Selenium (Se)

Selenium is a chemical element with the atomic number 34 in the periodic table. It occurs naturally in Earth’s crust, especially in carbonate rocks and volcanic and sedimentary soils. Being a member of the sulphur group of periodic table elements, this chalcogen has six electrons in the outermost shell and valences of -2, 4, and 6. 

As one of the essential trace minerals, selenium participates in numerous vital processes in the human body. On the other hand, high accumulations of this substance in bodily tissues and organs may lead to adverse health effects. 

Chemical and Physical Properties of Selenium

Symbol of SeleniumSe
Atomic Number of Selenium34
Group of SeleniumNon-Metal
Crystal Structure of SeleniumHexagonal
Atomic Weight of Selenium78.96
Shells of Selenium2,8,18,6
Orbitals of Selenium[Ar] 3d10 4s2 4p4
Valence of Selenium-2,4,6
Electronegativity according to Pauling2.55
Covalent Radius of Selenium1.16 Å
Ionic Radius of Selenium.50 (+4) Å
Atomic Radius of Selenium1.22 Å
Atomic Volume of Selenium16.45 cm³/mol
Name Origin of SeleniumGreek: selênê (moon).
Discovered ByJöns Jacob Berzelius
Pronounced of Seleniumsi-LEE-ni-em
Oxydation States of Selenium-2,(4),6
Density of Selenium4.79 g/cm³
Uses of SeleniumLight causes it to conduct electricity more easily. It is used in photoelectric cells, TV cameras, xerography machines and as a semiconductor in solar batteries and rectifiers. Also colors glass red.
Description of SeleniumRedish-brown liquid.
Physical stateSolid (non-metal) substance at room temperature
Half-lifeFrom less than 50 milliseconds to 0.97(5)×1020 years
Most characteristic isotope80Se
The energy of the first ionization940.7 kJ.mol-1
The energy of the second ionization2045 kJ.mol-1
The energy of the third ionization2973.7 kJ.mol-1

With the periodic table symbol Se, atomic number 34, atomic mass of 78.96 g.mol-1, and electron configuration [Ar] 3d104s24p4, actinium is ductile and reaches its boiling point at 685°C, 1265°F, 958 K, while the melting point is achieved at 220.8°C, 429.4°F, 494 K. This member of the sulfur family of elements in the periodic table has an electronegativity of 2.4 according to Pauling, whereas the atomic radius according to van der Waals is 0.14 nm. 

Classified under the element sulfur in Group 16 of the periodic table, selenium resembles the chemical properties of this group of chemicals. A member of the chalcogen family of elements, this metalloid displays both a and a photoconductive action (electrical resistance decreases with increased illumination), and a photovoltaic action (converts light to electricity).    

This chemical occurs in a form of several different allotropes. The crystalline hexagonal selenium is the most stable form with a metallic gray appearance. Crystalline monoclinic selenium occurs in a deep red color, while the amorphous selenium has is red in the powder form, but occurs as black in its vitreous form.

How Was Selenium Discovered?

In 1817, the distinguished Swedish chemist Jacob Berzelius (1779 – 1848) discovered selenium as an impurity in sulphuric acid. While studying the technical issues related to the production of sulfuric acid and nitric acid (aqua fortis), this accomplished experimenter’s curiosity was directed toward a reddish sludge obtained from a Falun pyrite sample.

Together with Johan Gottlieb Gahn (1745 – 1818), Berzelius exposed over 200 kg of sulfur to obtain nearly 3 grams of its precipitate. The analysis of the result from this experiment pointed out a presence of tellurium in the substance. But, familiar with the fact that this chemical element has never been traced in the minerals mined near Falun (a city in Sweden), Berzelius continued to experiment with the sludge. 

In the new round of chemical analysis, Jacob Berzelius observed similarities of the substance in front of him with sulphur, but some metallic properties too. That new substance was selenium. 

How Did Selenium Get Its Name?

Upon observing the striking resemblance of the chemical properties between tellurium (named after the Greek word for “earth”), Berzelius named his newly discovered chemical element selenium after the Greek word “selene”, meaning “moon”. Selene is also considered to be a Greek goddess of the moon. 

Where Can You Find Selenium?

In Earth’s crust, selenium commonly occurs as selenite, selenate, and selenides found in sulfide minerals, copper ores, and rare minerals (such as clausthalite, crooksite, and eucairite). The amino acids selenocysteine, methylselenocysteine, and selenomethionine also contain selenium as one of the key components of their structure. Selenium can be found in the soil (with different degrees of concentration) and ocean waters, as well. 

Smelting, extraction of sulfide ores, as well as coal burning also releases selenium. In its pure, elemental form, selenium can be obtained from the slimes and sludges formed during the production of sulfuric acid. 

For commercial purposes, this substance is obtained as a byproduct of electrolytic copper refining. Germany, United States, Japan, Belgium, Bolivia, and Russia are the leading countries in the world regarding the selenium refinement process. 

Selenide Minerals

The following list presents the minerals in which selenium can also be found: 

  • Achávalite
  • Aguilarite
  • Athabascaite
  • Berzelianite
  • Bukovite
  • Cadmoselite
  • Chrisstanleyite
  • Clausthalite
  • Crookesite
  • Drysdallite
  • Eskebornite
  • Ferroselite
  • Marthozite
  • Jolliffeite
  • Krut’aite
  • Oosterboschite
  • Penroseite
  • Sabatierite
  • Stilleite
  • Tiemannite
  • Trogtalite
  • Tyrrellite
  • Umangite
  • Verbeekite

Selenium in Everyday Life

Not only is selenium essential for our health, but it’s also a substance of choice for many industries that make wide use of its chemical properties.

  • Selenium is most extensively used as an additive to glass. It’s also used in the manufacturing of pigments for ceramics, paint, and plastics. This chemical is also considered to be an excellent additive to stainless steel;
  • The photoconductive and photoconductive properties of selenium make this chemical highly useful in photocells, solar cells, and photocopiers. By being able to convert AC electricity to DC electricity, selenium also finds its application in rectifiers. When exposed to light, the gray selenium conducts electricity better;
  • As an ingredient in anti-dandruff shampoos, selenium destroys the scalp fungus that causes dandruff;
  • The antifungal properties of selenium and its compounds are also used in the production of fertilizers;
  • This chemical element has a practical application in xerography to copy documents and in the manufacturing of photographic toners;
  • As a p-type semiconductor, selenium is widely used by the electronics industry.

Selenium and Health

Selenium as a trace mineral is one of the key components of selenoproteins and enzymes. These enzymes participate in reproduction and the metabolism of thyroid hormones. On the other hand, selenium possesses strong antioxidant properties that trigger the breaking down of peroxides. Peroxides, again, can damage the body’s tissues and the DNA, thus triggering inflammation and other health problems.

For this reason, selenium is extremely important for proper thyroid function, as well as for the body’s metabolic and homeostatic processes.

What Is Selenium Deficiency?

Selenium deficiency occurs when the selenium levels in the body are low. This may lead to various health and neurological problems, such as the potentially fatal Keshan disease and hypothyroidism.

What Are the Symptoms of Selenium Deficiency?

Individuals who experience selenium deficiency, most commonly report the following symptoms:

  • Hair loss;
  • Lethargy;
  • Discoloration of the nails and the skin;
  • Unexplained tiredness;
  • Foggy brain;
  • Seizures;
  • Cognitive problems;
  • Inability to concentrate on a task;
  • Fertility problems;
  • Lowered immunological defenses of the body;
  • Asthma exacerbation.

Selenium-Rich Foods

Food sources with high selenium content classify as powerful antioxidants. Their richness in this essential trace-mineral depends upon the quality of the soil the food has been grown in. 

Good sources of selenium-rich foods include:

  • Spinach;
  • Broccoli; 
  • Cabbage;
  • Mushrooms;
  • Onions ;
  • Soybeans;
  • Cashews;
  • Carrots;
  • Asparagus;
  • Raisins;
  • Berries;
  • Peaches;
  • Lentils;
  • Brown rice;
  • Sunflower seeds;
  • Bananas;
  • Brazil nuts; 
  • Mushrooms;
  • Peanut butter. 

Selenium Supplements

While most people get their RDA of selenium from food, some need to include dietary supplements of this trace mineral as a part of the daily selenium intake. Low selenium absorption can also be one of the health reasons for the administration of selenium in supplemental form.

Some observational studies on selenium supplementation provide evidence that this trace mineral may prevent heart diseases, as well as thyroid and cognitive problems, improve the clinical picture of chronic asthma, lower the risk of cancer (especially prostate cancer), reduce the side effects of radiation therapy, prevent HIV from progressing to AIDS, reduce the risk of miscarriage, etc. 

Selenium supplements offer the following health benefits: boosting the immune response of the body, improvement of hair and nail health, cancer prevention, cardiovascular disease prevention, as well as supporting the proper functioning of the thyroid gland. The daily recommended dietary allowance (RDA) for selenium is from 55 micrograms (mcg) to 400 micrograms per day for adults. Selenium intake should be higher in both pregnant and lactating women as the body needs more of it. 

Important: Please note that this selenium fact sheet is for educational and informative purposes only. This list is not complete and many other drugs may interact with selenium, including prescription and over-the-counter medicines, vitamins, and herbal products. We strongly advise that you seek medical advice from a qualified healthcare provider or trained health professionals before treating any medical problems or adding selenium supplements to your diet.

How Dangerous Is Selenium?

Despite being an essential trace element for humans and some animals, selenium compounds are highly toxic. The pure, elemental form of element 34 is not classified as a toxic substance. 

According to the United States Department of Health and Human Services, overconsumption of selenium often leads to acute selenium toxicity

What is Selenium Toxicity (Selenosis)?

Selenium toxicity, or selenium poisoning (also referred to as ‘selenosis’), occurs when the absorption of this trace mineral in the body’s tissues (especially the thyroid gland and the muscles) is far beyond the RDA. Extremely high levels of selenium in the body are more likely to occur in patients suffering from some thyroid disease, such as Hashimoto’s thyroiditis. Usually, the affected individuals experience some of the following symptoms:

  • Intestinal and neurological disorders; 
  • Heart attack; 
  • Myocardial infarction; 
  • Hair loss;
  • Brittle hair and nails;
  • Pneumonitis;
  • Bronchial asthma;
  • Muscle tenderness; 
  • Enlarged liver;
  • Tremors;
  • Fatigue;
  • Nausea;
  • Vomiting;
  • Metallic taste in the mouth;
  • Garlic-like breath;
  • Kidney failure;
  • Lightheadedness; 
  • Facial flushing;
  • Paralysis;
  • Death.

In a healthy individual, it takes 24 to 48 hours for the body to eliminate the excess selenium concentrations. Selenium toxicity can be avoided by controlling the daily intake of this trace mineral via food or supplements. 

Environmental Effects of Selenium

Since element 32 is widely spread in nature and also presents one of the most essential trace minerals in the human body, selenium’s biological and geological role in our environment is considered to be beneficial. Since this chemical element has an unstable electron configuration, its reactions in the environment are strongly dependent on environmental conditions.

However, the mines and industries that include selenium as one of the agents in their processes may present a great environmental hazard by contributing to selenium pollution

Isotopes of Selenium

Among the 30 isotopes of selenium, there are five stable, naturally occurring isotopes of this chemical element: selenium-74, selenium-76, selenium-77, selenium-78, and selenium-80.

Selenium-77 and selenium-78 isotopes are mainly used for the production of the therapeutic radioisotope Br-77, while selenium-76 is a form of the element 32 applied in the production of the medical isotopes Br-75 and Br-76.


[n 1]

ZNIsotopic mass (Da)

[n 2][n 3]


[n 4][n 5]



[n 6]



[n 7]

Spin and


[n 8][n 5]

Natural abundance (mole fraction)
Excitation energyNormal proportionRange of variation
65Se343164.96466(64)#<50 msβ+ (>99.9%)65As3/2−#  
β+, p (<.1%)64Ge
66Se343265.95521(32)#33(12) msβ+66As0+  
67Se343366.95009(21)#133(11) msβ+ (99.5%)67As5/2−#  
β+, p (.5%)66Ge
68Se343467.94180(4)35.5(7) sβ+68As0+  
69Se343568.93956(4)27.4(2) sβ+ (99.955%)69As(1/2−)  
β+, p (.045%)68Ge
70Se343669.93339(7)41.1(3) minβ+70As0+  
71Se343770.93224(3)4.74(5) minβ+71As5/2−  
72Se343871.927112(13)8.40(8) dEC72As0+  
73Se343972.926765(11)7.15(8) hβ+73As9/2+  
73mSe25.71(4) keV39.8(13) minIT73Se3/2−  
74Se344073.9224764(18)Observationally Stable[n 9]0+0.0089(4) 
75Se344174.9225234(18)119.779(4) dEC75As5/2+  
77mSe161.9223(7) keV17.36(5) sIT77Se7/2+  
79Se344578.9184991(18)3.27(8)×105 yβ79Br7/2+  
80Se344679.9165213(21)Observationally Stable[n 11]0+0.4961(41) 
81Se344780.9179925(22)18.45(12) minβ81Br1/2−  
82Se[n 12]344881.9166994(22)0.97(5)×1020 yββ82Kr0+0.0873(22) 
83Se344982.919118(4)22.3(3) minβ83Br9/2+  
84Se345083.918462(16)3.1(1) minβ84Br0+  
85Se345184.92225(3)31.7(9) sβ85Br(5/2+)#  
86Se345285.924272(17)15.3(9) sβ86Br0+  
87Se345386.92852(4)5.50(12) sβ (99.64%)87Br(5/2+)#  
β, n (.36%)86Br
88Se345487.93142(5)1.53(6) sβ (99.01%)88Br0+  
β, n (.99%)87Br
89Se345588.93645(32)#0.41(4) sβ (92.2%)89Br(5/2+)#  
β, n (7.8%)88Br
90Se345689.93996(43)#300# ms [>300 ns]β, n89Br0+  
91Se345790.94596(54)#270(50) msβ (79%)91Br1/2+#  
β, n90Br
92Se345891.94992(64)#100# ms [>300 ns]β92Br0+  
93Se345992.95629(86)#50# ms [>300 ns]  1/2+#  
94Se346093.96049(86)#20# ms [>300 ns]  0+  

Source: Wikipedia

List of Selenium Compounds 

Just like sulphur, selenium combines with metals and oxygen to form selenides (zinc selenide, ZnSe), selenates (calcium selenate, CaSeO4), selenites (silver selenite, Ag2SeO3), halogen, and chalcogen compounds. 

When selenium participates as one of the substances in a compound, it adopts the oxidation states of -2, +4, and +6. The aforementioned selenides are compounds of element 32 that contain selenium anion in the -2 oxidation state (Se-2). All selenium compounds are highly toxic by nature.

This chemical element most commonly occurs as a part of the following compounds:

  • Methaneseleninic acid
  • Selenium dibromide
  • Boroselenite
  • Copper(II) selenite
  • Hydrogenselenite
  • Selenite fluoride
  • Silver selenite
  • Sodium selenite
  • Selenium dichloride
  • Dimethyl selenide
  • 2C-Se
  • Benzeneselenol
  • Diethyl selenide
  • Diphenyl diselenide
  • Ebselen
  • Methaneseleninic acid
  • Methylselenocysteine
  • PhSeZn-halides
  • SeHCAT
  • Selenite (ion)
  • Selenium dioxide
  • Selenium oxybromide
  • Selenium oxydichloride
  • Selenium tetrachloride
  • Selenium tetrafluoride
  • Selenous acid
  • Selenenic acid
  • Seleninic acid
  • Selenomethionine
  • Selenophos
  • Selenopyrylium
  • Selenourea
  • Selone
  • Thioselenide
  • Woollins’ reagent
  • Benzeneselenol
  • Selenocysteine
  • Selenol

5 Interesting Facts and Explanations

  1. There are more than twenty genes coding for selenoproteins identified in humans. Some of them include DIO2, glutathione peroxidase, GPX1, GPX2 (gene), GPX3 (highly expressed in thyroid gland and kidneys), GPX4, GPX6, iodothyronine deiodinase, peptide-methionine (R)-S-oxide reductase, SECISBP2, SELS (gene), SELT, SEP15, SEPN1, SEPP1, SEPW1, SEPX1, SMCP, and TXNRD1.
  2. Brazil nuts are considered to be the best natural source of selenium. A quantity of six to eight nuts contains about 544 mcg of the trace mineral. In other words, only one brazil nut can provide the RDA of selenium. 
  3. Baron Jöns Jacob Berzelius is regarded as one of the founders of modern chemistry, together with John Dalton, Robert Boyle, and Antoine Lavoisier. He’s also one of the founders of Karolinska Institute. Apart from selenium, Berzelius also discovered cerium and thorium.
  4. Group 16 of the periodic table includes the elements oxygen, sulphur, selenium, tellurium, and polonium. The chemicals classified in this group are also labeled as chalcogens. 
  5. Believe it or not, but the scent of a skunk can be described with a chemical formula that includes selenium as one of the main participants in this smelly reaction. Namely, butyl seleno mercaptan (C4H9SeH) is a natural molecule contained in skunk’s spray which is produced when they are in a “defensive mode”.