Selenium

Selenium

Selenium is the third element in Group 16 of the periodic family. The members of this family are sometimes referred to as the chalcogens, meaning "ore-forming." Selenium has an atomic number of 34, an atomic mass of 78.96, and a chemical symbol of Se.

Properties

Selenium exists in a number of allotropic forms. One allotrope is an amorphous red powder, while a second allotrope has a bluish, metallic appearance. Other allotropes have properties intermediate between these two allotropes. The crystalline form of selenium has a melting point of 423°F (217°C), a boiling point of 1,265°F (685°C), and a density of 4.5 grams per cubic centimeter.

Chemically, selenium is a metalloid. It is a fairly reactive element that combines readily with hydrogen, fluorine, chlorine, and bromine. It also combines with a number of metals to form compounds known as selenides as, for example, magnesium selenide (MgSe). Selenium burns in oxygen with a bright blue flame to produce selenium dioxide (SeO₂), which has a characteristic odor of rotten horseradish.

Occurrence and Extraction

Selenium is a rare element with an abundance in the Earth's crust of about 0.05-0.09 parts per million. It is widely distributed in the Earth's crust and occurs in no one mineral from which it can be extracted with profit. It is most commonly obtained as a byproduct of the extraction of copper, iron, and lead from their ores. The major producers of selenium are Japan, Canada, Belgium, the United States, and Germany.

Discovery and Naming

Selenium was discovered in 1818 by the Swedish chemists Jöns Jakob Berzelius and J. G. Gahn (1745-1818). The two men were studying the chemicals used in making sulfuric acid at a plant where they had just become part owners. Among the chemicals they found a material they thought at first to be the element tellurium. Upon further analysis, they found that the material was not tellurium, but a new element with similar chemical properties. They suggested the name selenium for the element after the Greek word selene, meaning "moon." The name seemed appropriate because of the new element's close association with tellurium, which had been named from the Latin word tellus, meaning "Earth."

Uses

The two most important uses of selenium are in glass-making and in electronics. The addition of selenium to glass can either add a beautiful ruby red color to the final product or, if iron is present, it can balance the green color caused by the iron and produce a clear, colorless product. Selenium is of growing importance in the field of electronics, where it is used to make photovoltaic cells, laser printers, and plain-paper photocopiers.

Health Issues

Selenium has a dual biological effect on plants and animals. It is required in very small amounts to maintain good health of both plants and animals, but in larger amounts it can produce health problems. For example, birds visiting the Kesterson Reservoir in California are exposed to very high levels of selenium in the water and, over the past decades, have shown an increasing number of genetic deformities caused by the element.