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Selenium,  34Se
Selenium - black.jpg
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)
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
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
Naming After Selene, Greek goddess of the moon
Discovery and first isolation Jöns Jakob Berzelius and Johann Gottlieb Gahn (1817)
· references

Selenium is a chemical element with symbol Se and atomic number 34. It is widely used in the semiconductor industry.



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

Selenium dissolves in concentrated nitric acid with evolution of oxides of nitrogen:[1][2]

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.[3]

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.[4] Polyselenides form less readily than polysulfides. When strongly diluted, polyselenides hydrolyze into red selenium.[5]


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.



  3. Holleman-Wiberg, Lehrbuch der anorganischen Chemie, 102nd ed., page 622f.
  5. Holleman-Wiberg, Lehrbuch der anorganischen Chemie, 102nd ed., page 624

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