Selenium (revised)

SELENIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Selenium is a member of the chalcogen family. The chalcogens are elements in Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Other chalcogens are oxygen, sulfur, tellurium, and polonium. The name chalcogen comes from the Greek word chatkos, meaning "ore." The first two members of the family, oxygen and sulfur, are found in most ores.

Selenium is a metalloid. A metalloid is an element that has some characteristics of a metal and some of a non-metal.

Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties. They have many uses in common. In recent years, some important new uses have been found for selenium. It is now used in the manufacture of plain paper photocopiers and laser printers, in photovoltaic cells that convert sunlight into electricity, and in X-ray systems for medical applications.

SYMBOL
Se

ATOMIC NUMBER
34

ATOMIC MASS
78.96

FAMILY
Group 16 (VIA)
Chalcogen

PRONUNCIATION
suh-LEE-nee-um

Discovery and naming

Selenium was discovered in 1818 by Swedish chemists Jons Jakob Berzelius (1779-1848) and J. G. Gahn (1745-1818). The men were studying the chemicals used in making sulfuric acid at a plant where they had just become part-owners. Among these chemicals they found a material that they thought was the element tellurium. Tellurium had been discovered some 30 years earlier, mixed with some gold deposits in Hungary.

Tellurium is a rare element. Berzelius decided to study the sample more carefully. He took it back to his laboratory in Stockholm. There, he found that he and Gahn had been mistaken. The substance was similar to tellurium, but it also had different properties. They realized they had found a new element. Berzelius suggested naming the element selenium, from the Greek word selene, for "moon." The name seemed a good choice because the element tellurium is named after the Latin word tellus for "Earth." Just as the Earth and the Moon go together, so do tellurium and selenium.

Physical properties

Selenium exists in a number of allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope of selenium is an amorphous red powder. Amorphous means "without crystalline shape." A lump of clay is an example of an amorphous material. A second allotrope of selenium has a bluish, metallic appearance. A number of other allotropes have properties somewhere between these two forms.

The amorphous forms of selenium do not have specific melting points. Instead, they gradually become softer as they are heated. They may also change from one color and texture to another.

The crystalline (metallic) form of selenium has a melting point of217°C (423°F ) and a boiling point of 685°C (1,260°F). Its density is 4.5 grams per cubic centimeter.

Selenium from the Greek word for moon, selene.

Some of the most important physical characteristics of selenium are its electrical properties. For example, selenium is a semiconductor. A semiconductor is a substance that conducts an electric current better than non-conductors, but not as well as conductors. Semiconductors have many very important applications today in the electronics industry. Selenium is often used in the manufacture of transistors for computers, cellular phones, and hand-held electronic games.

Selenium is also a photoconductor, a material that changes light energy into electrical energy. Furthermore, it becomes better at making this conversion as the light intensity or brightness increases.

Chemical properties

Selenium is a fairly reactive element. It combines easily with hydrogen, fluorine, chlorine, and bromine. It reacts with nitric and sulfuric acids. It also combines with a number of metals to form compounds called selenides. An example is magnesium selenide (MgSe). One of its interesting reactions is with oxygen. It burns in oxygen with a bright blue flame to form selenium dioxide (SeO₂). Selenium dioxide has a characteristic odor of rotten horseradish.

Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties.

Occurrence in nature

Selenium is a very rare element. Scientists estimate its abundance at about 0.05 to 0.09 parts per million. It ranks among the 25 least common elements in the Earth's crust. It is widely distributed throughout the crust. There is no ore from which it can be mined with profit. Instead, it is obtained as a by-product of mining other metals. It is now produced primarily from copper, iron , and lead ores. The major producers of selenium in the world are Japan, Canada, Belgium, the United States, and Germany.

Isotopes

There are six naturally occurring isotopes of selenium, selenium-74, selenium-76, selenium-77, selenium-78, selenium-80, and selenium-82. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About a dozen radioactive isotopes of selenium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Only one radioactive isotope of selenium is used commercially, selenium-75. This isotope is used to study the function of two organs in the body, the pancreas and the parathyroid gland. (The pancreas helps with digestion and the parathyroid gland releases hormones.) The radioactive selenium is injected into the blood stream. It then goes primarily to one or both of these two organs. The isotope gives off radiation when it reaches these organs. A technician can tell whether the organs are functioning properly by the amount and location of radiation given off.

Extraction

Selenium is obtained as a by-product from other industrial processes. For example, when copper is refined, small amounts of selenium are produced as by-products. This selenium can be removed from the copper-refining process and purified. Selenium is also obtained as a secondary product during the manufacture of sulfuric acid.

The miracle of copying

W e sometimes forget what an amazing step forward the invention of the copy machine was. Hundreds of years ago, making a copy of a document was a long, difficult process. Some people spent their whole lives making copies of important documents. Each copy was written out by hand. The process was not only dull and monotonous, but it also resulted in many errors.

Even thirty years ago, copying was slow and difficult. For example, carbon paper allowed a person to make one or more copies while writing or typing. But every error had to be corrected on every copy. The copies were often messy and difficult to read.

Mimeograph machines made it possible to reproduce dozens of copies in a few minutes, but required handwritten or typed originals. The final product was printed in purple.

Then came the photocopy machine. Copies could be made by simply placing the original on a glass cover and pushing a button. What goes on inside a copy machine to make this happen?

An essential part of a photocopier is a drum-shaped unit or a wide moving belt. Fine selenium powder is spread on the surface of the drum or the belt. An electric charge is then applied to the selenium.

Another part of the photocopy machine consists of a set of mirrors. When the machine's "Copy" button is pushed, a bright light shines on the page being copied. The light reflects off the white parts of the page. But it is not reflected off the dark parts, such as text or images. The light reflects off the mirrors to the drum or belt.

Selenium is important because when light strikes the charged selenium, the charge disappears. The sections on the drum or belt struck by light have no charge. The sections not struck by light continue to have a charge.

Next, a toner is spread out over the surface of the drum or belt. A toner is usually finely-divided carbon. It sticks to the areas that still carry an electric charge. But it does not stick to the selection without a charge.

Finally, a piece of paper is pressed against the drum or belt. The toner sticks to the paper. A blast of heat causes the carbon to melt and stick tightly to the paper. A copy of the original document is produced by the machine.

Uses

The two most important uses of selenium are in glass-making and in electronics. Each accounts for about 30 to 35 percent of all the selenium produced each year. The addition of selenium to glass can have one of two opposite effects. First, it will cancel out the green color that iron compounds usually add to glass. If a colorless glass is desired, a little selenium is added to neutralize the effects of iron. Second, selenium will add its own color—a beautiful ruby red—if that is wanted in a glass product.

Selenium is also added to glass used in architecture. The selenium reduces the amount of sunlight that gets through the glass.

A growing use of selenium is in electronic products. One of the most important uses is in plain-paper photocopiers and laser printers. The element is also used to make photovoltaic ("solar") cells. When light strikes selenium, it is changed into electricity. A solar cell is a device for capturing the energy of sunlight on tiny pieces of selenium. The sunlight is then changed into electrical energy.

Currently, that process is not very efficient. Too much sunlight is lost without being converted into electricity. More efficient solar cells will be able to make use of all the free sunlight that strikes the planet every day.

About a third of all selenium produced is used as pigments (coloring agents) for paints, plastics, ceramics, and glazes. Depending on the form of selenium used, the color ranges from deep red to light orange.

Selenium is also used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The addition of selenium to a metal makes it more machinable. Machinability means working with a metal: bending, cutting, shaping, turning, and finishing the metal, for example.

About 5 percent of all selenium produced is used in agriculture. It is added to soil or animal feed to provide the low levels of selenium needed by plants and animals.

Compounds

Very few compounds of selenium have any important practical applications. One exception is selenium sulfide (SeS₂). This compound is used to treat seborrhea, or "oily skin." It is sometimes added to shampoos for people with especially oily hair. Another compound, selenium diethyldithiocarbonate (Se[SC(S)N(C₂H₅)₂]₄), is used as a vulcanizing ("toughening") agent for rubber products.

Health effects

Selenium has some rather interesting nutritional roles. It is essential in very small amounts for the health of both plants and animals. Animals that do not have enough selenium in their diets may develop weak muscles. But large doses of selenium are dangerous. In some parts of California, for example, selenium has been dissolved out of the soil by irrigation systems. Lakes accumulate unusually high levels of selenium and birds and fish in the area develop health problems.

A serious selenium problem occurred at the Kesterson Reservoir in Northern California. In the Late 1970s, scientists found that birds nesting in the reservoir were developing genetic deformities. They traced the problem to high levels of selenium in the water. A large artificial lake was built and the birds were moved to the artificial lake. They were no longer allowed to nest in the dangerous waters of the reservoir.