atoms of the same atomic number but different atomic weights, a result of having different numbers of neutrons in the atomic nucleus. Although chemically identical, isotopes often have slightly different reactivities because of the difference in mass.

Isotopes do not just differ in reactivity, however.

Which isotope of an element you are dealing with in nuclear physics is very important. For instance, deuterons in heavy water are used in particle accelerators. They are an isotope of hydrogen, and regular hydrogen nuclei simply will not do the same job.

Another example is in preparing fuel for nuclear reactors. Before purified uranium is made into fuel pellets, it must be enriched - a process that calls for increasing the percentage of the isotope U-235 in the fuel from less than one percent, up to about five percent.

The reason for this is the major constituent of uranium, the isotope U-238. Neutrons are a scarce resource in a nuclear reactor, and the feasibility of the fission process depends on getting a chain reaction to occur. When a U-235 nucleus fissions, it releases one or more neutrons. When one of these neutrons hits another U-235 nucleus, it causes that nucleus to fission also, releasing more neutrons, and so on.

U-238 nuclei, on the other hand, will simply 'gobble up' any neutron that hits them without fissioning. (They later decay to plutonium (?), but not fast enough for our purposes, and they do not emit neutrons when they do.) As a result, if there is too much U-238 present, the chain reaction simply will not proceed.

If the percentage of U-235 is increased, the chain reaction becomes sustainable, and generating power from fission becomes possible.

(Incidentally, breeder reactors make more fuel than they burn, by altering the process so that a lot of plutonium gets formed during the fission of enriched uranium fuel. After reprocessing, they can then use the plutonium as fuel.)

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An alternate form of an element that has the usual number of protons but a nonstandard number of neutrons; the fewer or additional neutrons give the isotope a different atomic weight than the regular element and may make the isotope radioactive, but otherwise an isotope has the same chemical action as the regular element. Because of this, isotopes (such as radioactive carbon) are used as tracers in biological systems or processes.


From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.

A chemical element is defined by its atomic number, which is the number of protons in its nucleus. However, the atomic number does not specify all of the properties of a given atom, as the nucleus can also contain neutrons that do not contribute (much) to the chemical properties of the atom. These variations due to differences in neutron number are called isotopes. An isotope of an element is usually identified by its mass number, the total number of particles in its nucleus. For example, carbon-14 has eight neutrons in addition to carbon's six protons.

When most people hear the word 'isotope', they think of radioactive isotopes of common elements, often used in medicine as diagnostic tracers or sources for radiation therapy. As the conditions on the stability of an isotope are quite stringent, these radioisotopes vastly outnumber non-radioactive isotopes as an overall fraction of the number of isotopes. As 'normal' matter is mostly non-radioactive, though, radioisotopes are rare in common experience and have thus developed a strong association with the word 'isotope'.

Elements can have multiple stable isotopes. The most famous cases of this are two elements at opposite ends of the periodic table: hydrogen and uranium. The stable isotopes of hydrogen are protium, whose nucleus consists of a bare proton, and deuterium, which contains both a proton and a neutron. Deuterium is important as it undergoes nuclear fusion much more readily than protium, making high-yield, sustained fusion reactions possible. The relative difference in mass between protium and deuterium is larger than that between any two other stable isotopes, which alters the properties of deuterium compounds more than any other equivalent substitution. Heavy water is water containing deuterium rather than protium, which has a significantly different melting point, viscosity, and density than normal water.

Uranium does not, strictly speaking, have a stable isotope, being too heavy for complete stability. However, a number of its isotopes have multi-billion-year lifetimes and are effectively stable. The most common uranium isotope is uranium-238, but it is incapable of self-sustaining nuclear fission as the neutrons released by fission do not cause other U-238 nuclei to fission. On the other hand, uranium-235 quite readily undergoes fission when struck by a neutron, and so it can be used to create self-sustaining fission reactions in both nuclear reactors and nuclear weapons. Thus, the most tightly-controlled material in the world is uranium 'enriched' with a greater fraction of uranium-235.

Specific isotopes can be produced by a variety of methods. Light isotopes with relatively large mass differences can have reasonable differences in chemical reaction rates, with deuterium undergoing electrolysis from water slower than protium. Large-scale electrolysis plants are thus used to produce heavy water. The two uranium isotopes are very difficult to separate chemically and uranium is usually enriched in centrifuges which collect more of the heavy U-238 at the outside. Short-lived radioisotopes cannot be enriched from natural materials and must be artificially produced. Commonly, a sample is bombarded with high-energy protons, some of which are captured by the nuclei in the sample to produce new isotopes.

Different isotopes are, of course, important in nuclear physics, where often the concepts of element and isotope are combined to describe 'nuclides'. Large-scale radioisotope beam facilities such as ISOLDE at CERN and ISAC at TRIUMF are invaluable tools for probing the structure and interactions of nuclei, with consequences for astrophysics, particle physics, and atomic physics, among other fields.


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This writeup is copyright 2008 D.G. Roberge and is released under the Creative Commons Attribution-NonCommercial-ShareAlike licence. Details can be found at http://creativecommons.org/licenses/by-nc-sa/3.0/ .

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