Atomic weight is the

• A. Number of protons
• B. Atomic number
• C. Sum of protons and neutrons
• D. Number of neutrons

Answer: (C)

Mass comes from the nucleus, which is made of protons and neutrons, and atomic weight reflects how heavy that nucleus is. The number of protons (atomic number) determines which element you have, but it’s the neutrons that add to the mass without changing the element’s identity. So the total number of nucleons—protons plus neutrons—gives the weight of the nucleus, which is what this simple definition uses for atomic weight. In more precise terms, atomic weight is the weighted average of the masses of all naturally occurring isotopes, so it isn’t always a whole number, but for a single isotope or basic understanding, the sum of protons and neutrons is the best description.