Total number of protons and neutrons in an atom

• A. Atomic number
• B. Isotopes
• C. Mass number
• D. Molar mass

Answer: (C)

The total number of protons and neutrons in a nucleus is the mass number. It is the count of all nucleons and is written as A, found by adding protons (the atomic number Z) and neutrons N: A = Z + N. This directly reflects how many particles make up the nucleus.

That’s why mass number is the best choice: it specifically counts the total number of nucleons. The atomic number, by contrast, counts only protons. Isotopes refer to atoms with the same number of protons but different numbers of neutrons. Molar mass is a bulk property in grams per mole and depends on isotopic composition, not simply the integer count of nucleons in a single nucleus. For example, carbon-12 has A = 12 (Z = 6, N = 6), while uranium-238 has A = 238 (Z = 92, N = 146).