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Selenium (Se)

Selenium is a chemical element of the periodic table with chemical symbol Se and atomic number 34 with an atomic weight of 78.9718 u and is classed as nonmetal and is part of group 16 (oxygen group). Selenium is solid at room temperature.

Selenium in the periodic table

Atomic number34
Group16 (Oxygen group)
AppearanceBlack, red, and gray (not pictured) allotropes
Color Gray
Number of protons34 p+
Number of neutrons45 n0
Number of electrons34 e-
From Wikipedia, the free encyclopediaSelenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.

Physical properties

Phase at STPSolid
Density4.81 g/cm3
Atomic weight78.9718 u

Thermal properties

Melting point494 K
220.85 °C
429.53 °F
Boiling point958 K
684.85 °C
1264.73 °F
Heat of vaporization26.32 kJ/mol

Atomic properties

Electronegativity (Pauling Scale)2.55
Electron affinity194.959 kJ/mol
Oxidation states−2, −1, +1, +2, +3, +4, +5, +6
(a strongly acidic oxide)
Ionization energies
  1. 941 kJ/mol
  2. 2045 kJ/mol
  3. 2973.7 kJ/mol
  4. 4144 kJ/mol
  5. 6590 kJ/mol
  6. 7880 kJ/mol
  7. 14990 kJ/mol

Electron configuration for selenium

Electron configuration
Shorthand configuration
[Ar] 3d10 4s2 4p4
Electron configuration
Full configuration
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
Electron configuration chart
Electrons per shell2, 8, 18, 6
Valence electrons 6
Valency electrons 2,4,6
Bohr model
SeleniumElectron shell for Selenium, created by Injosoft ABSe
Figure: Shell diagram of Selenium (Se) atom.
Orbital Diagram

The history of Selenium

Discovery and first isolationJöns Jakob Berzelius, Johan Gottlieb Gahn (1817)
Discovery of selenium
Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn. Both chemists owned a chemistry plant near Gripsholm, Sweden, producing sulfuric acid by the lead chamber process. The pyrite from the Falun Mine created a red precipitate in the lead chambers which was presumed to be an arsenic compound, so the pyrite's use to make acid was discontinued. Berzelius and Gahn wanted to use the pyrite and they also observed that the red precipitate gave off a smell like horseradish when burned. This smell was not typical of arsenic, but a similar odor was known from tellurium compounds. Hence, Berzelius's first letter to Alexander Marcet stated that this was a tellurium compound. However, the lack of tellurium compounds in the Falun Mine minerals eventually led Berzelius to reanalyze the red precipitate, and in 1818 he wrote a second letter to Marcet describing a newly found element similar to sulfur and tellurium. Because of its similarity to tellurium, named for the Earth, Berzelius named the new element after the Moon.


List of unique identifiers for Selenium in various chemical registry databases
CAS Number7782-49-2
ChemSpider ID4885617
EC number231-957-4
PubChem CID Number6326970