Question

what do all isotopes of an element have in common? what differs?

Explanation:

Isotopes are atoms of the same element. By definition, atoms of the same element have the same number of protons (atomic number). The difference lies in the number of neutrons. Since mass number = number of protons + number of neutrons, isotopes have different mass numbers.

Answer:

All isotopes of an element have the same number of protons (atomic number). The number of neutrons (and thus the mass number) differs.