Number of neutrons and mass number.
Your starting point here is the fact that the identity of an element depends exclusively on the number of protons it has in its nucleus, i.e. on its atomic number.
So if atomic number is something that isotopes of an element have in common, what would they differ in?
The first thing that sets isotopes apart from one another is the number of neutrons they have in their respective nuclei.
If two atoms have the same number of protons, but different number of neutrons in their nuclei, then they are isotopes.
As you know, an atom's mass number tells you the number of protons and neutrons an atom has in its nucleus.
Since isotopes differ in the number of neutrons they have in their nucleus, it follows that they will also have different mass numbers.
So there you have it, isotopes differ in
- the number of neutrons they have in the nucleus
- their mass number