Essential Properties and Practical Applications of Selenium
Selenium is a type of chemical element having atomic number 34 in the Periodic Table of the Elements. Selenium is in the fourth period of the sixth group of the main (A) subgroup of the Periodic Table. Selenium is one of the important elements for some species, including humans. Human bodies contain about 14 milligrams of cells, and every human body cell consists of more than a million selenium atoms. Having less amount of selenium can cause health problems to a human body, but too much is also very dangerous. Excess amounts of Selenium will disturb the development of an embryo or foetus.
Selenium is a chemical element that is placed in the 16th group and 4th period in the periodic table. The symbol of the selenium element is Se. It is a nonmetal element with the atomic number 34. It shows similar properties as sulphur, tellurium, and arsenic. Sulphur 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 Jöns 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 sulphide ores. For commercial use selenium is mostly produced as a byproduct of the 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.
Electronic Configuration of Selenium
Selenium atoms have a positively charged nucleus, inside of this nucleus there are 34 protons and 45 neutrons, and those 34 electrons move in three orbits. Selenium electron configuration can be defined as :
Se - 122s22p63s23p63d104s24p4
Notation of electronic configuration of Selenium is defined as :
Se - [Ar]3d104s24p4
Atoms of Selenium and Br+1 , Kr+2 have the same electronic configuration.
The shell filling order of a selenium atom with the electrons are :
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p.
Here, on the s sublevel, up to 2 electrons can be present, on the s level,up to 6 electrons, on the d level, up to 10 electrons and on the f level, up to 14 electrons.
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
Properties of Selenium
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Properties of Selenium includes :
It is a type of non-metal element having atomic number 34.
Atomic mass of Selenium is 78.96.
Selenium can easily be mixed up with different metals and nonmetals directly or with the help of aqueous solution.
Melting point of Selenium is 494K.
Boiling point of Selenium is 958K.
Density of Selenium is 4.81g/cm3.
Physical and Chemical Properties of Selenium
The physical and chemical properties of selenium are listed below:
It is found as solid at 20 ℃ temperature.
The relative atomic weight of selenium is 78.96.
Its melting point is 221 ℃.
The boiling point of selenium is 685 ℃.
It shows a hexagonal crystal structure.
Its density is 4.809 g cm-3.
It forms a strongly acidic oxide.
According to the 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.
Uses of Selenium
Selenium is used in various fields mainly due to its properties such as photoconductivity, semiconductor of electricity etc. A 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 dots and surges protectors.
A small amount of selenium salts are necessary for the 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 the 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.
Nowadays, the selenium compound (selenium sulphide) is widely used for the treatment of dandruff. It works as an antifungal agent.
Selenium element is used for making photocopiers and laser printers.
It is also used for improving the qualities of copper and steel that get tanned through the application of selenious acid.
Because of its semiconductor properties, selenium is also used in manufacturing of solar batteries and quantum dots.
Selenium is also used for the generation of Cadmium red, a dye for glass and plastics.
Disadvantages of Selenium
Selenium is an essential trace element, but its high concentrations are harmful to the environment. Selenium is a toxic element and its high concentrations are not only harmful to the environment but to human health as well. It affects our ecosystem as well. A 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 to selenium may cause bronchitis, asthma, vomiting, lungs dysfunction, diarrhoea, 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 the 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 the 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 by deficiency of other vitamins or other diseases.
Selenium is somewhat safe if its doses taken are less than 400 mcg daily, but selenium becomes unsafe if high doses are taken for a long time.
Selenium doses above 400 mcg daily will increase the risk of developing selenium toxicity.
Lower doses of selenium for a long time can increase the risk of developing diabetes.
Selenium is also responsible for causing stomach discomfort, headache, and rash.
High doses of selenium can even cause hair loss, fatigue, nausea, vomiting, and weight loss.
And the extremely high doses of selenium can lead to organ failure and death.
Selenium: Summary in Tabular form
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FAQs on Selenium in Chemistry: Key Facts for Students
1. What is selenium and where is it located in the periodic table?
Selenium (symbol Se) is a chemical element with atomic number 34. In the periodic table, it is located in Group 16 and Period 4, placing it among the chalcogens, along with oxygen, sulfur, tellurium, and polonium. Chemically, selenium is classified as a metalloid, meaning it exhibits properties that are intermediate between those of metals and nonmetals.
2. What are the main industrial applications of selenium?
Selenium has several important industrial uses due to its unique properties. The primary applications include:
Glass Manufacturing: It is used to decolorize glass by counteracting the green tint caused by iron impurities. In larger quantities, it imparts a deep red or bronze colour to glass and ceramics.
Electronics: Due to its photoconductive properties (conductivity increases with light exposure), it is crucial in photocopiers, photoelectric cells, light meters, and solar cells.
Pigments: Selenium compounds are used to create pigments for paints, plastics, and ceramics, producing a range of red colours.
Alloys: It is added to stainless steel to improve its machinability.
3. How does selenium's metalloid character influence its properties compared to other Group 16 elements like sulfur and tellurium?
Selenium's position as a metalloid places it between the nonmetal sulfur (above it) and the more metallic tellurium (below it). This results in intermediate properties. For instance, selenium is a semiconductor, unlike sulfur which is an insulator. Its electrical conductivity is much higher than sulfur's but lower than tellurium's. This transitional nature allows selenium to have unique photoelectric properties that are not as pronounced in sulfur or tellurium, making it ideal for specific electronic applications.
4. What are the different allotropic forms of selenium?
Selenium is known to exist in several allotropic forms, which are different structural modifications of the element. The most common forms are:
Grey (Trigonal) Selenium: This is the most stable form. It has a hexagonal crystal structure and exhibits metallic lustre. This allotrope is a notable photoconductor.
Red (Monoclinic) Selenium: This form is composed of cyclic Se₈ molecules, similar to the S₈ rings of sulfur. It is less stable and will slowly convert to the grey form upon heating.
Black (Amorphous) Selenium: This is a vitreous, or glassy, form that is irregular in structure. It is produced by rapidly cooling molten selenium.
5. Why does selenium commonly exhibit +4 and +6 oxidation states in its compounds?
Selenium's ability to show +4 and +6 oxidation states is determined by its valence electronic configuration, which is [Ar] 3d¹⁰ 4s² 4p⁴. When reacting with highly electronegative elements like oxygen or fluorine, selenium can lose electrons from its outermost shell. The +4 state arises from the loss of its four 4p electrons (e.g., in SeO₂). The +6 state, its maximum, occurs when it loses all six of its valence electrons (both 4s and 4p), as seen in compounds like SeF₆. The +2 state is less common and less stable compared to these higher oxidation states.
6. What is the biological importance of selenium in the human body?
Although toxic in large amounts, selenium is an essential trace mineral vital for human health. Its primary role is as a component of special proteins called selenoproteins. These proteins, such as glutathione peroxidases, function as powerful antioxidants that help protect cells from oxidative damage. Selenium is also crucial for proper thyroid hormone metabolism, DNA synthesis, and maintaining a healthy immune system.
7. How does the photoconductive property of selenium make it useful in devices like photocopiers?
The photoconductivity of grey selenium is the key to its use in older photocopiers. Here’s how it works: A cylindrical drum coated with selenium is given a uniform positive electrical charge in the dark. When an image of a document is projected onto the drum, the areas exposed to light (the white parts of the page) cause the selenium to become conductive, and the charge in these areas drains away. The dark areas (text and images) remain charged. A negatively charged powder (toner) is then dusted onto the drum and sticks only to the charged areas. This toner pattern is then transferred to a sheet of paper and fused with heat to create a copy.
8. What are some key chemical facts about the element selenium?
Here are some fundamental facts about selenium for students of chemistry:
Discovery: It was discovered in 1817 by the Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn.
Name Origin: Its name comes from the Greek word 'selene', meaning 'moon'. This was chosen because selenium was often found in nature alongside tellurium, which was named after 'tellus', the Latin word for 'Earth'.
Common Compounds: Selenium forms oxides like selenium dioxide (SeO₂) and selenates/selenites which are analogous to sulfates/sulfites.
Chemical Reactivity: It reacts with metals, nonmetals, and halogens, but does not react with non-oxidising acids.
