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From the New World Encyclopedia, "Technically, atomic mass is equal to the total mass of protons, neutrons, and electrons in the atom (when the atom is motionless), plus the mass contained in the binding energy of the atom's nucleus."

Mass number is the number of protons plus neutrons in an atom's nucleus. Mass number varies with individual isotopes due to the fact that atoms of the same element can have different numbers of neutrons.

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13y ago

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