Selenium Facts – Element Se or 34

Selenium Facts
Selenium is an element with symbol Se and atomic number 34.

Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between the elements above and below on the periodic table, sulfur and tellurium. People most often encounter selenium as a nutritional supplement or in dandruff shampoo.

Discovery and Naming

Selenium was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn. Its name is derived from the Greek word “selene,” meaning moon. The name comes from its similarity to tellurium, which takes its name from the Latin word for earth.

Appearance and Properties

Selenium Allotropes
These are the black glassy, gray, and red amorphous forms of selenium.

Selenium is appears in several allotropic forms and changes between them according to temperature. The most stable and dense form is gray metallic-looking selenium. Other allotropes include red and glassy black. Like sulfur, it essentially forms a polymer with its own atoms.

The properties of the allotropes differ markedly from each other. For example, gray selenium is photoconductive and a semiconductor, while red and black selenium are insulators.

Overall, the element shares properties most similar to those of sulfur and tellurium, although it is also somewhat similar to arsenic. Usually, it is considered a nonmetal, but the gray form acts as a metalloid.

Natural and Radioactive Isotopes

Selenium has seven natural isotopes: 74Se, 76Se, 77Se, 78Se, 80Se (all stable) and the radioisotopes 82Se and 79Se. . The most common is 80Se (49.6% abundance). There are also numerous synthetic isotopes, ranging from 64Se to 95Se.

Abundance and Sources

While selenium does occur in native form as a mineral, it only forms tiny crystals. The element is not very abundant in the Earth’s crust, with a concentration slightly lower than that of silver. Selenium occurs in sulfide ores like pyrite, where it partially replaces sulfur. Commercially, it is a byproduct from the refining of copper and other base metals. Coal burning is a common anthropogenic source of selenium pollution.

Uses of Selenium

Selenium has diverse uses, including:

Electron Levels of a Selenium Atom
  • As a replacement for lead in brass and as an alloying agent in steel.
  • As a photocopier component and in photographic toner.
  • In glassmaking, to remove color.
  • In electronics, for solar cells and photovoltaic devices. The first blue LEDs used zinc selenide.
  • As a nutritional supplement, due to its essential role in some enzymes.
  • In Li-Se batteries, which have higher electrical conductivity than Li-S batteries.
  • In the rubber industry, to improve the abrasion resistance.
  • As selenium dioxide for electroextraction of manganese.

Oxidation States

Selenium exhibits several oxidation states. The most common oxidation state is +4, found in compounds like selenides and selenates. The -2, +2, and +6 states also frequently occur.

Biological Role, Health Effects, and Toxicity

In humans and other animals, selenium is a vital component of key enzymes, including glutathione peroxidase. The element is critical for cells to activate and deactivate thyroid hormones. It is a component of the amino acids selenocysteine and selenomethionine. Dietary selenium helps protect against mercury toxicity. The element may act as an antioxidant that serves a chemo-preventative role. Selenium deficiency leads to health issues. However, excessive selenium intake is toxic, leading to symptoms like garlic breath odor, hair loss, nail brittleness, fatigue, and neurological abnormalities. Extreme deficiency is potentially fatal. Gauging absorption of the element is difficult, because it interacts with zinc, copper, iodine, and vitamin E.

Some plants strongly require selenium. Others use it as a defense against herbivores. Other plants do not require selenium.

Key Selenium Facts for Scientists

This table presents key information on the physical and chemical properties of selenium.

Atomic Number34
Atomic Weight78.971
Group16 (chalcogen)
Electron Configuration[Ar] 3d10 4s2 4p4
Electrons Per Shell2, 8, 18, 6
State at Room TemperatureSolid
Melting Point221 °C
Boiling Point685 °C
Density4.81 g/cm³ (gray form, near room temperature)
Heat of Vaporization95.48 kJ/mol
Heat of Fusion6.69 kJ/mol (gray)
Molar Heat Capacity25.363 J/(mol·K)
Oxidation States-2, -1, 0, +1, +2, +3, +4, +5, +6
Electronegativity2.55 (Pauling)
Ionization Energies1st: 941 kJ/mol, 2nd: 2045 kJ/mol
Atomic Radius120 pm
Covalent Radius120 pm
Van der Waals Radius190 pm
Crystal StructureTrigonal
Thermal Conductivity0.519 W/(m·K)
Shear Modulus3.7 GPa
Bulk Modulus8.3 GPa
Mohs Hardness2.0
Magnetic OrderingDiamagnetic

Interesting Selenium Facts

Here are some interesting and unusual facts about selenium that add to its fascinating character beyond the basic scientific details:

  • Color-Changing Properties: Selenium exhibits photochromic properties, meaning it changes color when exposed to light. This quality find use in photochromic lenses, which darken when exposed to sunlight.
  • Historical Medical Uses: In the 19th century, selenium compounds were used in medicines for diseases like syphilis, even before the understanding of its biological importance and potential toxicity.
  • Selenium in Rain: Selenium occurs in trace amounts in rainwater. This comes from the natural cycle of evaporation and precipitation, where selenium from soil, plants, and water bodies gets vaporized and then returns in rain.
  • Role in Glass Production: Beyond just removing color from glass, higher amounts of selenium impart a red or pink color to glass and enamels.
  • Selenium in Shampoos: Some anti-dandruff shampoos contain selenium sulfide, which is an effective treatment for dandruff and certain scalp dermatitis conditions due to its antifungal properties.
  • Space Applications: The photoelectric properties of selenium make it useful in space applications. It is also useful in the construction of solar cells for satellites.
  • Selenium and Mood: Some studies suggest a correlation between selenium intake and mood. Lower selenium levels are associated with increased risk of negative mood states like depression and anxiety.
  • Selenium in Fish: Fish is a rich source of selenium and the selenium content in fish is an indicator of pollution in the aquatic environment.
  • Selenium in Plants: Brazil nuts are the food that is highest in selenium. However, not all of the nuts contain it. Its presence and concentration depends on the soil in which the plants grow.
  • Selenium-Containing Enzymes in Defense Mechanisms: Certain enzymes that contain selenium play crucial roles in the defense mechanism of animals, including humans, protecting cells from damage caused by free radicals. Plants also use selenium as a defense, due to its toxicity to some organisms.
  • Selenium in Photocopiers: Before the digital era, selenium was crucial in photocopying process due to its photoconductive properties.


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