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Atomic weight:  The Name, its History, Definition, and Units

The widely used term “atomic weight” and its acceptance within the international system for measurements has been the subject of debate because an atomic weight as defined is relative value and is thus unitless (having a unit of 1). 

Atomic Weight—Love it or Leave it, N. E. Holden, Chem. Australia, 49, 135-136 (1982). [full text - pdf]{ At Wt--Love it or Leave it.pdf}

This debate is summarized in:
'Atomic weight': The name, its history, definition, and units, P. de Bièvre and H. S. Peiser, Pure Appl. Chem., 64, 1535-1543 (1992).   [full text - pdf 730 KB]{AW Name, History, Def, Units.pdf}

An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of an atom of 12C. 

It has to be noted that:

  • atomic weights can be defined for any sample
  • atomic weights are evaluated for atoms in their electronic and nuclear ground states
  • the average mass per atom in a specified source is the total mass of the element divided by the total number of atoms of that element
  • no stipulation is made for the element in the sample having the natural terrestrial isotopic composition
  • the IUPAC Tables of Standard Atomic Weights refer to the best knowledge of the elements in natural terrestrial sources


2010 Commission on Isotopic Abundances and Atomic Weights | a commission of the International Union of Pure and Applied Chemistry