The mass number of an atom is equal to what?

The mass number of an atom is determined by the total count of protons and neutrons present in its nucleus. Protons and neutrons are collectively referred to as nucleons. The mass number provides a way to identify the isotopes of an element, which are variations of the same element that have the same number of protons but different numbers of neutrons.

For example, if an atom has 6 protons and 6 neutrons, its mass number would be 12 (6 protons + 6 neutrons). Conversely, if it has 6 protons and 8 neutrons, its mass number would be 14. The number of electrons is not included in the mass number because electrons have a negligible mass compared to protons and neutrons. Additionally, while the number of protons determines the identity of the element (its atomic number), the mass number gives insight into the overall mass of the atom and its isotopes.

Thus, the correct answer reflects the fundamental relationship between the protons and neutrons within the nucleus of an atom.