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Duh... chemists are not that dumb. They've been
doing it right. The atomic masses on the periodic table
are the weighted averages (weighted by natural abundance)
of the atomic masses (not mass numbers) of the naturally
occurring isotopes of the elements. It is specifically done
this way to allow us to find average natural molecular masses
by simply adding the atomic masses listed in the periodic table.
It turns out chemical analyses of atomic weight
based on Avogadro yield only relative molecular weights.
Initially, Hydrogen was represented as 1, but a scale
based on Oxygen = 16 came to be preferred (as offering
many combinations.) When elements were found to be
a mix of isotopes however, and the standard, oxygen itself
was affected, physicists began to assign the common isotope
examined with the mass spectrometer the value 16, while
chemists continued with 16 for the natural mix, until the
'quasi-physical' definition of C = 12 for the most common
isotope of that material was adopted by IUPAP/IUPAC.
Hence when Michael gives us "correct" values, it would be
well to remember that he has in mind the values
currently defined to be "correct".