The hydrogen atom usually consists of a nucleus of single proton and its electron. There is an form of hydrogen called deuterium which has a proton and a neutron in its nucleus. Deuterium exists naturally, however only in a very small amount. All around us, only 0.015% of hydrogen atoms are deuterium. As the proton and neutron are almost equal in weight, the deuterium atom weighs about twice as much as the hydrogen atom. One often refers to the deuterium nucleus as a deuteron.
Water molecules are composed of two hydrogen atoms and one oxygen atom (H2O). If the hydrogen atoms of a water molecule are replaced by deuterium atoms, the result is heavy water (D2O).
An oxygen atom is about 16 times heavier than a hydrogen atom. So a water molecule is about 16+1+1=18 times heavier than a hydrogen atom, and a heavy water molecule is 16+2+2=20 times heavier than a hydrogen atom. In other words, heavy water is 20/18=1.11 times heavier than ordinary water.
Heavy water may contain as much as 100% D2O, and usually the term refers to water that is highly enriched in deuterium. The isotopic substitution with deuterium alters the bond energy of the hydrogen-oxygen bond in water, altering the physical, chemical, and especially biological properties of the pure or highly-enriched substance to a larger degree than is found in most isotope-substituted chemical compounds.
Relatively pure heavy water was produced in 1933, soon after the discovery of deuterium, the stable heavy isotope of hydrogen. With the discovery of nuclear fission in late 1938 and the need for a neutron moderator which captured few neutrons, heavy water soon achieved importance in relation to early nuclear programs during World War II. Due in part to German reliance on scarce heavy water for reactor research in this war, Germany did not succeed in producing a functioning reactor during World War II. Since this war, heavy water has played a part in a number of reactor designs, both in designs for power and for nuclear weapon-making. Reactors which use enriched uranium, however, are able to use normal 'light-water' for neutron moderation, and remain the most common type of reactor in use today.
Heavy water itself is not radioactive, and has physical properties similar to water save for being about 11% more dense. When the water in eukaryotic organisms is replaced by more than about 25 to 50% heavy water, they experience toxicity due to interference by the deuterium with the mitotic apparatus of these cells. Higher organisms, including mammals, if given only heavy water, soon become ill and die at the point that about half their body water has been replaced. Bacteria, however, are able to grow slowly in pure heavy water.
Small concentrations of heavy water are nontoxic. The adult human body naturally contains deuterium equivalent to the amount in about 5 grams of heavy water, and comparable doses of heavy water are still used as safe non-radioactive tracers for metabolic experiments in humans and other animals.
Heavy Water Wikipedia
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