Deuterium
Deuterium is an isotope of hydrogen with atomic mass of 2. It is represented by the symbols 2H or D. Deuterium is also known as heavy hydrogen. The nucleus of the deuterium atom, consisting of a proton and a neutron, is also known as a deuteron and is represented by the symbol d.
The possible existence of an isotope of hydrogen with atomic mass or two was suspected as early as the late 1910s after Frederick Soddy had developed the concept of isotopes. Such an isotope was of particular interest to chemists. Since the hydrogen atom is the simplest of all atoms --consisting of a single proton and a single electron--it is the model for most atomic theories. An atom just slightly more complex-one that contains a single neutron also--could potentially contribute valuable information to existing atomic theories.
Among those who sought for the heavy isotope of hydrogen was Harold Urey, at that time professor of chemistry at Columbia University. Urey began his work with the realization that any isotope of hydrogen other than hydogen-1 (also known as protium) must exist in only minute quantities. The evidence for that fact is that the atomic weight of hydrogen is only slightly more than 1.000. The fraction of any isotopes with mass greater than that value must, therefore, be very small. Urey designed an experiment, therefore, that would allow him to detect the presence of heavy hydrogen in very small concentrations.
Urey's approach was to collect a large volume of liquid hydrogen and then to allow that liquid to evaporate very slowly. His hypothesis was that the lighter and more abundant protium isotope would evaporate more quickly than the heavier hydrogen-2 isotope. The volume of liquid hydrogen remaining after evaporation was nearly complete, then, would be relatively rich in the heavier isotope.
In the actual experiment, Urey allowed 4.2 qt (4 l) of liquid hydrogen to evaporate until only 0.034 oz. (1 ml) remained. He then submitted that sample to analysis by spectroscopy. In spectroscopic analysis, energy is added to a sample. Atoms in the sample are excited and their electrons are raised to higher energy levels. After a moment at these higher energy levels, the electrons return to their ground state, giving off their excess energy in the form of light. The bands of light emitted in this process are characteristic for each specific kind of atom.
By analyzing the spectral pattern obtained from his 0.034 oz. (1 ml) sample of liquid hydrogen, Urey was able to identify a type of atom that had never before been detected, the heavy isotope of hydrogen. The new isotope was soon assigned the name deuterium. For his discovery of the isotope, Urey was awarded the 1934 Nobel Prize in chemistry.
Deuterium is a stable isotope of hydrogen with a relative atomic mass of 2.014102 compared to the atomic mass of protium, 1.007825. Deuterium occurs to the extent of about 0.0156% in a sample of naturally occurring hydrogen. Its melting point is 18.73 K (compared to 13.957 K or protium) and its boiling point is 23.67 K (compared to 20.39 K for protium).
Deuterium is most commonly found as part of the compound deuterium oxide (D2O), usually referred to as heavy water (an appropriate name considering that deuterium has twice the weight of the normal hydrogen atom). Given that the deuterium isotope occurs naturally in the ratio 1:4500; one would expect to find D2O at a level of about 1 in 20 million water molecules. Heavy water was first separated from ordinary water in 1932 by G N Lewis, a chemist at the University of California. Compounds containing deuterium have slightly different properties from those containing protium, and the melting and boiling points of heavy water (see below) are, respectively, 38.86°F (3.81°C) and 214.56°F (101.42°C).
Heavy water can be prepared by the prolonged electrolysis of water. During the electrolysis, molecular oxygen is generated at the anode, and molecular hydrogen at the cathode. The deuterium tends to remain behind, so the concentration of heavy water increases as electrolysis progresses. When electrolysis has been carried to completion, all that remains is pure heavy water. It takes approximately 100,000 gallons of water to produce a single gallon of pure heavy water by electrolysis.
Heavy water is a suitable and convenient moderator of neutrons in nuclear reactors. In heavy water reactors, plutonium can be bred from natural uranium. Therefore, the production of heavy water is closely monitored, and the material is export controlled. Nations seeking large quantities of heavy water immediately become suspect of wanting to use the material to moderate a reactor, with possible intentions of producing plutonium. However, Canada's CANDU reactors, which use heavy water, are designed and used for commercial electric power production.
Heavy water is not radioactive, nor is it especially dangerous to humans or other lifeforms. But seeds will not germinate in heavy water, and some animals, including tadpoles, cannot live in it. This is because replacing hydrogen with deuterium slows down the rate of any chemical reaction in which the chemical bond to the hydrogen atom is broken, including many chemical reactions occurring in biological systems. Hydrogen atoms from water end up in a large number of biomolecules, so any process involving these hydr ogen atoms will also be slowed down if heavy water is substituted for water. Thus, heavy water slows down many metabolic processes.
Deuterium has primarily two uses, as a tracer in research and in thermonuclear fusion reactions. A tracer is any atom or group of atoms whose participation in a physical, chemical, or biological reaction can be easily observed. Radioactive isotopes are perhaps the most familiar kind of tracer. They can be tracked in various types of changes because of the radiation they emit.
Deuterium is an effective tracer because of its mass. When it replaces protium in a compound, its presence can easily be detected because it weighs twice as much as the protium atom. Also, as mentioned above, the bonds formed by deuterium with other atoms are slightly different from those formed by protium with other atoms. Thus, it is often possible to figure out what detailed changes take place at various stages of a chemical reaction using deuterium as a tracer.
Deuterium plays a critical role in most thermonuclear fusion reactions. In the solar process, for example, the fusion sequence appears to begin when two protium nuclei fuse to form a single deuteron. The deuteron is used up in later stages of the cycle by which four protium nuclei are converted to a single helium nucleus.
In the late 1940s and early 1950s, scientists found a way of duplicating the process by which the sun's energy is produced in the form of thermonuclear fusion weapons, the socalled hydrogen bomb. The detonating device in this type of weapon was lithium deuteride, a compound of lithium metal and deuterium. The detonator was placed on the casing of an ordinary fission ("atomic") bomb. When the fission bomb detonated, it set off further nuclear reactions in the lithium deuteride that, in turn, set off fusion reactions in the larger hydrogen bomb.
For more than four decades, scientists have been trying to develop a method for bringing under control the awesome fusion power of a hydrogen bomb for use in commercial power plants. So far, the technical details for making such a process commercially viable have not been completely worked out.
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