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In page Heavy water:

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Semiheavy water, HDO, exists whenever there is water with light hydrogen (protium, 1H) and deuterium (D or 2H) in the mix. This is because hydrogen atoms (1H and 2H) are rapidly exchanged between water molecules. Water containing 50% 1H and 50% 2H in its hydrogen, is actually about 50% HDO and 25% each of H2O and D2O, in dynamic equilibrium. In normal water, about 1 molecule in 3,200 is HDO (one hydrogen in 6,400 is 2H), and heavy water molecules (D2O) only occur in a proportion of about 1 molecule in 41 million (i.e. one in 6,4002)[citation needed]. Thus semiheavy water molecules are far more common than "pure" (homoisotopic) heavy water molecules.