Isotopes of an element have the same chemical properties because they have the same number of electrons (and the same electron configuration). Chemical behaviour is determined by the electrons, not the neutrons. Isotopes are normally separated using physical properties (e.g., differences in mass, density, diffusion rate, or boiling point). Methods include gaseous diffusion, centrifugation, electromagnetic separation, or distillation. They cannot be easily separated by chemical methods because their chemical properties are almost identical. Isotopes of an element have the same number of protons in their nuclei (this is the definition — same atomic number, different mass numbers due to varying neutrons).