In natural water (see Standard Mean Ocean Water), almost all of the hydrogen atoms are of the isotope protium, 1H. Heavy water is water in which the hydrogen is replaced by its heavier isotope, deuterium,2H. It is chemically similar to normal water, but not identical. This is because the nucleus of deuterium is twice as heavy as protium, and thus causes noticeable differences in bonding energies and hydrogen bonding. Heavy water is used in the nuclear reactor industry to moderate (slow down) neutrons. By contrast with heavy water, the term light water (i.e., ordinary water, no special isotopes) designates water containing the most common form of hydrogen, the protium isotope. For example, light water reactor emphasizes that a reactor uses the less often found light water designhttp://www.seafriends.org.nz/oceano/oceans2.htm
