| | | | | manufacture of sulfuric acid. We sometimes forget |
| Selenium is both a chalcogen and a metalloid. The | | | | what an amazing step forward the invention of the |
| name chalcogen comes from the Greek word | | | | copy machine was. Hundreds of years ago, making a |
| chatkos, meaning "ore." The first two members of the | | | | copy of a document was a long, difficult process. |
| family, oxygen and sulfur, are found in most ores. | | | | Some people spent their whole lives making copies of |
| Selenium is a metalloid, an element that has some | | | | important documents. Each copy was written out by |
| characteristics of a metal and some of a non-metal. | | | | hand. The process was not only dull and monotonous, |
| Selenium and tellurium are often associated with each | | | | but it also resulted in many errors. |
| other. They tend to occur together in the Earth and | | | | Even thirty years ago, copying was slow and difficult. |
| have somewhat similar properties. They have many | | | | For example, carbon paper allowed a person to make |
| uses in common. In recent years, some important new | | | | one or more copies while writing or typing. But every |
| uses have been found for selenium. It is now used in | | | | error had to be corrected on every copy. The copies |
| the manufacture of plain paper photocopiers and laser | | | | were often messy and difficult to read. Mimeograph |
| printers, in photovoltaic cells that convert sunlight into | | | | machines made it possible to reproduce dozens of |
| electricity, and in X-ray systems for medical | | | | copies in a few minutes, but required handwritten or |
| applications. | | | | typed originals. The final product was printed in purple |
| Selenium was discovered in 1818 by Swedish chemists | | | | and then came the photocopy machine. Copies could |
| Jons Jakob Berzelius and J. G. Gahn. The men were | | | | be made by simply placing the original on a glass cover |
| studying the chemicals used in making sulfuric acid at a | | | | and pushing a button. We rarely think what goes on |
| plant where they had just become part-owners. | | | | inside a copy machine to make this happen? |
| Among these chemicals they found a material that | | | | An essential part of a photocopier is a drum-shaped |
| they thought was the element tellurium. Tellurium had | | | | unit or a wide moving belt. Fine selenium powder is |
| been discovered some 30 years earlier, mixed with | | | | spread on the surface of the drum or the belt. An |
| some gold deposits in Hungary. Tellurium is a rare | | | | electric charge is then applied to the selenium. Another |
| element. Berzelius decided to study the sample more | | | | part of the photocopy machine consists of a set of |
| carefully. He took it back to his laboratory in Stockholm. | | | | mirrors. When the machine's "Copy" button is pushed, a |
| There, he found that he and Gahn had been mistaken. | | | | bright light shines on the page being copied. The light |
| The substance was similar to tellurium, but it also had | | | | reflects off the white parts of the page. But it is not |
| different properties. They realized they had found a | | | | reflected off the dark parts, such as text or images. |
| new element. Berzelius suggested naming the element | | | | The light reflects off the mirrors to the drum or belt. |
| selenium; from the Greek word Selene, for "moon." | | | | Selenium is important because when light strikes the |
| The name seemed a good choice because the | | | | charged selenium, the charge disappears. The sections |
| element tellurium is named after the Latin word tellus | | | | on the drum or belt struck by light have no charge. The |
| for "Earth." Just as the Earth and the Moon go | | | | sections not struck by light continue to have a charge. |
| together, so do tellurium and selenium. | | | | Next, a toner is spread out over the surface of the |
| Selenium exists in a number of allotropic forms. One | | | | drum or belt. A toner is usually finely-divided carbon. It |
| allotrope of selenium is an amorphous red powder. | | | | sticks to the areas that still carry an electric charge. |
| Amorphous means "without crystalline shape." A lump | | | | But it does not stick to the selection without a charge. |
| of clay is an example of an amorphous material. A | | | | Finally, a piece of paper is pressed against the drum or |
| second allotrope of selenium has a bluish, metallic | | | | belt. The toner sticks to the paper. A blast of heat |
| appearance. A number of other allotropes have | | | | causes the carbon to melt and stick tightly to the |
| properties somewhere between these two forms. | | | | paper. A copy of the original document is produced by |
| The amorphous forms of selenium do not have | | | | the machine. |
| specific melting points. Instead, they gradually become | | | | The two most important uses of selenium are in |
| softer as they are heated. They may also change | | | | glass-making and in electronics. Each accounts for |
| from one color and texture to another. The crystalline | | | | about 30 to 35 percent of all the selenium produced |
| (metallic) form of selenium has a melting point | | | | each year. The addition of selenium to glass can have |
| of217°C (423°F) and a boiling point of 685°C | | | | one of two opposite effects. First, it will cancel out the |
| (1,260°F). Its density is 4.5 grams per cubic centimeter. | | | | green color that iron compounds usually add to glass. If |
| Selenium has come from serene, the Greek word for | | | | a colorless glass is desired, a little selenium is added to |
| moon. | | | | neutralize the effects of iron. Second, selenium will add |
| One of the most important physical characteristics of | | | | its own color—a beautiful ruby red—if that is |
| selenium is its electrical properties. For example, | | | | wanted in a glass product. Selenium plays a critical role |
| selenium is a semiconductor. Semiconductors have | | | | in the photocopying process. |
| many very important applications today in the | | | | Selenium is also added to glass used in architecture. |
| electronics industry. Selenium is often used in the | | | | The selenium reduces the amount of sunlight that gets |
| manufacture of transistors for computers, cellular | | | | through the glass. A growing use of selenium is in |
| phones, and hand-held electronic games. Selenium is | | | | electronic products. One of the most important uses is |
| also a photoconductor, a material that changes light | | | | in plain-paper photocopiers and laser printers. The |
| energy into electrical energy. Furthermore, it becomes | | | | element is also used to make photovoltaic ("solar") |
| better at making this conversion as the light intensity or | | | | cells. When light strikes selenium, it is changed into |
| brightness increases. | | | | electricity. A solar cell is a device for capturing the |
| Selenium is a fairly reactive element. It combines easily | | | | energy of sunlight on tiny pieces of selenium. The |
| with hydrogen, fluorine, chlorine, and bromine. It reacts | | | | sunlight is then changed into electrical energy. Currently, |
| with nitric and sulfuric acids. It also combines with a | | | | that process is not very efficient as too much sunlight |
| number of metals to form compounds called selenides. | | | | is lost without being converted into electricity. More |
| An example is magnesium selenide (MgSe). One of its | | | | efficient solar cells will be able to make use of all the |
| interesting reactions is with oxygen. It burns in oxygen | | | | free sunlight that strikes the planet every day. |
| with a bright blue flame to form selenium dioxide | | | | About a third of all selenium produced is used as |
| (SeO2). Selenium dioxide has a characteristic odor of | | | | pigments (coloring agents) for paints, plastics, ceramics, |
| rotten horseradish. Selenium and tellurium are often | | | | and glazes. Depending on the form of selenium used, |
| associated with each other. They tend to occur | | | | the color ranges from deep red to light orange. |
| together in the Earth and have somewhat similar | | | | Selenium is also used to make alloys. The addition of |
| properties. | | | | selenium to a metal makes it more machinable. |
| Selenium is a very rare element. Scientists estimate its | | | | Machinability means working with a metal: bending, |
| abundance at about 0.05 to 0.09 parts per million. It | | | | cutting, shaping, turning, and finishing the metal, for |
| ranks among the 25 least common elements in the | | | | example. About 5 percent of all selenium produced is |
| Earth's crust. It is widely distributed throughout the crust. | | | | used in agriculture. It is added to soil or animal feed to |
| There is no ore from which it can be mined with profit. | | | | provide the low levels of selenium needed by plants |
| Instead, it is obtained as a by-product of mining other | | | | and animals. |
| metals. It is now produced primarily from copper, iron, | | | | Very few compounds of selenium have any important |
| and lead ores. The major producers of selenium in the | | | | practical applications. One exception is selenium sulfide |
| world are Japan, Canada, Belgium, the United States, | | | | (SeS2). This compound is used to treat seborrhea, or |
| and Germany. | | | | "oily skin." It is sometimes added to shampoos for |
| There are six naturally occurring isotopes of selenium: | | | | people with especially oily hair. Another compound, |
| selenium-74, selenium-76, selenium-77, selenium-78, | | | | selenium diethyldithiocarbonate (Se[SC(S)N(C2H5)2]4), |
| selenium-80, and selenium-82. Isotopes are two or | | | | is used as a vulcanizing ("toughening") agent for rubber |
| more forms of an element. About a dozen radioactive | | | | products. Selenium has some rather interesting |
| isotopes of selenium are also known. Only one | | | | nutritional roles. It is essential in very small amounts for |
| radioactive isotope of selenium, selenium-75 is used | | | | the health of both plants and animals. Animals that do |
| commercially. This isotope is used to study the function | | | | not have enough selenium in their diets may develop |
| of two organs in the body, the pancreas and the | | | | weak muscles. But large doses of selenium are |
| parathyroid gland. (The pancreas helps with digestion | | | | dangerous. In some parts of California, for example, |
| and the parathyroid gland releases hormones.) The | | | | selenium has been dissolved out of the soil by irrigation |
| radioactive selenium is injected into the blood stream. It | | | | systems. Lakes accumulate unusually high levels of |
| then goes primarily to one or both of these two | | | | selenium and birds and fish in the area develop health |
| organs. The isotope gives off radiation when it | | | | problems. |
| reaches these organs. A technician can tell whether | | | | A serious selenium problem occurred at the Kesterson |
| the organs are functioning properly by the amount and | | | | Reservoir in Northern California. In the Late 1970s, |
| location of radiation given off. | | | | scientists found that birds nesting in the reservoir were |
| Selenium is obtained as a by-product from other | | | | developing genetic deformities. They traced the |
| industrial processes. For example, when copper is | | | | problem to high levels of selenium in the water. A large |
| refined, small amounts of selenium are produced as | | | | artificial lake was built and the birds were moved to |
| by-products. This selenium can be removed from the | | | | the artificial lake. They were no longer allowed to nest |
| copper-refining process and purified. Selenium is also | | | | in the dangerous waters of the reservoir. |
| obtained as a secondary product during the | | | | |
