Which atomic property is different in each isotope of an element?

In the question "Which atomic property is different in each isotope of an element?" The correct answer is the neutron and the mass number. Isotopes of an element are two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. Therefore, isotopes of an element have different number of electrons and protons but different number of neutrons and different mass number.