Skip to Main Content

About Selenium

Selenium Bohr

In 1817 the chemists Jons Jakob Berzelius and Johan Gottlieb Gahn decided to investigate a strange red precipitate found by workers at a sulfuric acid plant that they owned together. Initially, the material was believed to be a form of arsenic, which had led workers to discontinue use of the pyrite that had produced it, but the chemists noted that it did not have the expected properties for an arsenic compound. Tellurium was a closer fit, as the powder gave off a smell when burned that was known to be associated with that element, but further investigation showed that the element could not be tellurium either, and the chemists eventually recognized that they had discovered a previously unknown element. Tellurium’s name means “earth,” and Berzelius decided to name the new element for Selene, the moon, because of the similarities between the two.

Selenium is a semiconductor that can take a variety of crystalline structures depending on the conditions under which it is formed. The brick red powder found by Berzelius and Gohan was the form encountered most often as a result of chemical reactions, and when rapidly melted it produces a black vitreous solid often sold industrially, but the most stable form is a dense grey solid. In 1873, Willoughby Smith showed that the electric resistance of grey selenium varies predictably depending on incident light. This property is known as photoconductivity, and tellurium is now known to exhibit it as well, though to a lesser degree. The earliest major uses of selenium were in semiconductor devices such as rectifiers in radio and television tubes, which served to replace the previously used vacuum tubes and which preceded the silicon-based components used today.

Despite the rise of silicon as the major industrial semiconductor, selenium remains relevant to semiconductor technologies. While most selenium rectifiers have been replaced by other technologies, selenium remains in use for surge protection devices in some high-energy DC circuits. As a component of the compound semiconductors copper indium gallium selenium (CIGS), indium selenide, gallium selenide, cadmium selenide, and zinc selenide, selenium is essential for production of many thin-film solar cells, and is additionally found in electro-optical devices such as LEDs, lasers, and photoresistors. Notably, recently researchers have shown great interest in the possibilities of using cadmium selenide nanocrystals known as quantum dots in novel solar cells, more efficient LEDs, and in biomedical imaging applications.

Pure amorphous selenium was once a component of every photocopier, where its role as a photoconductor allowed for the production of images based on the areas of light shining through a printed document. Today organic photoconductors have largely replaced selenium in this role, but selenium can also produce images based on exposure to x-rays--images that may be transferred to paper, as in a photocopier, or read directly from charge patterns on the selenium into a computer via a thin film transistor array. This type of x-ray technology never rose to popularity during the age of x-ray films, but has seen renewed interest as digital x-ray imaging systems become the norm. Therefore, amorphous selenium is now is found in many flat-panel digital x-ray machines for medical and dental imaging.

Selenium serves several key functions outside its role as a photoconductive semiconductor. As a component of cadmium sulfoselenide pigments, it can impart a brilliant ruby red hue to materials which incorporate it. In glassmaking, selenium salts are added in small amounts because the red tone cancels out the green tinge lent by iron impurities. Selenium is a component of metal alloys, where it improves machinability of the final material and often replaces the more toxic metal lead.

Though excessive quantities of selenium can be toxic, the element is also an important micronutrient, as it is a necessary cofactor for several enzymes. Because of this key biological role, selenium is sometimes included in nutritional supplements. Notably, the major mechanism of mercury poisoning is the permanent inactivation of these essential enzymes caused by a reaction between mercury and selenium. Therefore the effects of some types of mercury exposure can be partially mitigated by sufficient selenium intake.

Selenium is produced primarily as a byproduct of copper refining, but is also recycled from scrap.

+ Open All
- Close All
Semiconductor & Optical
Sputtering Targets

High Purity (99.999%) Selenium (Se) Sputtering Target Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. Below its melting point, selenium is a p-type semiconductor and has many uses in electronics applications. Selenium is available as metal and compounds with purities from 99% to 99.999% (ACS grade to ultra-high purity). Elemental or metallic forms include pellets, rod, wire and granules for evaporation source material purposes. Selenium oxide is available in powder and dense pellet form for such uses as optical coating and thin film applications. Oxides tend to be insoluble. Selenium fluoride is another insoluble form for uses in which oxygen is undesirable such as metallurgy, chemical and physical vapor deposition and in some optical coatings. Selenium is also available in soluble forms including chlorides. These compounds can be manufactured as solutions at specified stoichiometries.

Selenium Properties

Selenide(Se) atomic and molecular weight, atomic number and elemental symbolSelenium is a Block P, Group 16, Period 4 element.The number of electrons in each of Selenium's shells is 2, 8, 18, 6 and its electron configuration is [Ar] 3d10 4s2 4p4. Selenium Bohr ModelThe selenium atom has a radius of and its Van der Waals radius is In its elemental form selenium's CAS number is 7782-49-2. Elemental SeleniumSelenium was discovered and first isolated by Jöns Jakob Berzelius and Johann Gottlieb Gahn in 1817. It is produced from selenide which is found in many sulfide ores. The origin of the name Selenium comes from the Greek word "Selênê" meaning moon.

Symbol: Se
Atomic Number: 34
Atomic Weight: 78.96
Element Category: nonmetal
Group, Period, Block: 16 (chalcogens), 4, p
Color: gray or red (crystalline), black or red (amorphous)
Other Names: Selen, Selenio
Melting Point: 220.8°C, 429.44°F, 493.95 K
Boiling Point: 685°C, 1265°F, 958.15 K
Density: 4808 kg·m3
Liquid Density @ Melting Point: 3.99 g·cm3
Density @ 20°C: 4.79 g/cm3
Density of Solid: 4819 kg·m3
Specific Heat: 0.767 Cal/g/K @ 25 °C
Superconductivity Temperature: N/A
Triple Point: N/A
Critical Point: 1766 K, 27.2 MPa
Heat of Fusion (kJ·mol-1): 5.1
Heat of Vaporization (kJ·mol-1): 90
Heat of Atomization (kJ·mol-1): 226.4
Thermal Conductivity: 0.00519 W/cm/K @ 298.2 K
Thermal Expansion: (25 °C) (amorphous) 37 µm·m-1·K-1
Electrical Resistivity: 106 nΩ-cm @ 0°C
Tensile Strength: N/A
Molar Heat Capacity: 25.363 J·mol-1·K-1
Young's Modulus: 10 GPa
Shear Modulus: 3.7 GPa
Bulk Modulus: 8.3 GPa
Poisson Ratio: 0.33
Mohs Hardness: 2
Vickers Hardness: N/A
Brinell Hardness: 736 MPa
Speed of Sound: (20 °C) 3350 m·s-1
Pauling Electronegativity: 2.55
Sanderson Electronegativity: 3.01
Allred Rochow Electronegativity: 2.48
Mulliken-Jaffe Electronegativity: 2.60 (16.7% s orbital)
Allen Electronegativity: 2.424
Pauling Electropositivity: 1.45
Reflectivity (%): N/A
Refractive Index: 1.000895
Electrons: 34
Protons: 34
Neutrons: 45
Electron Configuration: [Ar] 3d10 4s2 4p4
Atomic Radius: 120 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius: 120±4 pm
Covalent Radius (Å): 1.18
Van der Waals Radius: 190 pm
Oxidation States: 6, 4, -2
Phase: Solid
Crystal Structure: hexagonal
Magnetic Ordering: diamagnetic
Electron Affinity (kJ·mol-1) 194.997
1st Ionization Energy: 940.97 kJ·mol-1
2nd Ionization Energy: 2044.54 kJ·mol-1
3rd Ionization Energy: 2973.74 kJ·mol-1
CAS Number: 7782-49-2
EC Number: 231-957-4
MDL Number: MFCD00134090
Beilstein Number: N/A
SMILES Identifier: [Se]
InChI Identifier: InChI=1S/Se
PubChem CID: 6326970
ChemSpider ID: 4885617
Earth - Total: 9.6 ppm
Mercury - Total:  0.79 ppm
Venus - Total: 5.4 ppm
Earth - Seawater (Oceans), ppb by weight: 0.45
Earth - Seawater (Oceans), ppb by atoms: 0.035
Earth -  Crust (Crustal Rocks), ppb by weight: 50
Earth -  Crust (Crustal Rocks), ppb by atoms: 10
Sun - Total, ppb by weight: N/A
Sun - Total, ppb by atoms: N/A
Stream, ppb by weight: 0.2
Stream, ppb by atoms: 0.003
Meterorite (Carbonaceous), ppb by weight: 13000
Meterorite (Carbonaceous), ppb by atoms: 2900
Typical Human Body, ppb by weight: N/A
Typical Human Body, ppb by atom: N/A
Universe, ppb by weight: 0.1
Universe, ppb by atom: 0.001
Discovered By: Jöns Jakob Berzelius and Johann Gottlieb Gahn
Discovery Date: 1817
First Isolation: Jöns Jakob Berzelius and Johann Gottlieb Gahn (1817)

Health, Safety & Transportation Information for Selenium

The EPA does not classify selenium as carcinogenic, although selenium sulfide is a probable carcinogen. Selenates and selenites which are compounds of selenium, are highly toxic. Hydrogen selenide gas (SeH2) is the most acutely toxic compound of selenium. Safety data for Selenium and its compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific material or compound referenced in the Products tab. The below information applies to elemental Selenium.

Safety Data
Material Safety Data Sheet MSDS
Signal Word Danger
Hazard Statements H301-H331-H373-H413
Hazard Codes T
Risk Codes 23/25-33-53
Safety Precautions 20/21-28-45-61
RTECS Number VS7700000
Transport Information N/A
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)
Skull and Crossbones-Acute Toxicity  Health Hazard

Selenium Isotopes

Selenium (Se) has six naturally occurring isotopes. Five of these are stable: 74Se, 76Se, 77Se, 78Se, and 80Se.

Nuclide Isotopic Mass Half-Life Mode of Decay Nuclear Spin Magnetic Moment Binding Energy (MeV) Natural Abundance
(% by atom)
65Se 64.96466(64)# <50 ms ß+ to 65As; ß+ + p to 64Ge 3/2-# N/A 520.5 -
66Se 65.95521(32)# 33(12) ms ß+ to 66As 0+ N/A 536.97 -
67Se 66.95009(21)# 133(11) ms ß+ to 67As; ß+ + p to 66Ge 5/2-# N/A 549.7 -
68Se 67.94180(4) 35.5(7) s ß+ to 68Br 0+ N/A 566.17 -
69Se 68.93956(4) 27.4(2) s ß+ to 69As; ß+ + p to 68Ge (1/2-) N/A 576.11 -
70Se 69.93339(7) 41.1(3) min EC to 70As 0+ N/A 589.78 -
71Se 70.93224(3) 4.74(5) min EC to 71As 5/2- N/A 598.79 -
72Se 71.927112(13) 8.40(8) d EC to 72As 0+ N/A 611.53 -
73Se 72.926765(11) 7.15(8) h EC to 73As 9/2+ N/A 620.54 -
74Se 73.9224764(18) Observationally Stable - 0+ N/A 632.34 0.89
75Se 74.9225234(18) 119.779(4) d EC to 75As 5/2+ 0.67 640.42 -
76Se 75.9192136(18) STABLE - 0+ N/A 651.29 9.37
77Se 76.9199140(18) STABLE - 1/2- 0.53506 659.37 7.63
78Se 77.9173091(18) STABLE - 0+ N/A 669.31 23.77
79Se 78.9184991(18) 2.95(38)E+5 y ß- to 79Br 7/2+ -1.018 676.46 -
80Se 79.9165213(21) Observationally Stable - 0+ N/A 686.4 49.61
81Se 80.9179925(22) 18.45(12) min ß- to 81Br 1/2- N/A 693.55 -
82Se 81.9166994(22) 97(5)E+18 y - to 82Kr 0+ N/A 702.56 8.73
83Se 82.919118(4) 22.3(3) min ß- to 83Br 9/2+ N/A 707.84 -
84Se 83.918462(16) 3.1(1) min ß- to 84Br 0+ N/A 716.85 -
85Se 84.92225(3) 31.7(9) s ß- to 85Br (5/2+)# N/A 721.21 -
86Se 85.924272(17) 15.3(9) s ß- to 86Br 0+ N/A 727.42 -
87Se 86.92852(4) 5.50(12) s ß- to 87Br; ß- + n to 86Br (5/2+)# N/A 731.77 -
88Se 87.93142(5) 1.53(6) s ß- to 88Br; ß- + n to 87Br 0+ N/A 737.06 -
89Se 88.93645(32)# 0.41(4) s ß- to 89Br; ß- + n to 88Br (5/2+)# N/A 740.48 -
90Se 89.93996(43)# 300# ms [>300 ns] ß- + n to 91Br; ß- to 92Br 0+ N/A 745.76 -
91Se 90.94596(54)# 270(50) ms ß- to 91Br; ß- + n to 90Br 1/2+# N/A 748.25 -
92Se 91.94992(64)# 100# ms [>300 ns] ß- to 92Br 0+ N/A 752.6 -
93Se 92.95629(86)# 50# ms [>300 ns] Unknown 1/2+# N/A 754.16 -
94Se 93.96049(86)# 20# ms [>300 ns] Unknown 0+ N/A 758.51 -