Ratio of the relative frequency of a certain isotopeAtoms of the same atomic number (i.e. the same chemical elem... in an isotopeAtoms of the same atomic number (i.e. the same chemical elem... mixture to the relative frequency of this isotopeAtoms of the same atomic number (i.e. the same chemical elem... in the natural isotopeAtoms of the same atomic number (i.e. the same chemical elem... mixture.