Editing Dalton (unit) (section)

=== Joint definition by IUPAP and IUPAC ===

The existence of two distinct units with the same name was confusing, and the difference (about {{val|1.000282}} 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 to [[Josef Mattauch]] that the [[carbon-12]] be adopted as mass scale because of carbon's use as a secondary standard in [[mass spectrometry]]. Also, carbon-12 implied acceptable relative changes in the atomic weight scale, i.e., 42 parts-per-million (ppm) compared to 275 ppm for [[oxygen-16]] which would not acceptable to chemists. 

Following the approval of the [[International Union of Pure and Applied Physics]] (IUPAP) General Assembly at Ottawa, Canada in 1960 and the [[International Union of Pure and Applied Chemistry]] (IUPAC) General Assembly at Montreal, Canada in 1961, the atomic weights were officially given on the carbon-12 scale for the first time.<ref name=petley1989/><ref name=hold2004/>

The new unit was named the "unified atomic mass unit" and given a new symbol "u", to replace the old "amu" that had been used for the oxygen-based unit.<ref name=goldbUnAtMaUn/> However, the old symbol "amu" has sometimes been used, after 1961, to refer to the new unit, particularly in lay and preparatory contexts.

With this new definition, the [[standard atomic weight]] of [[carbon]] is about {{val|12.011|u=Da}}, and that of oxygen is about {{val|15.999|u=Da}}. These values, generally used in chemistry, are based on averages of many samples from [[Earth's crust]], its [[atmosphere]], and [[organic materials]].