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Huh. I always thought that metric units of mass were defined by the mass of a mole of whatever atom you were interested in: a mole of carbon masses exactly 12 grams, a mole of oxygen masses 16 grams, a mole of hydrogen masses 1 gram.

EDIT: corrected atomic mass of carbon. Not sure what I was thinking there ...

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Unless they have changed the definition, back when I studied it the mole was defined as the number of particles found in 12 grams of C (C-12), 6.022*10^23.

So the mole depended on the gram.

As it turns out, a mole of carbon doesn't mass exactly 12/16 of a mole of oxygen or 12× a mole of hydrogen. (Carbon's average atomic mass is 12, not 14.) So you'd have to pick an atom.

At present, the definition runs the other direction: a dalton is defined as 1/(Avogadro's number) of a gram. But Avogadro's number is a little difficult to measure directly; what you end up doing is picking an element and measuring its atomic mass. They picked carbon.

It seems to me that the definition you describe would indeed be a reasonable one, since Avogadro's number is known to seven significant figures at this point, which seems comparable to the mass stability of the international prototype kilogram.

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