Final answer:
The mass number of an atom is defined by the total number of protons and neutrons in its nucleus.
Explanation:
Mass Number
The mass number (A) of an atom is the total number of protons and neutrons in its nucleus. Protons and neutrons are much heavier than electrons, and because electrons have virtually no mass, the mass of an atom is concentrated in the nucleus. Therefore, an atom's mass number is determined by the total number of protons and neutrons in its nucleus.
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