Two isotopes are two atoms with the same number of protons but a different number of neutrons. (In other words the same atomic number but a different mass number)

Isotopes are atoms of the same element with different relative masses, that is to say the atoms have the same number of protons, the same number of electrons, and different numbers of neutrons.

An example is chlorine... Cl 35 and Cl 37 are the two common isotopes. An atom of each has 17 protons, has 17 electrons but Cl 35 has 18 neutrons whereas Cl 37 has 20 neutrons.