Isotopes are two or more atoms that have the same number of protons (ie they are of the same element), but different numbers of neutrons. For example, the most common isotope of hydrogen has a single proton and no neutrons in its nucleus. A very small proportion of hydrogen atoms are of the isotope deuterium (with one proton and one neutron) or tritium (with one proton and two neutrons). Isotopes can be very difficult to separate because they have the same external electron shell configuration, which gives them basically identical chemical beheavior. This means that they must be separated by physical means such as diffusion, centrifugation, or sometimes fractional distillation.