atomic weightn.
/əˌtɒmɪk ˈweɪt/, U.S. /əˌtɑmɪk ˈweɪt/ Chiefly Chemistry.
The average weight 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). Also: the sum of the atomic weights of the atoms in a molecule or radical (now rare; = molecular weight n. at molecular adj. Compounds). Cf. atomic mass n.In the early 19th cent. it was observed that the atomic weights of many elements, as inferred from the proportions in which they combine with one another, were close to whole-number multiples of the atomic weight of hydrogen (Prout's hypothesis). Hydrogen was therefore used as the basis of atomic weights, with the assigned value 1; oxygen was also used, with a value taken as 1 or 8, or later (following a suggestion by Ostwald in 1885) 16. In 1960 the isotope carbon-12 was adopted as the basis of atomic weights, with an assigned atomic weight of 12.The term is being superseded in technical use by atomic mass.
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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
1805 J. Dalton in Mem. Literary & Philos. Soc. Manch. 2nd Ser. 1 286 An enquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new.
1810 J. Dalton New Syst. Chem. Philos. I. ii. 275 From the relative weights in the mass, the relative weights of the ultimate particles or atoms of the bodies might have been inferred.
1813 T. Thomson in Ann. Philos. 2 36 Knowing the weight of an atom of oxygen and of an atom of hydrogen, we have it in our power to determine the weight of an atom of the other substances which unite with oxygen, or with hydrogen.]
1819 Ann. Philos. 14 295 (title) On ferro-chyazate of potash, and on the atomic weight for iron.
1827 A. Fyfe Elements Chem. II. 493 The atomic weights are given both according to the oxigen and hydrogen scale.
1830 J. G. Macvicar Elements Econ. Nature iii. 444 The atomic weight of the lighter gas..is 110, composed of 80 silex and 30 fluorine.
1868 H. Watts Dict. Chem. V. 785 Thorinum, or Thorium. Atomic Weight, 115·72; Symbol, Th.
1916 Science 21 July 102/2 Neptunium..is described as belonging to the ‘tantalum group’, of the atomic weight 118.
1942 J. D. Stranathan ‘Particles’ of Mod. Physics v. 169 Atomic weights are in general not quite whole numbers; but they are very close to whole numbers.
1964 L. H. Van Vlack Elements Materials Sci. (ed. 2) ii. 19 Atomic weights range from 1.008 for hydrogen, which has only one proton, to nearly 250 for some of the unstable transuranium elements.
2001 O. Sacks Uncle Tungsten xxiii. 285 The emanations of radium and thorium and actinium..were chemically identical, all the same element, though with slightly different atomic weights.
This entry has been updated (OED Third Edition, December 2008; most recently modified version published online December 2021).
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