What is the mass number of an atom?

The mass number of an atom is defined as the sum of the number of protons and neutrons located in the nucleus. Protons and neutrons are both present in the nucleus and together they contribute to the overall mass of the atom. This is why the mass number is specifically concerned with these two types of nucleons, rather than just protons or just neutrons alone.

The mass number is crucial in understanding the identity of an element and its isotopes. Isotopes of an element have the same number of protons (which defines the element) but different numbers of neutrons, leading to different mass numbers.

While the other options provide important information about atomic structure, they do not accurately represent what the mass number is. For example, the number of protons alone characterizes the atomic number, while the total number of subatomic particles includes electrons, which are not counted in the mass number as they have negligible mass compared to protons and neutrons. The weight measured in atomic mass units relates to the mass of the atom, but it’s not the definition of mass number itself. Hence, the correct and precise definition revolves around the sum of protons and neutrons.