Difference between revisions of "Selenium"
From Sciencemadness Wiki
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===Physical=== | ===Physical=== | ||
Selenium is a solid nonmetal at standard conditions. It has several allotrope forms: | Selenium is a solid nonmetal at standard conditions. It has several allotrope forms: | ||
| − | *'''Amorphous''': Has a brick-red color, as is the most common form of selenium obtained from chemical reactions. | + | *'''Amorphous''' or '''red''': Has a brick-red color, as is the most common form of selenium obtained from chemical reactions. |
*'''Black''': Also known as '''vitreous''' selenium, this form is a dull grey solid, obtained by rapidly melting its amorphous form. The structure of black selenium is irregular and complex, consisting of polymeric rings. It is slightly soluble in [[carbon disulfide]]. | *'''Black''': Also known as '''vitreous''' selenium, this form is a dull grey solid, obtained by rapidly melting its amorphous form. The structure of black selenium is irregular and complex, consisting of polymeric rings. It is slightly soluble in [[carbon disulfide]]. | ||
*'''Gray''': Obtained by heating black selenium to 180°C. This is the most dense and stable form of selenium. It is not soluble in carbon disulfide or other organic solvents. Unlike the black variety, its structure is hexagonal crystal lattice consisting of helical polymeric chains. Gray selenium is a semiconductor, that also shows appreciable photoconductivity. | *'''Gray''': Obtained by heating black selenium to 180°C. This is the most dense and stable form of selenium. It is not soluble in carbon disulfide or other organic solvents. Unlike the black variety, its structure is hexagonal crystal lattice consisting of helical polymeric chains. Gray selenium is a semiconductor, that also shows appreciable photoconductivity. | ||
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==Availability== | ==Availability== | ||
Selenium is sold by chemical suppliers. It can also be purchased from Ebay and Metallium. | Selenium is sold by chemical suppliers. It can also be purchased from Ebay and Metallium. | ||
| + | |||
| + | Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals. | ||
==Preparation== | ==Preparation== | ||
Elemental selenium can be made by reducing selenous acid with sulfur dioxide. | Elemental selenium can be made by reducing selenous acid with sulfur dioxide. | ||
| + | |||
| + | The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS<sub>2</sub>). | ||
A good tutorial to make selenium allotropes can be found [http://woelen.homescience.net/science/chem/exps/selenium/index.html here]. | A good tutorial to make selenium allotropes can be found [http://woelen.homescience.net/science/chem/exps/selenium/index.html here]. | ||
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[[Category:Chalcogens]] | [[Category:Chalcogens]] | ||
[[Category:P-block]] | [[Category:P-block]] | ||
| + | [[Category:Minerals]] | ||
Revision as of 17:30, 11 May 2020
 Selenium powder - black | |||||
| General properties | |||||
|---|---|---|---|---|---|
| Name, symbol | Selenium, Se | ||||
| Allotropes | Black, red, gray | ||||
| Appearance | Black, red, gray solid | ||||
| Selenium in the periodic table | |||||
| |||||
| Atomic number | 34 | ||||
| Standard atomic weight (Ar) | 78.971(8) | ||||
| Group, block | , p-block | ||||
| Period | period 4 | ||||
| Electron configuration | [Ar] 3d10 4s2 4p4 | ||||
| Physical properties | |||||
| Black, gray, red | |||||
| Phase | Solid | ||||
| Melting point | 494 K (221 °C, 430 °F) | ||||
| Boiling point | 958 K (685 °C, 1265 °F) | ||||
| Density near r.t. |
4.81 g/cm3 (gray) 4.39 g/cm3 (alpha) 4.28 g/cm3 (vitreous) | ||||
| when liquid, at | 3.99 g/cm3 | ||||
| Critical point | 1766 K, 27.2 MPa | ||||
| Heat of fusion | 6.69 kJ/mol (gray) | ||||
| Heat of | 95.48 kJ/mol | ||||
| Molar heat capacity | 25.363 J/(mol·K) | ||||
| pressure | |||||
| Atomic properties | |||||
| Oxidation states | 6, 5, 4, 3, 2, 1, −1, −2 (a strongly acidic oxide) | ||||
| Electronegativity | Pauling scale: 2.55 | ||||
| energies |
1st: 941.0 kJ/mol 2nd: 2045 kJ/mol 3rd: 2973.7 kJ/mol | ||||
| Atomic radius | empirical: 120 pm | ||||
| Covalent radius | 120±4 pm | ||||
| Van der Waals radius | 190 pm | ||||
| Miscellanea | |||||
| Crystal structure | Hexagonal | ||||
| Speed of sound thin rod | 3350 m/s (at 20 °C) | ||||
| Thermal expansion | 37 µm/(m·K) (amorphous) | ||||
| Thermal conductivity | 0.519 W/(m·K) (amorphous) | ||||
| Magnetic ordering | Diamagnetic | ||||
| Young's modulus | 10 GPa | ||||
| Shear modulus | 3.7 GPa | ||||
| Bulk modulus | 8.3 GPa | ||||
| Poisson ratio | 0.33 | ||||
| Mohs hardness | 2.0 | ||||
| Brinell hardness | 736 MPa | ||||
| CAS Registry Number | 7782-49-2 | ||||
| History | |||||
| Naming | After Selene, Greek goddess of the moon | ||||
| Discovery and first isolation | Jöns Jakob Berzelius and Johann Gottlieb Gahn (1817) | ||||
Selenium is a chemical element with symbol Se and atomic number 34. It is widely used in the semiconductor industry.
Contents
Properties
Chemical
Like sulfur, selenium will burn in the presence of oxygen to yield selenium dioxide.
- Se + O2 → SeO2
Unlike sulfur dioxide, selenium dioxide is a solid.
Selenium will be oxidized by chlorine in water to give selenic acid and hydrochloric acid.
- Se + 4 H2O + 3 Cl2 → H2SeO4 + 6 HCl
Physical
Selenium is a solid nonmetal at standard conditions. It has several allotrope forms:
- Amorphous or red: Has a brick-red color, as is the most common form of selenium obtained from chemical reactions.
- Black: Also known as vitreous selenium, this form is a dull grey solid, obtained by rapidly melting its amorphous form. The structure of black selenium is irregular and complex, consisting of polymeric rings. It is slightly soluble in carbon disulfide.
- Gray: Obtained by heating black selenium to 180°C. This is the most dense and stable form of selenium. It is not soluble in carbon disulfide or other organic solvents. Unlike the black variety, its structure is hexagonal crystal lattice consisting of helical polymeric chains. Gray selenium is a semiconductor, that also shows appreciable photoconductivity.
Availability
Selenium is sold by chemical suppliers. It can also be purchased from Ebay and Metallium.
Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.
Preparation
Elemental selenium can be made by reducing selenous acid with sulfur dioxide.
The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS2).
A good tutorial to make selenium allotropes can be found here.
Projects
- Selenium halides
- Made cadmium selenide
- Make red glass
Handling
Safety
Selenium and most of its compounds tend to be toxic if ingested in high quantities.
Storage
In closed containers.
Disposal
As selenium is not cheap and not easy to come by, it's best to try to recycle it.
