Atomic mass equals the sum of which particles?

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

Answer: (A)

Atomic mass comes from the particles in the nucleus—the protons and neutrons. The mass of an electron is so small compared with a proton or neutron that it doesn’t noticeably affect the total mass, so you add up the numbers of protons and neutrons to get the atomic mass (the mass number). That’s why the sum of protons and neutrons is the correct description. Protons plus electrons would give a mass that’s off because electrons contribute almost nothing to the mass. Neutrons plus electrons would miss the mass contribution from protons. Protons only would ignore the neutrons, which add substantial mass to many nuclei.