- The average mass of all the isotopes of an element, expressed in atomic mass units. It is the combined mass, or weight, of the protons, neutrons, and electrons of an atom. The atomic weight of carbon, for example, is approximately 12.011 atomic mass units, not 12. The reason is that carbon has three naturally occurring isotopes, of which carbon-12 is by far the most common. Also called relative atomic mass.
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