# atomic mass unit

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atomic mass unit
(u or amu)
the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules. Early in the nineteenth century, scientists discovered that each chemical element is composed of atoms, and that each chemical compound is composed of molecules in which atoms are combined in a fixed way. No one knew then just how small atoms and molecules really are, but as long as the relative weights of the different atoms were known, the outcome of chemical reactions could be predicted. These relative masses were determined by careful study of various reactions. The general idea was that atoms of hydrogen, known to be the lightest element, should have a mass of 1 amu, and all the other atoms should have masses which are multiples of this (then unknown) mass of the hydrogen atom. For a long time, physicists and chemists disagreed on the details of this definition. In 1960 they agreed on the definition of the unified atomic mass unit as 1/12 the mass of the most common atoms of carbon, known as carbon-12 atoms. (Most elements are mixtures of atoms which have different masses because they contain different numbers of neutrons; these varieties are called isotopes.) Careful experiments have measured the size of this unit; the currently accepted value (1998) is 1.660 538 782 x 10-27 kilograms. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 028 MeV (see electronvolt). In biochemistry, the atomic mass unit is called the dalton (Da).

Dictionary of units of measurement. 2015.

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• atomic mass unit — n a unit of mass for expressing masses of atoms, molecules, or nuclear particles equal to 1/12 the mass of a single atom of the most abundant carbon isotope 12C called also dalton * * * (u) (amu) the unit mass equal to the mass of the nuclide of… …   Medical dictionary

• atomic mass unit — n. a unit of mass, exactly one twelfth the mass of a neutral atom of the most abundant isotope of carbon, C12: one atomic mass unit equals 1.6605 x 10 24 gram and is equivalent to 931.48 MeV in energy: symbol amu or u …   English World dictionary

• atomic mass unit — ► NOUN ▪ a unit of mass used to express atomic and molecular weights, equal to one twelfth of the mass of an atom of carbon 12 …   English terms dictionary

• Atomic mass unit — unified atomic mass unit Unit system: SI recognized unit Unit of... mass Symbol: u Unit conversions 1 u in... is equal to...    dalton …   Wikipedia

• Atomic mass unit — Einheit Norm Internationales Einheitensystem Einheitenname atomare Masseneinheit Einheitenzeichen u Beschriebene Größe(n) Masse Größensymbol(e) …   Deutsch Wikipedia

• atomic mass unit — atominės masės vienetas statusas T sritis Standartizacija ir metrologija apibrėžtis Santykinės atominės masės matavimo vienetas lygus nuklido ¹²C pagrindinės būsenos neutraliojo atomo rimties masės 1/12 daliai. Žymimas u: 1 u = (1,660 538 86 ±… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• Atomic mass unit — Unité de masse atomique Pour les articles homonymes, voir U, uma et AMU. L unité de masse des atomes unifiée (symbole u ou uma, en anglais : Atomic mass unit AMU) est une unité de mesure standard, utilisée pour mesurer la masse des atomes et …   Wikipédia en Français

• atomic mass unit — atominės masės vienetas statusas T sritis fizika atitikmenys: angl. amu; atomic mass unit vok. atomare Masseneinheit, f; Atommasseneinheit, f rus. атомная единица массы, f; единица атомной массы, f pranc. unité de masse atomique, f …   Fizikos terminų žodynas

• atomic mass unit — Physics. 1. Also called dalton. a unit of mass, equal to 1/12 the mass of the carbon 12 atom and used to express the mass of atomic and subatomic particles. 2. (formerly) a unit of mass, equal to 1/16 the mass of an oxygen atom having atomic mass …   Universalium

• atomic mass unit — noun unit of mass for expressing masses of atoms or molecules • Hypernyms: ↑mass unit …   Useful english dictionary