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By 1970, selenium in rectifiers had largely been replaced by silicon,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
- For
electronics industry.
- As
a nutritional supplement.
- In
glass industry.
- As
component of pigments in plastics, paints, enamels, inks, and
rubber.
- In
the preparation of pharmaceuticals.
- As
a nutritional feed additive for poultry and livestock.
- In
pesticide formulations.
- As
an ingredient in anti-dandruff shampoos.
- As
a constituent of fungicides.
- For
toning of photographs.
- Radioactive
selenium is used in diagnostic medicine.
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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