The average mass of an atom of a given element, allowing for the relative abundance of different isotopes. It is stated in terms of the mass of a proton.
In chemistry, the mass of one atom of an element expressed in atomic mass units. It is the average mass of all the isotopes of an element. One atomic mass unit is exactly one-twelfth the mass of the neutral atom of carbon-12 and is equal to 1.6605 × 10-24 grams.
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