mass number
Sum of the numbers of protons and neutrons in the nucleus of an atom. It is used along with the atomic number (the number of protons) in nuclear notation: in symbols that represent nuclear isotopes, such as 146C, the lower number is the atomic number, and the upper number is the mass number. Since the masses of the electrons in an atom are negligible, the total number of protons is neutrons (mass number) in an atom determines its mass.
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