How is mass number related to isotopes?
Mass number can help identify an isotope.
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. An atom is unlikely to ever change its proton number, because that would imply that it is a new element - one with the new proton number. There are cases where protons have been removed or added without changing the element itself, but that is not within the scope of a general chemistry course, even within the AP level. So that leaves us with the idea that only the number of neutrons change, when the number of neutrons change each of those new amounts of neutrons are correlated to a different isotope.