atomic mass

atomic mass
the mass of an isotope of an element measured in units formerly based on the mass of one hydrogen atom taken as a unit or on 1/16 the mass of one oxygen atom, but after 1961 based on 1/12 the mass of the carbon-12 atom. Abbr.: at. m.
[1895-1900]

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      the quantity of matter contained in an atom of an element. It is expressed as a multiple of one-twelfth the mass of the carbon-12 atom, 1.99264648 × 10−23 gram, which is assigned an atomic mass of 12 units. In this scale 1 atomic mass unit (amu) corresponds to 1.66053873 × 10−24 gram.

      The observed atomic mass is slightly less than the sum of the masses of the protons (proton), neutrons (neutron), and electrons (electron) that make up the atom. The difference, called the mass defect, is accounted for during the combination of these particles by conversion into binding energy, according to an equation in which the energy (E) released equals the product of the mass (m) consumed and the square of the velocity of light in vacuum (c); thus, E = mc2. See also atomic weight.

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Universalium. 2010.

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