Atomic Number: 34
Atomic Mass: 78.96 amu (atomic mass units)
Melting Point: 217.0 C (490.15 K, 422.6 F)
Boiling Point: 684.9 C (958.05 K, 1264.82 F)
Number of Protons: 34
Number of Electrons: 34
Number of Neutrons: 45
Crystal Structure: Hexagonal
Density @ 293 K: 4.79 grams per cubic centimeter
Selenium was discovered in 1817 by Jons Jacob Berzelius. This Swedish chemist was investigating an impurity in sulfuric acid samples being produced by a Swedish factory. He at first believed the contaminant to be tellurium but found it to be a new element. This element was named selenium after the Greek moon goddess "Selene".
Selenium has two crystalline forms the most stable is hexagonal and it is gray in color. Monoclinic crystals of selenium are a deep red color. It can also be prepared with an amorphous structure. The element belongs to a group 16 of the periodic table. This group is sometimes known as chalcogens. The other chalcogens are oxygen, sulfur, tellurium, polonium and ununhexium.
The element is considered essential to plants and mammals in very small quantities. It is thought to stimulate metabolism as well as protect against heavy metals and free radicals. Too much selenium is toxic this can be seen in animals grazing on land rich in selenium. The plant Astragalus also known as locoweed concentrates the selenium and animals eating this go "loco" because of their increased selenium levels. However sheep grazed on pasture with too little selenium develop a condition known as white muscle disease. Some of selenium's compounds, such as hydrogen selenide, are extremely toxic and resemble arsenic in their action.
The element has six naturally occurring isotopes five are stable while the sixth, selenium-82, has an extremely long half life. The most abundant isotope is selenium-80 at 49.61%. There are twenty nine other unstable isotopes of selenium with half lives that range from about 1.3 microseconds to 119.79 days.
Selenium is found in the mineral ores eucairite, crooksite and clausthalite. These ores are too rare to make mining them economical so most selenium is produced as a by-product of the refining of copper.
Selenium has a number of properties that make it a very useful element.
* The amount of resistance to electricity exhibited by selenium depends on the amount of light shining upon the crystal. This makes it useful in the production of photo electric cells as well as light meters used in cameras and copiers.
* It is used in the production of solar panels as it can produce electricity from sunlight.
* The element is a semiconductor and as such is used in the production of some solid state electronics. It is also used in rectifiers which convert alternating current to direct current.
* It is used to impart a ruby red color into decorative glassware and enamels.
* Selenium is also used as a photocopying toner.
* Selenium sulfide shampoos are used to treat dandruff and skin conditions caused by the yeast Pityrosporum.