Selenium: The Essential Semiconductor
Atomic Number: 34 | Symbol: Se | Discovered: 1817 | Group 16, Period 4, p-block
Selenium Crystal Structure
Gray selenium crystallizes in a hexagonal structure. Each selenium atom forms two covalent bonds with its neighbors in helical chains, with weak interchain interactions.
📸 PHOTOCOPIERS • 💊 ANTIOXIDANTS • 🔌 SEMICONDUCTORS • 🥛 GLASS MANUFACTURING • ⚡ RECTIFIERS • 🧬 ESSENTIAL NUTRIENT
Nonmetal • Multiple Allotropes • Essential Trace Element • Photoconductor • Semiconductor • 1.7-2.6 eV Band Gap
The Discovery of the Moon Element
Selenium was discovered in 1817 by Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn. They were investigating a red sediment from the Gripsholm sulfuric acid factory that was causing health problems among workers. Initially believing it contained tellurium, Berzelius eventually realized it was a new element. He named it "selenium" from the Greek word "selēnē" (σελήνη), meaning "moon," to complement tellurium which was named after Earth ("tellus" in Latin). For decades, selenium remained a laboratory curiosity with no practical applications until its photoconductive properties were discovered in the 1870s, leading to its use in early light-sensing devices.
Basic Properties of Selenium
Selenium is characterized by its multiple allotropes, photoconductive properties, and essential biological role as an antioxidant nutrient.
Interactive 3D Bohr Model of a Selenium Atom
Click and drag to rotate • Scroll to zoom • Nucleus: 34 protons (red), 45 neutrons (blue) • Electron shells: 2, 8, 18, 6
Selenium's band gap varies with allotrope: gray selenium (1.7 eV), amorphous red selenium (2.6 eV), making it a versatile semiconductor material.
Selenium Allotropes: A Material with Multiple Forms
Gray (Metallic) Selenium
Structure: Hexagonal crystal lattice composed of helical chains
Properties: Most stable allotrope at room temperature. Metallic gray appearance but actually a semiconductor. Photoconductive - electrical conductivity increases 1000-fold when exposed to light. Band gap of 1.7 eV.
Applications: Used in photocopiers, photodetectors, and xerography. Also in rectifiers and solar cells.
Red Selenium
Structure: Monoclinic crystals composed of Se₈ rings
Properties: Bright red color. Less stable than gray selenium, converts to gray form upon heating. Insoluble in water and most solvents. Band gap of approximately 2.0 eV.
Applications: Used as a pigment (cadmium selenide gives red color). Also used in some semiconductor applications and as a source for preparing other selenium compounds.
Black Selenium
Structure: Vitreous (glass-like) form with disordered structure
Properties: Obtained by rapid cooling of molten selenium. Brittle and glassy. Semiconductor with properties intermediate between gray and red forms. More soluble than other allotropes.
Applications: Used in some electronic applications and as a precursor for making other selenium compounds. Also studied for phase-change memory applications.
Amorphous Selenium
Structure: No long-range order, random arrangement of atoms
Properties: Can be red or black depending on preparation. Red amorphous selenium consists of Se₈ rings. Black amorphous selenium has polymeric chains. High resistivity in dark but becomes photoconductive when exposed to light.
Applications: Used in xerography (photocopiers) and as a photoconductive layer in flat-panel X-ray detectors for medical imaging.
Photoconductor
Selenium's electrical conductivity increases dramatically when exposed to light (up to 1000-fold). This property made it essential for early photocopiers and light sensors.
Essential Antioxidant
Selenium is a crucial component of glutathione peroxidase and other antioxidant enzymes that protect cells from oxidative damage. It's an essential trace element for humans and animals.
Semiconductor
Gray selenium is a p-type semiconductor with a band gap of 1.7 eV. It was used in early solid-state rectifiers before being replaced by silicon and germanium.
Colorant
Selenium compounds provide colors in glass and ceramics: cadmium selenide (red), zinc selenide (yellow), and selenium itself produces a ruby red color in glass.
The Oxygen Group: Selenium's Chemical Family
Selenium belongs to Group 16 (chalcogens) along with oxygen, sulfur, tellurium, and polonium. These elements have six electrons in their outer shell.
| Property | Sulfur (S) | Selenium (Se) | Tellurium (Te) | Polonium (Po) |
|---|---|---|---|---|
| Atomic Number | 16 | 34 | 52 | 84 |
| Melting Point (°C) | 115 | 221 | 450 | 254 |
| Band Gap (eV) | 2.6 (orthorhombic) | 1.7-2.6 | 0.33 | Metal |
| Crystal Structure | Orthorhombic | Hexagonal (gray) | Hexagonal | Simple cubic |
| Biological Role | Amino acids (cysteine, methionine) | Antioxidant enzymes | No known essential role | Highly radioactive, toxic |
| Primary Uses | Fertilizers, chemicals, vulcanization | Electronics, glass, photocopiers, nutrition | Alloys, semiconductors, solar cells | Static eliminators, heat sources |
Important Selenium Compounds
Selenium forms diverse compounds with applications ranging from electronics to nutrition.
Selenium Dioxide (SeO₂)
Properties: White crystalline solid, sublimes at 315°C
Toxicity: Toxic, causes selenium poisoning
Uses: Oxidizing agent in organic chemistry, reagent for selenium compounds
Hydrogen Selenide (H₂Se)
Properties: Colorless gas, foul odor (like rotten horseradish)
Toxicity: Extremely toxic, more toxic than HCN
Uses: Preparation of metal selenides, semiconductor doping
Selenium Oxychloride (SeOCl₂)
Properties: Colorless liquid, powerful solvent
Toxicity: Highly toxic and corrosive
Uses: Solvent for specialized applications, chlorinating agent
Selenocysteine (Sec)
Properties: 21st amino acid, contains selenium
Toxicity: Non-toxic in proteins
Uses: Active site of glutathione peroxidase and other selenoenzymes
Key Properties That Define Selenium
- The Original Photocopier Material: Selenium's photoconductive properties (discovered in 1873) made it essential for the first photocopiers (xerography) and early light sensors, revolutionizing document reproduction.
- Essential Antioxidant Nutrient: Selenium is a crucial component of glutathione peroxidase and other antioxidant enzymes that protect cells from oxidative damage. It's the 21st amino acid (selenocysteine) in the genetic code.
- U-Shaped Dose Response: Selenium has a narrow therapeutic window. Both deficiency and excess cause serious health problems, creating a U-shaped relationship between selenium status and health.
- Multiple Allotropes: Selenium exists in several allotropes: gray (metallic, hexagonal), red (monoclinic, Se₈ rings), and black (vitreous). Gray selenium is the most stable and exhibits photoconductivity.
- Glass Decolorizer and Colorant: Selenium removes green tint from glass (caused by iron impurities) and, in higher concentrations, imparts a ruby red color. Cadmium selenide provides bright red pigments.
- Early Semiconductor: Selenium rectifiers were used in early electronics before silicon and germanium became dominant. Selenium's semiconductor properties vary with allotrope (1.7-2.6 eV band gap).
- Named for the Moon: Discovered by Berzelius in 1817, selenium was named after the Greek moon goddess Selene to complement tellurium (named for Earth).
- Geographical Health Disparities: Selenium deficiency causes Keshan disease (heart disease) and Kashin-Beck disease (bone disease) in low-selenium regions of China, while excess causes selenosis in high-selenium areas.
Selenium Toxicity and Nutrition
Selenium has a narrow therapeutic window with both deficiency and excess causing serious health problems. The recommended daily allowance for adults is 55 μg/day, with an upper limit of 400 μg/day. Selenium deficiency causes Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy) in low-selenium regions of China and Tibet. It also increases risk of certain cancers and impairs immune function. Selenium excess (selenosis) causes symptoms including garlic breath odor, hair and nail loss, neurological abnormalities, and in severe cases, cirrhosis and pulmonary edema. Brazil nuts are exceptionally high in selenium (one nut can contain 68-91 μg). Industrial exposure to hydrogen selenide or selenium dioxide can cause acute poisoning. Organic selenium compounds (selenomethionine, selenocysteine) are less toxic and better absorbed than inorganic forms (selenate, selenite).
Historical Timeline: From Discovery to Essential Nutrient
Discovery: Swedish chemist Jöns Jacob Berzelius discovers selenium in the red sediment from a sulfuric acid factory in Gripsholm, Sweden. He names it after the Greek moon goddess Selene.
Photoconductivity Discovery: Willoughby Smith discovers that selenium's electrical resistance decreases when exposed to light, opening the door to photoelectric applications.
Early Applications: Selenium cells used in photometers, burglar alarms, and early television systems. Alexander Graham Bell's "photophone" used selenium to transmit sound on a beam of light.
Selenium Rectifiers: Selenium rectifiers developed for converting AC to DC in power supplies, used in radios, televisions, and other electronics until replaced by silicon in the 1970s.
Toxicology Studies: Recognition of selenium as both toxic and potentially essential. "Blind staggers" and "alkali disease" in livestock linked to selenium excess.
Essentiality Established: Klaus Schwarz demonstrates that selenium prevents liver necrosis in vitamin E-deficient rats, establishing selenium as an essential trace element.
Glutathione Peroxidase Discovery: Rotruck et al. discover that selenium is an essential component of glutathione peroxidase, explaining its antioxidant function.
Keshan Disease Link: Chinese researchers demonstrate that selenium deficiency causes Keshan disease, a fatal cardiomyopathy endemic in low-selenium regions of China.
Selenocysteine as 21st Amino Acid: Discovery that selenocysteine is incorporated into proteins via a unique mechanism, making it the 21st proteinogenic amino acid.
Nutrition and Cancer Research: Large-scale studies on selenium's role in cancer prevention, with mixed results. Recognition of U-shaped dose-response relationship.
Selenium in the Modern World: Essential Applications
Photocopiers and Printers
Amorphous selenium photoconductive drums in xerography (photocopiers, laser printers). Selenium's photoconductivity enables electrostatic image formation and toner transfer.
Glass Manufacturing
Decolorizer to remove green tint from glass (iron impurities). Colorant for ruby red glass. Also used in architectural glass to reduce solar heat transmission.
Nutrition and Supplements
Essential trace element as selenomethionine or sodium selenite in supplements and fortified foods. Added to animal feed to prevent selenium deficiency diseases.
Electronics
Cadmium selenide in quantum dots for displays and solar cells. Zinc selenide for blue LEDs and lasers. Selenium in some photovoltaic cells and photodetectors.
Metallurgy
Added to stainless steel and copper alloys to improve machinability. Selenium in lead alloys improves mechanical properties. Also used in manganese electrolysis.
Pigments and Colors
Cadmium selenide (CdSe) gives bright red, orange, and yellow pigments for plastics, ceramics, and paints. Zinc selenide (ZnSe) used for optical materials.
Medical Imaging
Amorphous selenium flat-panel detectors for digital radiography and mammography. Selenium-75 radioisotope used in gamma radiography for industrial inspection.
Rectifiers and Surge Protectors
Selenium rectifiers historically used to convert AC to DC. Selenium surge protectors still used in some telecommunications equipment.
GLUTATHIONE PEROXIDASE • XEROGRAPHY • QUANTUM DOTS • RUBY GLASS • KESHAN DISEASE • SELENOMETHIONINE
Approximately 40% of selenium production is used in glass manufacturing, 30% in electronics, 15% in chemicals, and 10% in metallurgy
Production: From Copper Refining to Recycling
Most selenium is recovered as a byproduct of copper refining, with significant production also from lead and nickel processing.
Primary Sources
Recovered from anode slimes in copper electrolytic refining. Also from lead and nickel processing. Main ores contain selenides of copper, lead, and silver.
Extraction Process
Anode slimes roasted with soda ash to convert selenium to soluble selenite (Na₂SeO₃). Acidified to precipitate selenium dioxide, then reduced with sulfur dioxide to elemental selenium.
Purification
Distillation produces high-purity selenium (99.99%). Zone refining for semiconductor grade. Vacuum distillation for ultra-high purity.
Major Producers
Japan, Germany, Belgium, Russia, Canada. Global production ~3,000 tons annually. Recycling from photocopier drums and electronics increasing.
Selenium Isotopes and Nuclear Applications
Naturally occurring selenium consists of six stable isotopes, with several radioactive isotopes used in research and medicine.
Selenium-74 (⁷⁴Se)
Natural Abundance: 0.89%
Nuclear Properties: Stable
Special Note: Double beta decay candidate
One of six stable selenium isotopes. Being studied for neutrinoless double beta decay experiments to determine if neutrinos are their own antiparticles.
Selenium-80 (⁸⁰Se)
Natural Abundance: 49.61%
Nuclear Properties: Stable
Special Note: Most abundant isotope
The most abundant stable isotope of selenium. Used as the base material for producing selenium-75 and other radioisotopes through neutron activation.
Selenium-82 (⁸²Se)
Natural Abundance: 8.73%
Nuclear Properties: Radioactive, very long-lived
Special Note: Primordial radionuclide
Undergoes double beta decay with a half-life of 1.08×10²⁰ years. Used in experiments searching for neutrinoless double beta decay.
Selenium-75 (⁷⁵Se)
Half-life: 119.8 days
Decay: Electron capture to arsenic-75
Use: Industrial radiography, medical research
Used in gamma radiography for non-destructive testing of welds and castings. Also used in medical research to study selenium metabolism and biochemistry.
Selenium in Biology and Medicine
Selenium is an essential trace element with crucial roles in antioxidant defense, thyroid hormone metabolism, and immune function.
Essential Antioxidant Nutrient
Selenium is a key component of glutathione peroxidase and thioredoxin reductase, antioxidant enzymes that protect cells from oxidative damage by reducing hydrogen peroxide and lipid hydroperoxides. Selenoprotein P transports selenium in blood and may have antioxidant functions. At least 25 selenoproteins have been identified in humans, with roles in antioxidant defense, thyroid hormone metabolism, DNA synthesis, and reproduction. Selenium deficiency increases susceptibility to oxidative stress and contributes to various diseases.
The 21st Amino Acid: Selenocysteine
Selenocysteine (Sec) is the 21st proteinogenic amino acid, incorporated into selenoproteins via a unique translation mechanism. The UGA codon, normally a stop codon, encodes selenocysteine when followed by a selenocysteine insertion sequence (SECIS) in the mRNA. This makes selenium unique among trace elements - it is incorporated into proteins as an amino acid rather than as a cofactor. Selenocysteine's selenium atom has a lower pKa (5.2) than cysteine's sulfur (8.3), making it more reactive at physiological pH, which is crucial for its enzymatic functions.
Geographical Health Disparities
Selenium status varies dramatically with geography due to soil selenium content. Keshan disease (cardiomyopathy) and Kashin-Beck disease (osteoarthropathy) are endemic in low-selenium regions of China and Tibet. Conversely, selenosis occurs in high-selenium areas like parts of China, the US, and Venezuela. Soil selenium content affects food selenium levels: grains from North American soils are generally selenium-rich, while those from New Zealand and parts of Europe and China are low. Brazil nuts from selenium-rich soils can contain extremely high levels (up to 2,500 μg per nut).
Fun Facts and Historical Anecdotes
Fascinating Facts About Selenium
- The First Practical Photocopier: Chester Carlson's 1938 invention of xerography used a selenium-coated plate. This evolved into the modern photocopier, revolutionizing document reproduction.
- Bell's Photophone: Alexander Graham Bell's 1880 "photophone" used selenium to transmit sound on a beam of light, predating fiber optics by nearly a century.
- Moon Element: Selenium was named after the Greek moon goddess Selene by Berzelius, who had previously named tellurium after Earth ("tellus" in Latin).
- Brazil Nut Phenomenon: Brazil nuts are the richest dietary source of selenium, but levels vary dramatically by region. One nut from selenium-rich soil can provide over 10 times the daily requirement.
- The Selenium Rectifier Revolution: Before silicon diodes, selenium rectifiers were essential for converting AC to DC in power supplies for radios, televisions, and other electronics from the 1930s to 1970s.
- Keshan Disease Solution: In the 1970s, Chinese researchers solved the mystery of Keshan disease (a fatal heart condition) by linking it to selenium deficiency and preventing it with selenium supplements.
- Ruby Glass Secret: Selenium gives glass a beautiful ruby red color. Combined with cadmium sulfide, it produces a range of colors from yellow to deep red.
- Quantum Dot Pioneer: Cadmium selenide quantum dots, first synthesized in the 1980s, revolutionized nanotechnology with their size-tunable optical properties.
- Garlic Breath Clue: The garlic-like odor on the breath is a classic sign of selenium poisoning, caused by dimethyl selenide excretion.
Selenium Statistics and Global Impact
The Future of Selenium: Innovation and Challenges
As technology and nutrition science advance, selenium continues to find new applications while facing sustainability challenges.
Advanced Materials and Nanotechnology
Cadmium selenide quantum dots for high-efficiency displays, solar cells, and biomedical imaging. Selenium nanomaterials for drug delivery and cancer therapy. Selenium-containing polymers and coatings with antioxidant properties. Selenium in 2D materials like selenium nanosheets for electronics and catalysis.
Precision Nutrition and Medicine
Personalized selenium supplementation based on genetic polymorphisms in selenoproteins. Selenium nanoparticles for targeted cancer therapy. Selenium compounds as antiviral agents (including against HIV and SARS-CoV-2). Understanding selenium's role in aging, neurodegeneration, and immune function.
Sustainable Agriculture
Biofortification of crops with selenium to address deficiencies. Development of selenium-efficient crop varieties. Selenium nanoparticles as nanofertilizers and nanopesticides. Understanding selenium cycling in agricultural ecosystems.
Supply Chain and Sustainability
Selenium classified as critical raw material by EU due to supply concentration and essential applications. Increased recycling from end-of-life products (photocopiers, electronics, glass). Development of extraction from alternative sources (coal fly ash, wastewater). Exploration of selenium recovery from industrial byproducts.
Conclusion: The Element of Light and Life
Selenium stands as a remarkable element that bridges the physical and biological worlds. From its discovery as an industrial byproduct to its recognition as an essential nutrient, selenium's story illustrates how scientific understanding can transform our relationship with an element. Its unique properties - photoconductivity that revolutionized document reproduction, semiconductor behavior that powered early electronics, and biochemical roles that protect life from oxidative damage - make selenium truly multidisciplinary in its significance.
The selenium story teaches us about balance and context. As a nutrient, it demonstrates the principle of hormesis - beneficial at low doses but toxic at high doses. This U-shaped dose-response relationship reminds us that in biology, more is not always better. The geographical disparities in selenium status, from deficiency diseases in China to toxicity in Venezuela, highlight how elemental cycles connect geology to human health.
Technologically, selenium's journey from photocopier drums to quantum dots shows how materials can find new applications as science advances. The same element that made office automation possible in the 20th century now enables nanotechnology and advanced medical imaging in the 21st. Selenium's continued relevance in both established industries (glass, metallurgy) and cutting-edge technologies (nanomaterials, biomedicine) demonstrates its versatility.
As we face challenges in sustainable nutrition, advanced electronics, and environmental stewardship, selenium will continue to offer solutions and raise questions. From biofortifying crops to prevent deficiency diseases to developing more efficient solar cells, from understanding selenoprotein genetics to recycling selenium from industrial waste, this element remains at the intersection of human needs and scientific discovery.
In selenium, we find an element that literally brings light to our world - through photocopiers that reproduce knowledge, through antioxidant enzymes that protect life, and through quantum dots that create vivid displays. Its story reminds us that elements are not just entries in a periodic table but dynamic participants in both technology and biology, with stories that continue to unfold as we deepen our understanding of their properties and potential.