Atomic mass is the sum of which particles?

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

Answer: (B)

Atomic mass represents the total mass of the nucleus, coming from the protons and neutrons inside it. The electrons orbiting the nucleus have mass, but it’s extremely small compared with the protons and neutrons, so it’s not included when we talk about atomic mass. The mass number of an atom equals the number of protons plus the number of neutrons, which is why atomic mass is often described as the sum of those nucleons. Isotopes differ in the number of neutrons, which changes their mass while keeping the same number of protons. So the correct idea is that atomic mass is the sum of protons and neutrons.