Which property of the isotopes must be different

chemical properties of isotopes are different. but their valencies are same so the gain or lose electrons the same

physical properties

Explanation:

Isotopes have the same number of electrons as an atom of that element. Because of that they have similar chemical properties. However they have different numbers of neutrons, which affects the mass number. Mass number determines the physical properties such as boiling/melting/density etc. Therefore, the physical properties of the isotopes must be different.