Selenium is a chemical element which is placed in the 16th group and 4th period in the periodic table. Symbol of the selenium element is Se. It is a nonmetal element with the atomic number 34. It shows similar properties as sulfur, tellurium, and arsenic. Sulfur is placed above selenium and tellurium is placed below selenium in the 16th group of the periodic table. Arsenic is found at the left of selenium in the 4th period and 15th group. As it is found in the 16th group so, it is a p – block element. Due to its various properties sometimes, it is considered as a metalloid as well.
Selenium was discovered and isolated by Swedish Chemists Jons Jacob Berzelius and Johann Gottlieb Gahn in 1817. The word selenium is derived from the Greek word ‘selene’ which means moon. It is rarely found as pure ore or in pure elemental form. It is generally found in metal sulfide ores. For commercial use selenium is mostly produced as a byproduct of refining of metal sulfide ores.
Selenium is found as red, grey and black allotropes of it. Its 7 isotopes are available in nature. Apart from these it has many synthetic isotopes as well, although all of its synthetic isotopes are unstable in nature. Out of seven naturally occurring isotopes, 5 are stable and two are unstable.
Atomic number of selenium is 34. Its electronic configuration is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4 or it can be written as [Ar] 3d10 4s2 4p4. It has 2 electrons in K – shell, 8 electrons in L – shell, 18 electrons in M – shell and 6 electrons in its outermost shell N.
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It is found as solid at 20 ℃ temperature.
Relative atomic weight of selenium is 78.96.
Its melting point is 221 ℃.
Boiling point of selenium is 685 ℃.
It shows hexagonal crystal structure.
Its density is 4.809 g cm-3.
It forms a strongly acidic oxide.
According to Pauling scale, its electronegativity is 2.55
Its crystal structure is hexagonal.
It has many isotopes. Its 5 isotopes are stable which are 74Se, 76Se, 77Se, 78Se, 80Se. although its key isotope is 80Se.
It forms several isotopes.
Optical Properties – Selenium conducts some amount of electricity. Thus, it is a semiconductor. Selenium is also a photoconductor. It can change light energy into electrical energy. It has been found in research that its conductivity increases in presence of light which means its photoconductivity increases as the intensity of light increases.
Selenium is used in various fields mainly due to its properties such as photoconductivity, semiconductor of electricity etc. Few of its uses are listed below –
It is widely used in glassmaking and pigmentation. These are the main commercial uses of selenium in today’s world.
It is used in photocells as it is a photoconductor.
In the past selenium used to dominate the electronics market as it is a semiconductor but presently silicon has almost replaced selenium.
It is used in fluorescent quantum dot and surge protectors.
Small amount of selenium salts is necessary for proper functioning of cells in many organisms.
It is an ingredient of infant formula.
It is a component of many enzymes such as thioredoxin reductase, deiodinase enzymes.
Selenium dioxide is used in manganese electrolysis.
It is used to make alloys. Such as in brasses selenium is used in place of lead which is more toxic than selenium. It is used in copper alloys as well.
It is used in Li – Se batteries.
It is used in thin – film solar cells of copper indium gallium selenide.
It is used in flat panel x – ray detectors.
Organoselenium compounds are used in vulcanization of rubber.
It is used in photocopying machines.
Zinc selenide is used in blue LEDs.
Selenium-75 is used in industrial radiology as a gamma source.
Now a days, the selenium compound (selenium sulfide) is widely used for the treatment of dandruff. It works as an antifungal agent.
Selenium is an essential trace element, but its high concentrations are harmful for the environment. Selenium is a toxic element and its high concentrations are not only harmful for the environment but for human health as well. It affects our ecosystem as well. High concentration of selenium in fishes may cause swelling of gills and blood flow within the gills. Selenium may cause cataracts, damage to liver tissues, respiratory problems etc.
Overexposure of selenium may cause bronchitis, asthma, vomiting, lungs dysfunction, diarrhea, abdominal pain, shortness of breath and in severe cases death.
Generally required or necessary amounts of selenium is consumed by animals and humans through food, but more amount of selenium is consumed by them through its accumulation in the environment. Selenium enters the environment as selenium dioxide through combustion of coal and oils.
High concentrations of selenium may cause reproductive failures, defects in infants. Although its deficiency may also cause many adverse effects on health. Selenium deficiency may cause Kashin – beck disease, stress, low functioning of brain, white muscle disease in ruminants, dysfunction of thyroid gland etc. Although after research it has been said that all these effects of selenium deficiency are generally accompanied with deficiency of other vitamins or other diseases.
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