If two atoms are isotopes of the same element the atoms must have

Same number of protons but different number of neutrons

Explanation:

Isotopy is a phenomena in which atoms of the same element differs by the number of neutrons in their nucleus. While it is important to note that they have the same number of protons and hence the same atomic number, they have different mass numbers.

They have the same atomic numbers because an element can only have one atomic number. Hence, all atoms of that element are composed of the same number of protons but different neutron numbers.

See the explanation below, please.

Explanation:

If two atoms are isotopes of the same element, the atoms must have the same atomic number (Z) and a different mass number (A).

Isotopes differ from each other by the number of neutrons. Example: Hydrogen (A = 1 and Z = 1) and Deuterium (A = 2 and Z = 1).