Periodic Table -> Selenium


Selenium Details

Selenium Symbol: Se

Selenium Atomic Number: 34

Selenium Atomic Weight: 78.96

What is Selenium?

Selenium (atomic number 32, symbol Se) is a nonmetal and a chemical element that is added to dietary supplements and is an essential mineral. It was first observed by the Swedish scientist Jons Jacob Berzelius. The element occurs in copper ores and rare minerals such as clausthalite, crooksite, and eucairite. It is also found in amino acids such as methylselenocysteine, selenocysteine, and selenomethionine. Selenium is found in ocean waters and some soils as well. Commercial production at mining sites and coal burning also release selenium, including smelting and extraction of sulfide ores. The element is obtained through the process of electrolytic metal refining. There are two forms red powder and a semi-metal that is silver in color.

Physical and Chemical Properties
This element is semiconductor and conducts electricity. Black selenium is solid, lustrous, and brittle and forms red selenium through evaporation. Five stable isotopes occur in nature Se-80, Se-78, Se-77, Se-76, and Se-74. Se-80 is a key isotope. The element also forms different compounds such as selenides, halogen compounds, and chalcogen compounds, including sodium selenite, silver selenite, selenium trioxide, selenous acid, and others. Selenides also form, for example, copper indium gallium, zinc, lead, and mercury selenides. These compounds are semiconductors. Other compounds are explosive. One example is etraselenium tetranitride. The element is also found in different inorganic forms such as minerals that contain selenite, selenate, and selenide acid. Selenium dichloride is an example of an inorganic compound, but it has no commercial application. The element itself is usually obtained from lead, nickel, copper, and other sulfide ores. Its physical properties resemble the properties of tellurium and sulfur. Common oxidation states include -2, +4, and +6. It is solid at room temperature. The boiling point is 1,265 F (685C), and the melting point is 428.9F (220.5C).

Commercial Applications and Health
Selenium has a number of applications in different industries. It is used to produce solar cells, photocells, alloys, and glass. It has photoconductive and photovoltaic properties. The element is added to stainless steel and used as a gamma source and in photographic toners. It is also added to rectifiers, storage batteries, and metal alloys. Some of its compounds, for example, selenium sulfide are added to anti dandruff shampoos. It is used in food supplements and animal feeds as well.
In fact, selenium is found in organic and inorganic forms and many foods, including beef, rice, cottage cheese, and turkey. Other foods that are rich in selenium include shrimp, ham, sardines, Brazil nuts, and boiled and frozen spinach.

Health and Environmental Hazards and Overexposure
Deficiency increases the risk for medical conditions such as the Kashin-Beck disease, infertility and reproductive problems, mental fatigue, hyperthyroidism, and others. Side effects include sensitivity to light, emotional disturbance, and heart palpitations. Deficiency is more common in countries and regions that are low in selenium, including Siberia, Tibet, China, and others. Deficiency is uncommon in countries such as Canada and the U.S., but some groups are at risk. In addition to populations living in low-selenium regions, HIV-positive patients and patients on kidney dialysis are at a higher risk. Deficiency also increases the risk for cretinism in babies. Research studies show that proper nutrition and intake may help prevent diseases and chronic conditions such as thyroid disease, cognitive problems, cardiovascular problems, and cancer. One of the reasons is that this element has antioxidant properties. Selenium supplements lower the risk of cardiac death and coronary heart disease. Overexposure can be a health hazard, however, as the element is toxic if taken in large doses. Exposure occurs through inhalation, contact with soil, and water and food. Overexposure to fumes leads to abdominal pain, sore throat, fever, nausea, bronchial asthma, and enlarged liver. In animals, it leads to birth defects and fertility problems. In general, people who work at mining sites and selenium-recovery industries must exercise caution. The tolerable intake levels depend on age and sex.

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