Atomic Mass Number (A) is defined as which of the following?

• A. Sum of protons and neutrons in the nucleus
• B. Number of protons
• C. Number of electrons
• D. Sum of protons and electrons

Answer: (A)

The mass number is the total count of nucleons in the nucleus: protons plus neutrons. The electrons sit outside the nucleus and contribute negligible mass, so they aren’t included in A. Therefore A = Z + N, where Z is the number of protons and N is the number of neutrons. This is why isotopes of the same element have different mass numbers—they share the same protons but have different numbers of neutrons. For example, carbon-12 has 6 protons and 6 neutrons (A = 12), while carbon-14 has 6 protons and 8 neutrons (A = 14). The other options describe either just protons, just electrons, or protons plus electrons, none of which matches how the mass number is defined.