Selenium
Selenium is a metal-like element signified by the atomic symbol Se. The average atomic weight of its isotopes is 78.96, and its atomic number is 34. The element exists in at least three allotropic forms, which may be either amorphous or crystalline and red or black in color. Both its chemical and physical properties are similar to those of sulfur, the element just above it in the periodic table.
Selenium was discovered in 1817 by the famous Swedish chemist, Jöns Berzelius. Berzelius found the element in the sludge deposited at the bottom of a tank in a sulfuric acid factory. In a letter to a friend dated September 23, 1817, Berzelius described how some of the sulfur burned in the factory escaped oxidation and, instead, deposited at the bottom of the lead chamber. He noted that this sulfur-based sludge also seemed to contain a small amount of tellurium, an element discovered in gold ores by Müller von Reichenstein in 1783.
Five months later, Berzelius followed up with a second letter in which he announced that he had been wrong about the strange material in the lead chamber. It was not tellurium, but an entirely new element, in the sludge. Berzelius named the element selenium, from the Greek word selene, for "moon. " He chose the name as an analogy with tellurium (from the Latin, tellus, for "earth"). Berzelius' confusion about selenium is easy to understand since it lies just above tellurium in the periodic table and is chemically very similar to it.
Selenium occurs rarely in minerals such as clausthalite (PbSe) and tiemannite (HgSe), but most commonly as an impurity in sulfide ores. It is obtained commercially as a by-product of the electrolytic refining of copper.
Selenium is somewhat unusual among the elements in that small amounts are needed to maintain the health of plants and animals, but, in larger amounts, it is toxic to organisms. Scientists think that the human body, for example, incorporates selenium into certain proteins and enzymes that occur in muscles. A lack of selenium can result in the degeneration of skeletal muscles.
On the other hand, even small excesses of selenium appear to have harmful effects on plants and animals. In the late 1970s, for example, scientists observed a large increase in the number of waterfowl who were born with various genetic defects in the Kesterson Reservoir in northern California. They traced these defects to selenium which had been released from industrial plants and agricultural wastes, traveled through the state's waterways, and accumulated in Kesterson.
Selenium has a great many commercial applications. The element is photovoltaic, meaning that it is able to convert light directly into electricity. It is also photoconductive, meaning that its resistance to the flow of electrical current decreases when it is exposed to light. Both properties make the element useful in photocells, exposure meters, and solar cells. Since it converts alternating current to direct current, it can also be used in rectifiers. The element is also a semiconductor, which makes it useful in electronic and solid-state appliances. Photocopying machines also use selenium in toners. Finally, compounds of selenium are used to impart pink, red, or orange color to glass.
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