Atomic Number: 34
Atomic Symbol: Se
Atomic Weight: 78.96
Melting Point: 428.9 F (220.5 C)
Boiling Point: 1,265 F (685 C)
Word origin: Selenium is named after the Greek word for moon, Selene.
Discovery: The element was discovered by Swedish chemist Jöns Jacob Berzelius in 1817. Berzelius also noted the similarities between selenium and the metal tellurium, named after the Earth.
Properties of selenium
Selenium has several allotropic forms, but only three are generally recognized. Amorphous selenium is either red, in powder form, or black, in vitreous, or glassy, form. The most stable form of the element, crystalline hexagonal selenium, is a metallic gray, while crystalline monoclinic selenium is a deep red. The element resembles sulfur in its makeup.
Elemental selenium is an essential nutritional element found in many plants. However, hydrogen selenide (H2Se) and other selenium compounds are highly toxic. Hydrogen selenide can be intolerable to humans at concentrations of 1.5 ppm.
Selenium has 21 known isotopes. Of these, six are stable.
Sources of selenium
Selenium can be found naturally in the minerals crooksite and clausthalite. Selenium is a byproduct of copper sulfide ores. Today, it is mostly obtained from the anode metal from electrolytic copper refineries. Selenium is recovered by roasting the sludge or mud with sulfuric acid or soda. It can also be recovered by smelting with soda and niter.
Selenium combines with proteins to make selenoproteins, which are important antioxidant enzymes. Selenoproteins help prevent cellular damage from free radicals, which are natural byproducts of oxygen metabolism that may contribute to the development of chronic diseases such as cancer and heart disease. Other selenoproteins help regulate thyroid function and play a role in the immune system.
Plant foods are the major dietary sources of selenium in most countries throughout the world. Selenium also can be found in some meats and seafood. In the United States, meats and bread are common sources of dietary selenium. Some nuts are also sources of selenium.
Uses of selenium
The photovoltaic and photoconductive properties of selenium make it useful for a variety of items. Selenium is used:
- in photocells and light meters
- in photocopying processes and in photo toner
- to decolorize glass
- to create a ruby-red enamels or glass product
- in rectifiers, as it can conduct AC power to DC power
- as an additive to stainless steel
- for a variety of electronic applications, when dropped below its melting point.