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May 28th, 2009 |

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Positive and Negative Aspects of Radioactive Elements

The actinides (or actinoids) are the chemical elements with atomic numbers between 90 and 109 inclusively. They occur between Groups 3 and 4 in Period 7 of the periodic table. All elements in this family are radioactive. Five actinides namely: thorium, protoactinium, uranium, neptunium, and plutonium have been found in nature. The other actinides have been produced artificially in nuclear reactors or particle accelerators. For many years, the list of chemical elements known to scientists ended with atomic number 92, uranium. Scientists were uncertain as to whether elements heavier than uranium would ever be found. Then, in 1940, a remarkable discovery was made while University of California physicists Edwin McMillan and Philip Abelson were studying nuclear fission. During their research, the duo found evidence for the existence of a new element with atomic number 94, two greater than that of uranium. This new element was the first transuranium (heaver than uranium) element ever discovered. McMillan and Abelson named it neptunium, after the planet Neptune, just as uranium had been named after the planet Uranus. Later in the same year, McMillan and his two other colleagues found a second transuranium element, which they named plutonium, after the planet Pluto. At that point, the race was on to develop more synthetic transuranium elements, but the research process was not easy. The approach was to fire subatomic particles or small atoms, like those of helium, at a very large nucleus by means of a particle accelerator. If the smaller particle could be made to merge with the larger nucleus, a new atom would be produced. Over time, techniques became more and more sophisticated, and ever-heavier elements were created: americium (number 95) and curium (number 96) in 1944; berkelium (number 97) in 1949; californium (number 98) in 1950; einsteinium (number 99) and fermium (number 100) in 1952; mendelevium (number 101) in 1955; nobelium (number 102) in 1958; and lawrencium (number 103) in 1961. Studies of the actinide elements are among the most ingenious in all of chemistry. In some cases, no more than one or two atoms of a new element have been produced. Yet scientists have been able to study those few atoms well enough to discover basic properties of the elements. These studies are made even more difficult because most actinide isotopes decay quickly, with half-lives of only a few days or a few minutes. With the discovery of lawrencium, the actinide family of elements is complete. Scientists have also found elements heavier than lawrencium, but these elements belong to the lanthanide family (or rare earth elements). Uranium is a dull gray metallic element, relatively abundant in Earth's crust, ranking number 47 among the elements. Although perhaps not as well known, it is actually more abundant than more familiar elements such as tin, silver, mercury, and gold. By far the most important property of uranium is its radioactivity. Natural uranium consists of three isotopes of mass numbers 234, 235, and 238. All three isotopes are radioactive. Its most abundant isotope, uranium-238, decays by emitting an alpha particle with a half-life of 4.47 × 109 years. The half-life of uranium-238 is about equal to the age of Earth. That means that about one-half of all the uranium found on Earth at its moment of creation is still here. The other one-half has decayed to other elements. Knowing the half life of uranium-238 scientists can estimate the age of rocks. The amount of uranium-238 found in any particular rock is compared to the amount of daughter isotopes found with it. A daughter isotope is an isotope formed when some parent isotope, such as uranium-238, decays. The more daughter isotope present in a sample, the older the rock; the less daughter isotope, the younger the rock. The second most abundant isotope of uranium, uranium-235, has the rare property of being fissionable, meaning that its atomic nuclei will break apart when bombarded by neutrons. The fission of a uranium-235 nucleus releases very large amounts of energy, additional neutrons, and two large fission products. The fission products are the atomic nuclei formed when a fissionable nucleus such as uranium-235 breaks apart. The fission of uranium-235 nuclei has become extremely important in the manufacture of nuclear weapons and in the operation of nuclear power plants. In fact, these applications account for the primary applications of uranium in everyday life. Thorium is a soft metal with a bright silvery luster when freshly cut. It is relatively soft, with hardness about equal to that of lead. It is even more abundant than uranium, ranking number 39 in abundance among the elements in Earth's crust. No more than a few hundred tons of thorium is produced annually. About one-half of this production goes to the manufacture of gas mantles, insulated chambers in which fuel is burned. The rest goes for use as nuclear fuel, in sunlamps, in photoelectric cells or light sensitive cathode, and in the production of other alloys. At one time, the actinides other than uranium were no more than scientific curiosities. They were fascinating topics of research for scientists but of little practical interest. That situation has now changed, and all of the actinides that can be prepared in large enough quantities have found some use or another. Plutonium, for example, is used in the manufacture of nuclear weapons and as the power source in nuclear power plants. On a smaller scale, it is also used as a power source in smaller devices such as the heart pacemaker. Californium is used in smoke detectors, curium is a power source in space vehicles, and americium is utilized in the treatment of cancer. Most of the actinides are trans-uranium. They never occur free in nature. These metals are similar because their atomic structures are similar; all form compounds with the most common oxidation state 3. They are metals, with high luster and electrical conductivity and find uses in petroleum and electronics industries, manufacture of super conductors, permanent magnets, ceramics, glass, and metal alloys. Radioactive elements have a lot of important uses and they also cause some dangerous problems if they are not handled properly. Henri Becquerel first found out about radioactive elements late in the 19th century by placing some photographic film under uranium salts. The film was in a light-tight envelope, and it was exposed where he put the uranium on it. This behavior was eventually found to be caused by the emission of radiation from the decaying uranium which penetrated the paper envelope and exposed the film. Ever since then, more elements have been investigated for their radioactivity, and different isotopes of elements have different radioactive behavior. Many are used commercially and medically, and others are just nuisances. Small amounts of radioactive materials can be ingested as "radiotracers" to see how certain chemicals are taken up by the body. If a health researcher is interested in how a certain element is distributed by the body after it is ingested, he can choose to use a radioactive isotope of a common element, mix it in, and then use sensitive radiation detectors to see where it ends up in the body. These are often used in studies to see how medications are absorbed and transported within the body. Thorium, a naturally occurring radioactive element, is used in making mantles for gas and kerosene lamps because thorium oxide glows brightly when heated. The radioactive elements uranium and plutonium are used in the generation of electricity in nuclear power plants. Small radioactive sources of particles are used in many home smoke detectors. These elements are also used in the production of nuclear weapons. One can propose that the presence of nuclear weapons has prevented war, but also that they have made the consequences of possible war much worse than before. Depleted uranium, that is, naturally occurring uranium with the U235 taken out, is mostly U-238, which is a bit less radioactive than the natural material. This material is very dense and hard, however, and otherwise useless, so the army uses it to make bullets and other shells. These can pierce steel armor. Whether this is a good use or a bad use depends on which side of the gun one is standing on. Some radioactive elements glow because of their radioactive decays. They emit electrons or alpha particles, changing from one kind of element to another, and as the electrons in the atoms rearrange themselves to the new atom’s configuration, they emit light. Radium was used for watch dials because it glows green. Tritium can also be used as a backlight in watches because it too glows green. Tritium is still used in small quantities in small vials on watch hands and to mark the hour positions on watch dials. Radium isn’t used anymore, however. Radiation, even in small doses, can cause cancer in humans and other living things. Fast moving photons (gamma rays), electrons (beta rays) and helium nuclei (alpha particles) can crash into other molecules and change their structure. If this happens to a DNA molecule, it can damage the genetic information, and sometimes turn a cell cancerous. Radiation also causes burns, much like sunburn, in large doses over short amounts of time. Usually we can walk away from radioactive substances, lowering our risk. But if we ingest radioactive elements, they stay with us. Particularly nasty radioactive elements include radon and radioactive iodine. Radon is a chemically inert gas with a short half-life (and therefore decays rapidly, emitting radiation faster than other elements). It is produced naturally as a decay product of longer-lived radioactive elements in rock and soil. It may diffuse through basement walls and into people’s homes. It increases the rate of lung cancer when people breathe it in. It is a good idea to ventilate basements and have them checked, particularly in areas of the country where radon is common. Radioactive iodine is also readily absorbed by the body and becomes incorporated in bones, and is therefore difficult to eliminate from the body. The radiation it emits can cause bone cancer over long periods of time. The radium on watch dials was incorporated in paint. Workers used to paint the watch dials by hand, and some would even lick their paint brushes to make a sharper tip. They ingested radon paint, and some became ill with cancer. Naturally occurring uranium also was used to make bright yellow paint, but now this too has been stopped. Some people complain about radiation emitted by those depleted-uranium bullets and shells left over in wars. Residents of areas where such munitions have been used are concerned about the long-term health effects of the radioactivity. There is some concern that the main dangers from the leftover uranium dust may be due to chemical poisoning rather than radiation. Plutonium, while radioactive, also happens to be just plain poisonous. Human bodies do not deal well with heavy metals: lead, mercury, and arsenic come to mind as things not to ingest because they are poisonous. Plutonium may well be the most poisonous of the lot.
About the Author

Dr.Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.

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