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Selenium Se

Selenium is an essential micronutrient in all known forms of life.

The History Says

By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced.

The Present Scenario

Notable Occurrences of selenium are Jerome, Yavapai County, Arizona; Gold Quarry Mine and Willard Mine, Nevada and the Darwin Mine, California, USA; Moctezuma, Sonora, Mexico; Monte Vesuvius, Italy; Harz Mountains, Germany; Potosi, Bolivia and Los Llantenes, Argentina.

Selenium is a naturally occurring mineral element. It is distributed widely in nature in most rocks and soils. When pure, it exists as metallic gray to black hexagonal crystals. In nature, it occurs combined (usually) with sulfide or with silver, copper, lead, and nickel minerals.

It has a minimum of 29 isotopes, out of which 5 are stable and 6 are nuclear isomers. Free selenium is a nontoxic but most of its compounds are very toxic whose actions are similar to that of arsenic. For example, hydrogen selenide is very toxic. Plants which are grown in selenium-rich soils, like locoweed, are toxic and can cause effects on animals if they are feeded on it.

The word "Selenium" has been derived from a Greek word "selene," which mean "Moon." It was discovered by Jons Jakob Berzelius in the year 1817. He founded the element associated with tellurium.

Physical Properties of Selenium

Phase Solid
Density (gray) 4.81 g/cm³
(alpha) 4.39 g/cm³
(vitreous) 4.28 g/cm³
Liquid density 3.99 g/cm³
Melting point 494 K
(221 °C, 430 °F)
Boiling point 958 K
(685 °C, 1265 °F)
Heat of fusion (gray) 6.69 kJ/mol
Heat of vaporization 95.48 kJ/mol
Heat capacity (25 °C) 25.363 J/(mol-K)

Atomic Properties of Selenium

Crystal structure Hexagonal
Oxidation states ±2, 4, 6
(strongly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies 1st: 941.0 kJ/mol
2nd: 2045 kJ/mol
3rd: 2973.7 kJ/mol
Atomic radius 115 pm
Atomic radius (calc.) 103 pm
Covalent radius 116 pm
Van der Waals radius 190 pm

Uses of Selenium

Occurrences of Selenium

Selenium occurs as selenide in many sulfide ores, like copper, silver, or lead. The muds, a byproduct, in the processing of these ores from the anode mud of copper refineries and the lead chambers of sulfuric acid plants, are further processed by a number of means to obtain free selenium.

Selenium and Health

Though selenium deficiency is relatively rare in healthy people, it may occur in patients with several compromised intestinal function, or then those undergoing a total parenteral nutrition. People who are dependent on food being sourced from selenium-deficient soil are at risk. Only 55 micrograms of selenium is recommended per day for adults in diet. More than 400 micrograms of it per day may lead to toxicity.

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