Atomic mass is the number of protons and neutrons.

• A. The Number of Protons and Neutrons
• B. The Number of Protons
• C. The Total Electrons
• D. The Charge of the Atom

Answer: (A)

Mass of an atom comes mainly from the nucleus, which contains protons and neutrons. Each of these nucleons has about the same mass, and electrons contribute far less to the total mass—so their mass is negligible in comparison. Therefore, the atomic mass is essentially the sum of protons and neutrons, the mass number. The number of protons alone would tell you the atomic number, not the mass. The total number of electrons and the atom’s charge relate to electrical properties rather than mass, and don’t define the mass either. In reality, the atomic mass listed for an element is a weighted average of isotopes, but it still reflects the protons plus neutrons that make up the nucleus.