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Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons-the number of protons plus neutrons.
Quick Answer. The isotopes of an element differ from each other in their mass numbers and the number of neutrons they contain. All of the isotopes of an element, however, contain the same number of protons, which is equal to that element's atomic number. The atomic number, because it does not reflect the number of neutrons in an atom, does not distinguish between the isotopes of a given element, which will all occupy the same position in the periodic table. Continue Reading