Selenium powder - black
|Name, symbol||Selenium, Se|
|Allotropes||Black, red, gray|
|Appearance||Black, red, gray solid|
|Selenium in the periodic table|
|Standard atomic weight (Ar)||78.971(8)|
|Group, block||, p-block|
|Electron configuration||[Ar] 3d10 4s2 4p4|
|Black, gray, red|
|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)|
|Oxidation states||6, 5, 4, 3, 2, 1, −1, −2 (a strongly acidic oxide)|
|Electronegativity||Pauling scale: 2.55|
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|
|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)|
|Young's modulus||10 GPa|
|Shear modulus||3.7 GPa|
|Bulk modulus||8.3 GPa|
|Brinell hardness||736 MPa|
|CAS Registry Number||7782-49-2|
|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.
- Se + O2 → SeO2
Unlike sulfur dioxide, selenium dioxide is a solid.
- Se + 4 H2O + 3 Cl2 → H2SeO4 + 6 HCl
- 4 HNO3 + Se → H2SeO3 + 4 NO2 + H2O
When selenous acid is reduced with sulfur dioxide or metabisulfite red selenium is formed.
In concentrated sulfuric acid selenium dissolves to give a deep green solution.
- Se8 + 3 H2SO4 → Se82+ + 2 HSO4- + SO2 + 2 H2O
Diluting this solution in water forms red selenium.
When selenium is dissolved in hot concentrated sodium or potassium hydroxide, deep red polyselenides form. Polyselenides form less readily than polysulfides. When strongly diluted, polyselenides hydrolyze into red selenium.
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.
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.
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.
- Selenium halides
- Made cadmium selenide
- Make red glass
- Make red selenium
- Make the green Se82+
- Make polyselenides
- Make selenites and selenates
Selenium and most of its compounds tend to be toxic if ingested in high quantities.
In closed containers.
As selenium is not cheap and not easy to come by, it's best to try to recycle it.
- Holleman-Wiberg, Lehrbuch der anorganischen Chemie, 102nd ed., page 622f.
- Holleman-Wiberg, Lehrbuch der anorganischen Chemie, 102nd ed., page 624