atomic mass 
noun
The mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (the mass number) or to the average number allowing for the relative abundances of different isotopes.The atomic mass of an isotope is the total number of protons and neutrons in its nucleus: here 20 and 22 respectively....
- Isotopes are atoms of the same element that have slightly different atomic masses due to the presence of differing numbers of neutrons in the nucleus.
- He postulated that all the atoms of the same element have the same atomic mass, while the atoms of a different element have a different atomic mass.
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