How do isotopes of an element differ?

Isotopes of an element differ primarily in their mass numbers, which is why the choice referring to different mass numbers is correct. Isotopes are variants of the same chemical element that have the same number of protons, meaning they share the same atomic number and are chemically identical. However, they differ in the number of neutrons in their atomic nuclei, which affects their mass. This difference in neutron count leads to varying mass numbers, which is the sum of protons and neutrons in the nucleus.

Because isotopes have the same number of protons, they maintain the same chemical properties since chemical behavior is largely determined by the arrangement of electrons and the nuclear charge, both of which remain unchanged in isotopes of an element. This is why options that refer to differences in protons, different atoms, or different chemical properties are not correct. In summary, the defining characteristic that distinguishes isotopes is their varying mass numbers due to the different neutron counts.