What does the mass number of an atom represent?

The mass number of an atom is defined as the total count of protons and neutrons present in its nucleus. Protons and neutrons are collectively known as nucleons, and since they contribute to the mass of the atom, the mass number provides an essential measure of the atom's overall mass. Each element has a unique number of protons, which determines its atomic number, while the number of neutrons can vary among different isotopes of the same element. Understanding the mass number helps in distinguishing between isotopes and plays a critical role in domains such as chemistry and nuclear physics.

In contrast, the other options do not accurately represent the definition of mass number. The count of protons alone only gives the atomic number, while the number of atoms in a molecule pertains to molecular composition rather than an individual atom's mass number. Lastly, atomic weight, which may be expressed in grams, takes into account isotopic distribution and is not a direct measure of mass number, which is simply a whole number reflecting proton and neutron totals. Thus, the correct representation of mass number focuses on the sum of protons and neutrons.