Isotopes are atoms of the same element with different atomic masses. Since they are of the same element, they must have equal numbers of protons. Therefore, to have different atomic masses (number of protons plus neutrons), they must have different numbers of neutrons. The number of electrons depends on the atom’s ionization state; if both isotopes are electrically neutral, they will have the same number of electrons. Isotopes cannot differ in the number of nuclei; every atom has one nucleus (though that nucleus may contain numerous protons and neutrons).