Editing Dalton (unit) (section)

=== Isotopic variation ===
The discovery of isotopes of oxygen in 1929 required a more precise definition of the unit. Two distinct definitions came into use. Chemists choose to define the AMU as {{sfrac|1|16}} of the average mass of an oxygen atom as found in nature; that is, the average of the masses of the known isotopes, weighted by their natural abundance. Physicists, on the other hand, defined it as {{sfrac|1|16}} of the mass of an atom of the isotope oxygen-16 (<sup>16</sup>O).<ref name=hold2004/>