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| atomic mass unit | |
|---|---|
| Name | atomic mass unit |
| Quantity | mass |
| SI | kilogram |
| Value | 1.66053906660×10^−27 kg (defined) |
| Uncertainty | exact (by definition) |
| Firstproposed | nineteenth century |
atomic mass unit
The atomic mass unit is a unit of mass used to express atomic and molecular masses. It underpins quantitative work in Chemistry, Molecular biology, Nuclear physics, Physical chemistry and Mass spectrometry and serves as a bridge between laboratory-scale masses and macroscopic measures in SI contexts. Developed through contributions by figures associated with institutions such as the Royal Society, International Committee for Weights and Measures and national metrology institutes, it provides a practical scale for discussing isotopes, molecules, and nuclear masses.
The concept emerged in the nineteenth century when practitioners at the Royal Institution and laboratories influenced by John Dalton sought a convenient mass scale tied to hydrogen or oxygen; early proposals involved standards promoted by the Royal Society and chemists linked to the Lavoisier tradition. Debates between proponents in France and United Kingdom led to competing scales using the mass of an oxygen isotope versus hydrogen, with involvement from organizations such as the International Union of Pure and Applied Chemistry and later the International Union of Pure and Applied Physics to harmonize definitions. In the twentieth century, the need for concordance with nuclear physics and mass spectrometry drove adoption of a unified scale anchored to a specific nuclide, with decisive input from laboratories like the National Institute of Standards and Technology and researchers associated with the Cavendish Laboratory and Institut Laue–Langevin.
Two historically used names and symbols coexist: the dalton and the unified atomic mass unit, each reflecting standardization efforts by organizations such as the International Committee for Weights and Measures and the International Organization for Standardization. The dalton (symbol Da) is promoted by biochemical and pharmaceutical communities and organizations including the World Health Organization and European Molecular Biology Laboratory, while the unified atomic mass unit (symbol u) has been widely used in older physics literature and by metrology laboratories such as the Physikalisch-Technische Bundesanstalt. Both terms denote the same quantity defined relative to a specified nuclide and adopted in resolution documents from bodies like the General Conference on Weights and Measures.
Determination of the unit’s value relies on precision experiments from groups at institutions such as the National Institute of Standards and Technology, Massachusetts Institute of Technology, Max Planck Institute for Chemistry and national laboratories including the Oak Ridge National Laboratory. Techniques include Penning trap mass spectrometry developed by teams linked to the CERN and the GSI Helmholtz Centre for Heavy Ion Research, time-of-flight methods used in facilities like Lawrence Berkeley National Laboratory, and X-ray crystal density comparisons applied historically at establishments such as the Bureau International des Poids et Mesures. Interlaboratory comparisons coordinated by the IUPAP and data compilations by the International Avogadro Project and committees associated with the CODATA Task Group yield evaluated values and uncertainties.
Atomic masses of nuclides and molecular masses of compounds are expressed in daltons or unified atomic mass units; tables compiled by entities like the International Union of Pure and Applied Chemistry and databases maintained by the National Center for Biotechnology Information provide standardized values for elements and isotopes. Conversions to SI kilograms are anchored by experiments from metrology centers such as the National Physical Laboratory and are reconciled with constants endorsed by CODATA and the International Committee for Weights and Measures. The unit directly relates to the mass of nucleons as studied at facilities like the Joint Institute for Nuclear Research and to binding energies interpreted via the Liquid drop model and measurements from accelerators such as those at Brookhaven National Laboratory.
In Biochemistry and Proteomics, molecular weights of proteins and nucleic acids are reported in daltons by facilities like the European Bioinformatics Institute and journals overseen by the American Chemical Society. In Nuclear physics and Astrophysics, masses of isotopes and mass excess values are central to modeling processes studied by collaborations at TRIUMF, RIKEN and the Institute for Nuclear Research. Mass spectrometers produced by companies cooperating with research centers such as Thermo Fisher Scientific are calibrated using standards traceable to metrology institutes including the Physikalisch-Technische Bundesanstalt and the National Institute of Standards and Technology.
High-precision mass measurements at facilities like CERN’s ISOLDE, GSI and ORNL reveal isotopic mass differences and mass excess values used in nuclear reaction network calculations in Stellar nucleosynthesis research. Isotopic composition standards promulgated by the International Organization for Standardization and compilations by the International Union of Geological Sciences affect reported atomic weights; metrological work at the International Avogadro Project constrains the kilogram linkage. Evaluations and mass tables published by the Atomic Mass Data Center and groups within the National Nuclear Data Center provide mass excesses with uncertainties for use in applications ranging from decay-energy calculations at Lawrence Livermore National Laboratory to isotope geochemistry at the United States Geological Survey.
Conventions on symbols (Da, u) and nomenclature are maintained by standard-setting bodies such as the International Union of Pure and Applied Chemistry, the International Organization for Standardization and the International Committee for Weights and Measures. Scientific publishers including the Royal Society of Chemistry, Nature Publishing Group, and the American Chemical Society adopt style guides that reflect these conventions, while databases at institutions like the National Institutes of Health and the European Bioinformatics Institute implement unit labels consistently to ensure interoperability across disciplines.