Which measurement reflects the total number of protons and neutrons in an atom?

• A. Atomic Number
• B. Element
• C. Isotope
• D. Mass Number

Answer: (D)

The total number of protons and neutrons in a nucleus is captured by the mass number. This value, often denoted A, counts all nucleons in the nucleus, so it equals the number of protons (the atomic number, Z) plus the number of neutrons (N): A = Z + N. That’s why isotopes of the same element have different mass numbers—they share the same number of protons but have different numbers of neutrons. The atomic number describes protons alone and identifies the element, while the term isotope refers to variants with different neutron counts. Atomic mass, on the other hand, is a weighted average of all isotopes and isn’t just a simple integer count of nucleons.