1⁄12 of the mass of a free carbon-12 atom at rest in its ground state. [1]
The atomic mass unit (AMU or amu) of an element is a measure of its atomic mass. Also known as the dalton (Da) or unified atomic mass unit (u), the AMU expresses both atomic masses and molecular masses.
Jun 30, 2019 · In chemistry, an atomic mass unit or AMU is a physical constant equal to one-twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses .
