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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 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.
  • 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.


Elemental selenium can be made by reducing selenous acid with sulfur dioxide.

A good tutorial to make selenium allotropes can be found here.


  • Selenium halides
  • Made cadmium selenide
  • Make red glass



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.


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