abundance. Physicists, on the other hand, defined it as of the mass of an atom of the isotope oxygen-16 (¹⁶O).

Joint definition by IUPAP and IUPAC

The existence of two distinct units with the same name was confusing, and the difference (about in relative terms) was large enough to affect high-precision measurements. Moreover, it was discovered that the isotopes of oxygen had different natural abundances in water and in air.

In April 1957 Alfred O. C. Nier suggested

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