weight, by several mass units.

Relative isotopic masses are always close to whole-number values, but never (except in the case of carbon-12) exactly a whole number, for two reasons:
* protons and neutrons have different masses, and different nuclides have different ratios of protons and neutrons.
* atomic masses are reduced, by the binding energies between the protons and neutrons.

The ratio of atomic mass to mass number (number of nucleons)

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