These are the nuclei that belong to the same element and have the same number of protons. They occupy the same position in the periodic table and hence have the same atomic number. The isotopes of given element contains same number of electrons and hence have the same chemical properties, thus it is difficult to separate them from one another using chemical methods. However the Isotopes of given element differ in mass dependent physical properties like rate of diffusion and thus can be separated from one another using physical properties.