Which statement is true about atomic mass number A?

• A. A equals Z plus N
• B. A equals Z
• C. A equals N
• D. A equals Z plus electrons

Answer: (A)

The mass number A counts all the nucleons in the nucleus—the total number of protons and neutrons. Since Z is the number of protons and N is the number of neutrons, the mass number is their sum: A = Z + N. This is why the statement is true. Electrons sit outside the nucleus and have negligible mass, so they don’t contribute to A. That’s why A isn’t just Z or just N, and it isn’t Z plus electrons. For example, carbon-12 has Z = 6 and N = 6, so A = 12; carbon-14 has Z = 6 and N = 8, so A = 14.