atomic massn.
/əˌtɒmɪk ˈmas/, U.S. /əˌtɑmɪk ˈmæs/ Chiefly Physics.
The average mass of an atom of an element, expressed by comparison to a standard atom (formerly one of hydrogen or oxygen, now one of the isotope carbon-12: cf. atomic mass unit n. at Compounds). Also: the corresponding mass for a particular isotope, usually rounded to the nearest whole number (the mass number: see mass number n. at mass n.2 Compounds 2). Cf. atomic weight n.
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the world > matter > physics > atomic physics > [noun] > atomic mass
atomic mass1884
the world > matter > physics > atomic physics > [noun] > atomic weight
atomic weight1819
equivalent number1826
atomic mass1884
relative atomic mass1892
the world > matter > chemistry > atomic chemistry > [noun] > atoms > atomic weight or mass
atomic weight1819
Prout's hypothesis1860
atomic mass1884
1884 Proc. Royal Soc. 37 109 We denote the atomic mass by A.
1898 S. W. Holman Matter, Energy, Force & Work 153 The terms atomic masses, combining masses, etc., are likewise used instead of ‘atomic weights’, [etc.].
1903 O. Lodge Mod. Views on Matter 7 Their mass is of the order one-thousandth of the atomic mass of hydrogen.
1946 Electronic Engin. 18 88/2 Neptunium and plutonium are isobares (same atomic mass but different atomic number).
1985 W. A. Fowler in M. S. Roberts Astron. & Astrophysics xxi. 303 No stable nuclei exist at atomic mass 5 and 8.
2000 New Phytologist 147 242/1 There are at least 21 isotopes of Cs, mostly in the range of atomic masses 123–144.
Compounds
atomic mass unit n. a unit used for expressing atomic masses, formerly equal to 1/ 16 of the average mass of an atom of oxygen, or of the precise mass of an atom of the isotope oxygen-16, but since 1960 equal to 1/ 12 of the mass of an atom of carbon-12; abbreviated amu.
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the world > matter > physics > atomic physics > [noun] > atomic mass > unit of
mass unit1942
atomic mass unit1954
the world > matter > chemistry > atomic chemistry > [noun] > atoms > atomic weight or mass > atomic mass unit
dalton1938
atomic mass unit1954
1954 Science 24 Dec. 1078/2 (table) Atomic mass unit..amu..1 amu=931 Mev.
1958 W. K. Mansfield Elem. Nucl. Physics i. 10 The atomic mass M, not to be confused with the molecular weight, is the mass of an isotope in atomic mass units.
1977 P. R. Ehrlich et al. Ecoscience iii. 72 One atomic mass unit is approximately the mass of a proton.
1993 Discover July 14 The world has a new champion in the hydrocarbon division: a knobby, lacy, spherical molecule made up of 1,134 atoms of carbon and 1,146 of hydrogen and weighing in at 14,776 atomic mass units.
This entry has been updated (OED Third Edition, December 2008; most recently modified version published online December 2021).
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