Selenium is a non-metal and was discovered in 1817. It has been widely used in electronic devices. It is toxic in high amount but is an essential micronutrient for all living things.
History and Discovery
Selenium was discovered by John Gottleib Gahn and Jons Jacob Berzelius in 1817. He observed that the novel element greatly resembled in various properties to the previously discovered element, tellurium. The name selenium has been derived from a Greek word selene that means moon [1]. In early days selenium was widely used in making electronic devices and in 1930s it was used to make rectifiers and replaced the use of copper oxide in conventional rectifiers. Later it was identified as a hazardous element as industrial workers showed signs of selenium toxicity.
Selenium
| Periodic Table Classification | Group 16 Period 4 |
|---|---|
| State at 20C | Solid |
| Color | Black, red, and gray |
| Electron Configuration | [Ar] 3d10 4s2 4p4 |
| Electron Number | 34 |
| Proton Number | 34 |
| Electron Shell | 2, 8, 18, 6 |
| Density | 4.79 g.cm-3 at 20°C |
| Atomic number | 34 |
| Atomic Mass | 78.96 g.mol -1 |
| Electronegativity according to Pauling | 2.55 |
Occurrence
Selenium is not an abundant element. Selenium is not commonly found in its free or elemental form. it exists in the form of ores and minerals. The most common ore of selenium are the sulfide ores. In mineral form, selenium is present in the form of selenite or selenide compounds. Selenium is also found in amino acids in living organisms, where it exists in the form of seleno-methionine, and selenocysteine [2]. High amount of selenium is also present in oceans [3]. In the human body, selenium content ranges between 13-20 milligram [4].
Physical Characteristics
Selenium is a non-metal and is sometimes considered as a metalloid. There are various allotropic forms of selenium, that interconvert into each other with varying temperature. Selenium exists in an amorphous powder form in chemical reactions. And when subjected to rapid melting, selenium acquires a black glass like allotropic form. Black selenium is converted to grey selenium when temperature is increased to around 180°C. red selenium has here sub-forms, alpha, beta and gamma. Grey allotropic form of selenium is the most stable one and is resistant to oxidation when exposed to air. It does not dissolve in nonoxidizing acids.
Chemical Characteristics
There are four common oxidation states of selenium, -2, +2, +4 and +6. Selenium reacts with oxygen and forms selenium dioxide and selenium trioxide. Selenium reacts with sodium and silver to form salts, termed as selenites. Selenous acid reacts with hydrogen sulfide to form selenium disulfide. Concentrated selenic acid reacts with gold to form gold selenite at high temperature. Among the halogens, selenium only reacts with chloride to form a stable compound, selenium monochloride.
Significance and Uses
- Selenium has been used in electronic industry but has been greatly replaced by silicon.
- Selenium is widely used as a semiconductor. It is used to make photocells.
- Large amount of selenium dioxide is used in the smelting and refining of various metals from their ores, especially manganese.
- Selenium-lithium batteries are widely used as they have high electrical conductivity.
- Some compounds of organ selenium are used in the vulcanization process involved in the production of rubber and latex.
- Selenium is used as a toner for photographing purpose.
Health Hazards
Certain salts of selenium are toxic, but in large amounts. Long term and exposure to high dose of selenium lead to a condition termed as selenosis [5]. It is characterized by bad garlic like odor in breath, fatigue, neurological and gastrological disorders. Very high doses of selenium can be fatal. In trace quantities, however, selenium is an essential nutrient for the living organisms, including animals, plants and humans. Most multivitamins, and infant formula milk contain minute quantities of selenium. Selenium is a component of vital enzymes, including thioredoxin reductase and glutathione peroxidase that function to control oxidative stress in the living cells. Selenium also aids in the normal functioning of the thyroid gland. Certain plants, such as Stanleya sp. (prince’s plum) and those belonging to Astragalus species require high amounts of selenium to grow and thrive. These plants are considered as selenium indicators as their growth depicts high selenium content of the soil.
Isotopes of Selenium
There are seven isotopes in naturally occurring selenium, these include selenium-74, selenium-76, selenium-77, selenium-78, selenium-79, selenium-80 and selenium-82.
REFERENCES
[1]. Berzelius, J.J. (1818). “Lettre de M. Berzelius à M. Berthollet sur deux métaux nouveaux” [Letter from Mr. Berzelius to Mr. Berthollet on two new metals]. Annales de Chimie et de Physique. 2nd series (in French). 7: 199–206. From p. 203: “Cependant, pour rappeler les rapports de cette dernière avec le tellure, je l’ai nommée sélénium.” (However, in order to recall the relationships of this latter [substance (viz, selenium)] to tellurium, I have named it “selenium”.)
[2]. Birringer, Marc; Pilawa, Sandra; Flohé, Leopold (2002). “Trends in selenium biochemistry”. Natural Product Reports. 19 (6): 693–718. doi:10.1039/B205802M. PMID 12521265.
[3]. Amouroux, David; Liss, Peter S.; Tessier, Emmanuel; et al. (2001). “Role of oceans as biogenic sources of selenium”. Earth and Planetary Science Letters. 189 (3–4): 277–283. Bibcode:2001E&PSL.189..277A. doi:10.1016/S0012-821X(01)00370-3.
[4]. A common reference for this is Schroeder, H. A.; Frost, D. V.; Balassa, J. J. (1970). “Essential trace metals in man: Selenium”. Journal of Chronic Diseases. 23 (4): 227–43. doi:10.1016/0021-9681(70)90003-2. OSTI 6424964. PMID 4926392.
[5]. “Dietary Supplement Fact Sheet: Selenium”. National Institutes of Health; Office of Dietary Supplements. Retrieved 2009-01-05.