The sum of the protons and neutrons in the nucleus is called what?

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

Answer: (D)

The number you get when you add up all the protons and neutrons in the nucleus is the mass number. It’s the total count of nucleons and serves as the identifier for different isotopes of an element. The atomic number, by contrast, is only the number of protons and determines which element it is. Neutrons change the mass but not the element, so isotopes have the same atomic number but different mass numbers. Radioactivity is the process of radioactive decay, not a count of particles. Atomic mass refers to the actual mass of the atom in atomic mass units and is related to the mass number but isn’t just the simple sum, since it includes effects like binding energy and the distribution of isotopes. So the sum of protons and neutrons is described by the mass number.