How is mass number related to isotopes?

An isotope can be identified using its mass number.

Although there are instances where protons have been added or removed without changing the element itself, those situations are outside the purview of a general chemistry course, let alone the AP level. Thus, the idea that only the number of neutrons changes is left. When the number of neutrons changes, each of those new amounts of neutrons is correlated to a different isotope. The mass number is essentially the addition of the number of neutrons an atom has to the number of protons it has in its standard state.

The mass number of an isotope is the sum of the number of protons and neutrons in the nucleus of an atom. Isotopes of an element have the same number of protons (and therefore the same atomic number), but they differ in their number of neutrons and thus have different mass numbers.