definitions
strictly speaking, the referance of the atomic mass is 1/12 of carbon-12, so approximately, the atomic mass is the sum of the protons and nuetrons in nuclei. What's more, every element has one or more isotopes in nature, so we have to calculate the contribution of each isotope, that is to say, the average mass of all the isotopes of the same element gives the final atomic mass of one element.
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