Selenium
Essential trace element and semiconductor discovered in 1817.
Selenium is a chemical element with the symbol Se and atomic number 34. It was discovered in 1817 by Jöns Jacob Berzelius, who observed its resemblance to the previously known element tellurium. In its pure form, selenium is rarely found in nature; it appears in several distinct physical forms, including a brick-red powder, a black vitreous solid, and a grey, metallic-looking substance. Commercially, it is obtained as a byproduct during the refining of metal sulfide ores, where it often replaces sulfur. Pure selenide or selenate minerals are uncommon. Selenium is a semiconductor and exhibits photoconductivity, making it useful in photocells and certain types of DC power surge protectors, though many of its former electronic applications have been superseded by silicon. It remains important in glassmaking and pigments. The element has seven naturally occurring isotopes, five of which are stable, with selenium-80 being the most abundant. It also has a long-lived primordial radionuclide, selenium-82, and a non-primordial isotope, selenium-79, found in uranium ores. Selenium forms several allotropes that change with temperature; the most stable and dense form is grey, with a chiral hexagonal lattice of helical chains. Other allotropes include a black, brittle vitreous form and red monoclinic forms containing cyclooctaselenium rings. Selenium compounds exist in oxidation states of -2, +2, +4, and +6. It is a nonmetal, sometimes considered a metalloid, with properties intermediate between sulfur and tellurium. Its oxides include selenium dioxide and the thermodynamically unstable selenium trioxide. While trace amounts are essential for cellular function in many animals, including humans, both elemental selenium and its salts are toxic in even small doses, causing selenosis with symptoms such as diarrhea, fatigue, hair loss, and joint pain. It is an ingredient in multivitamins and infant formula and is a component of antioxidant enzymes like glutathione peroxidase.
- symbol
- Se
- atomic_number
- 34
- discovered_by
- Jöns Jacob Berzelius
- year_discovered
- 1817
- category
- nonmetal (more rarely considered a metalloid)
- common_allotropes
- brick-red powder, vitreous black solid, grey metallic-looking form
Lore & Background
Selenium seldom occurs in its elemental state or as pure ore compounds in Earth's crust. It is found in metal sulfide ores, where it substitutes for sulfur, and is produced commercially as a byproduct in ore refining. The element's name derives from the Greek word for 'moon', reflecting its similarity to tellurium, named for the Earth. Selenium forms several allotropes that interconvert with temperature changes. The most stable form is gray selenium, a semiconductor with a chiral hexagonal crystal lattice. Other allotropes include amorphous brick-red powder and vitreous black solid. Selenium is used in glassmaking, pigments, photocells, DC power surge protectors, and fluorescent quantum dots. Selenium is a component of antioxidant enzymes such as glutathione peroxidase and thioredoxin reductase, and is listed as an ingredient in multivitamins, dietary supplements, and infant formula. However, both elemental selenium and its salts are toxic in even small doses, causing selenosis with symptoms including diarrhea, fatigue, hair loss, and joint pain.
Reader's Guide
Selenium occupies a unique position as both an essential nutrient and a toxic substance. Its role in antioxidant enzymes underscores its biological importance, while its semiconductor properties made it valuable in early electronics, though largely replaced by silicon. The element's allotropes exhibit diverse physical properties, from insulating red powder to photoconductive gray metal. Commercially, selenium remains important in glassmaking and pigments, and finds niche applications in surge protectors and quantum dots. Its discovery by Berzelius in 1817 added a new element to the periodic table, bridging sulfur and tellurium. The dual nature of selenium—necessary for life yet dangerous in excess—illustrates the delicate balance of trace elements in biology. Its presence in infant formula and multivitamins reflects careful regulation to provide benefit without harm.
Did You Know?
- Selenium was discovered in 1817 by Jöns Jacob Berzelius, who named it after the Greek word for 'moon'.
- Gray selenium is a semiconductor and shows appreciable photoconductivity, unlike other selenium allotropes.
- Selenium is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase.
- Selenium disulfide, with an approximate composition of SeS2, has been used in shampoo as an anti-dandruff agent.
Frequently Asked Questions
Who is Selenium?
Selenium is element 34 on the periodic table, a nonmetal (occasionally grouped as a metalloid) with the symbol Se. It belongs to group 16, sharing a chemical family with oxygen and sulfur.
What are Selenium's powers and role?
Selenium serves as a semiconductor in electronics and appears in several distinct allotropes, including a brick-red powder, a glassy black solid, and a grey metallic-looking form. In biology it acts as a critical trace nutrient for humans and many animals.
How was Selenium discovered?
Swedish chemist Jöns Jacob Berzelius identified selenium in 1817 after observing its close resemblance to tellurium. He named it from the Greek word for moon, pairing it with tellurium's Earth-based name.
Why is Selenium important?
In small trace amounts, selenium is essential for proper thyroid function and antioxidant defense in humans and other animals. However, exceeding the safe range makes it toxic, so the balance between benefit and harm is narrow.
What does Selenium look like in different forms?
Selenium can present as a brick-red powder, a vitreous black solid, or a grey form with a metallic sheen. These allotropes give it a surprisingly varied visual identity compared to most elements on the table.
More in Chemical Elements & Metals 1-20
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