Atomic mass unit
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- AMU redirects here, but may also refer to the Arab Maghreb Union
The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. It is defined to be 1/12 of the mass of one atom of carbon-12.
- 1 u = 1/NA gram = 1/(1000 NA) kg (where NA is Avogadro's number)
- 1 u ≈ 1.66053886 × 10−27 kg ≈ 931.49 MeV/c2
See 1 E-27 kg for a list of objects which have a mass of about 1 u.
The symbol amu for atomic mass unit can sometimes still be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied. In biochemistry and molecular biology literature (particularly in reference to proteins), the term "dalton" is used, with the symbol Da. Because proteins are large molecules, they are typically referred to in kilodaltons, or "kDa", with one kilodalton being equal to 1000 daltons. The unified atomic mass unit is not an SI unit of mass, although it is (only by that name, and only with the symbol u) accepted for use with SI.
The unit is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains n protons and neutrons will have a mass approximately equal to n u. (The reason is that a carbon-12 atom contains 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison.) This is only a rough approximation however, since it does not account for the mass contained in the binding energy of an atom's nucleus; this binding energy mass is not a fixed fraction of an atom's total mass.
Another reason the unit is used is that it is experimentally much easier and more precise to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Masses are compared with a mass spectrometer (see below).
Avogadro's number (NA) and the mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1 gram. For example, the molecular mass of water is 18.01508 u, and this means that one mole of water has a mass of 18.01508 grams, or conversely that 1 gram of water contains NA/18.01508 ≈ 3.3428 × 1022 molecules.
History
The chemist John Dalton was the first to suggest the mass of one atom of hydrogen as the atomic mass unit. Francis Aston, inventor of the mass spectrometer, later used 1/16 of the mass of one atom of oxygen-16 as his unit.
Before 1961, the physical atomic mass unit was defined as 1/16 of the mass of one atom of oxygen-16, while the chemical atomic mass unit was defined as 1/16 of the average mass of an oxygen atom (taking the natural abundance of the different oxygen isotopes into account). Both units are slightly smaller than the unified atomic mass unit, which was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961.
Mass spectrometry is overwhelmingly used by archaeologists (and forensic scientists) to determine the ages of old man-made objects based upon the isotopic proportions of carbon. Having the atomic mass unit based upon oxygen added needless error; now that it is based upon an isotope of carbon this is obviated.
External links
ast:Unidá de masa atómica bg:Атомна маса (единица) br:Unanenn mas atomek ca:Unitat de massa atòmica cs:Atomová hmotnostní konstanta de:Atomare Masseneinheit es:Unidad de masa atómica eo:Unuigita atommasa unuo fr:Unité de masse atomique ko:원자 질량 단위 it:Unità di massa atomica he:יחידת מסה אטומית hu:Atomi tömegegység nl:Atomaire massa-eenheid ja:原子質量単位 pl:Jednostka masy atomowej pt:Unidade de massa atômica ru:Атомная единица массы sk:Atómová hmotnostná jednotka sl:Atomska enota mase sr:Јединица атомске масе sv:Atommassenhet th:หน่วยมวลอะตอม vi:Đơn vị khối lượng nguyên tử zh:原子质量单位