Which of the following correctly defines the mass number of an atom?
Sum of protons and neutrons
The nucleus of an atom contains protons and neutrons, together called nucleons.
The mass number (A) is the total number of nucleons: A = number of protons + number of neutrons.
The number of protons alone is the atomic number, and electrons add almost nothing to the mass.
Hence, the answer is the sum of protons and neutrons.
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