The statement of mass number of an atom is "the number of protons plus the number of neutrons in a nucleus."
The total number of protons and neutrons in an atomic nucleus is known as the mass number, often known as the atomic mass number or nucleon number. Each atom also has a mass number, which is represented by the symbol A. The atom's mass number equals the sum of the number of protons and neutrons in the nucleus.
Protons + neutrons = mass number
Hence the correct answer is the number of protons plus the number of neutrons in a nucleus.
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