What is defined as the sum of the numbers of protons and neutrons in one atom of an element?

• A. Mass number
• B. Atomic number
• C. Isotopic mass
• D. Nucleon count

Answer: (A)

This is about counting the particles that make up the nucleus. The mass number is defined as the total number of nucleons—protons plus neutrons—in one atom, so A = Z + N. It is always an integer because you can’t have a fraction of a nucleon. The atomic number counts only the protons and identifies the element, not the total nucleons. Isotopic mass refers to the actual mass of an isotope, usually measured in atomic mass units, which isn’t just a simple count. Nucleon count is a descriptive idea, but the standard term for the sum of protons and neutrons is mass number.