Selenium is a chemical element with the symbol Se and atomic number 34. It is a metalloid (more rarely considered a nonmetal) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic. It seldom occurs in its elemental state or as pure ore compounds in Earth's crust. Selenium (from Ancient Greek σελήνη (selḗnē) 'moon') was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth).

Selenium is found in metal sulfide ores, where it partially replaces the sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores, most often during production. Minerals that are pure selenide or selenate compounds are known but rare. The chief commercial uses for selenium today are glassmaking and pigments. Selenium is a semiconductor and is used in photocells. Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent quantum dot.

Although trace amounts of selenium are necessary for cellular function in many animals, including humans, both elemental selenium and (especially) selenium salts are toxic in even small doses, causing selenosis. Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in infant formula, and is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) as well as in three deiodinase enzymes. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any.

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Atomic properties

Standard atomic weight78.971 ±0.008
Atomic mass78.9718 u

Atomic radii

Radius (empirical)120 pm
Radius (calculated)103 pm
Covalent radius120 ±4 pm
Van der Waals radius190 pm

Atomic shell

Electron configurationAr 3d10 4s2 4p4
Ionization energy(1st) 9.752392 eV
(2nd) 21.196 eV
(3rd) 31.697 eV
(4th) 42.947 eV
(5th) 68.3 eV
(6th) 81.83 eV
Shell model

Physical properties

Densityblack: 4.28 g·cm−3 (333.1 K)
gray: 4.819 g·cm−1 (298.1 K)
red: 4.48 g·cm−1 (298.1 K)
Molar volume1.642·10-5 m3·mol−1
Speed of sound3,350 m·s−1 (293.1 K)


Melting point494 K
Boiling point958 K
Liquid range464 K
Critical point1,766 K @ 27.2 MPa
Transition temperature7 K


Melting enthalpy5.4 kJ·mol-1
Enthalpy of vaporization26 kJ·mol-1
Binding energy227 kJ·mol-1

Heat and conductivity

Thermal conductivity0.52 W·m-1·K-1
Work function5.9 eV


Mohs hardness2
Brinell hardness736 NM·m-2

Elastic properties

Young’s modulus10 GPa
Shear modulus3.7 GPa
Bulk modulus8.3 GPa
Poisson’s ratio0.33


Magnetic susceptibility-2.501·10-5 cm3·mol−1 (298 K)

Optical properties

Refractive index1

Chemical properties

Basicitystrongly acidic
Oxidation state±2, 4, 6
Standard potential-0.67 V (Se + 2e- → Se2-)


Pauling scale2.55
Sanderson scale3.01
Allred-Rochow scale2.48
Mulliken scale2.6
Allen scale2.424
Ghosh-Gupta scale5.874 eV
Boyd-Edgecombe scale2.46
Nagle scale2.31
Pearson absolute negativity5.89 eV

Other properties

Natural occurrenceprimordial
Crystal structureHexagonal
Goldschmidt Classificationchalcophile
Superconductorwith transition tempperature under special conditions
Price/kg21.4 USD

Natural abundances

30 ppb ≈ 3.01·1011 M☉
13,000 ppb ≈ 13 g
Earth’s crust
50 ppb ≈ 1,380 Mt
0.45 ppb ≈ 616 kt
Flowing water
0.2 ppb ≈ 3.2 kt
Human body
50 ppb ≈ 3.5 mg