The sum of the protons and neutrons in the nucleus is known as the

• A. Mass Number
• B. Atomic Mass Unit
• C. Atomic Number
• D. Nuclide

Answer: (A)

The total number of protons and neutrons in a nucleus is the mass number. It’s defined as A = Z + N, where Z is the number of protons (the atomic number) and N is the number of neutrons. This integer helps identify the specific isotope of an element, since different isotopes have the same Z but different N, changing A. It’s not the unit used to measure mass (that would be the atomic mass unit), and it’s not the atomic number (which counts only protons), nor a nuclide (which refers to a specific nucleus defined by Z and N). So the sum of protons and neutrons is the mass number.