Atomic mass is defined as the number of what?

• A. Protons and Neutrons
• B. Protons
• C. Neutrons
• D. Electrons

Answer: (A)

Atomic mass is the total number of nucleons in the nucleus—the protons and neutrons. The mass of electrons is so small that it doesn’t affect the atom’s mass in any meaningful way, so we count only protons and neutrons to get the atomic mass. This is why the correct answer is the number of protons and neutrons. Protons alone would give the atomic number (identity of the element), and neutrons alone wouldn’t account for the positive charge or the full mass. Electrons do exist, but their mass is negligible in this context.