Quick Facts
| Property | Value |
|---|---|
| Symbol | Se |
| Atomic Number | 34 |
| Atomic Mass | 78.971 u |
| Category | Nonmetal |
| Period | 4 |
| Group | 16 |
| Block | p |
| Electron Configuration | [Ar] 3d¹⁰ 4s² 4p⁴ |
Physical Properties
| Property | Value |
|---|---|
| State at 20°C | Solid |
| Density | 4.81 g/cm³ (gray selenium) |
| Melting Point | 221°C (494 K) |
| Boiling Point | 685°C (958 K) |
| Appearance | Gray (metallic), red (amorphous) |
Atomic Properties
| Property | Value |
|---|---|
| Electron Configuration | [Ar] 3d¹⁰ 4s² 4p⁴ |
| Electronegativity | 2.55 (Pauling scale) |
| First Ionization Energy | 941.0 kJ/mol |
| Atomic Radius | 120 pm |
| Covalent Radius | 120 pm |
| Van der Waals Radius | 190 pm |
History and Discovery
Discovered by: Jöns Jacob Berzelius Year of Discovery: 1817 Location: Stockholm, Sweden
Etymology
Named after "Selene," the Greek goddess of the Moon, because of its similarity to tellurium (named after Earth, "Tellus").
Discovery Story
Jöns Jacob Berzelius discovered selenium in 1817 while analyzing residues from sulfuric acid production. He initially thought it was tellurium but soon recognized it as a new element. He noted its beautiful red color in some forms.
Isotopes
| Isotope | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|
| ⁷⁴Se | 0.89% | Stable | - |
| ⁷⁶Se | 9.37% | Stable | - |
| ⁷⁷Se | 7.63% | Stable | - |
| ⁷⁸Se | 23.77% | Stable | - |
| ⁸⁰Se | 49.61% | Stable | - |
| ⁸²Se | 8.73% | 1.08 × 10²⁰ years | β⁻β⁻ |
Occurrence
Natural Abundance
Selenium is rare, comprising about 0.05 ppm of Earth's crust. It is often associated with sulfide ores, particularly those of copper. Main sources are copper refining byproducts. Major producers are Japan, Germany, Belgium, and Russia.
Extraction and Production
- Byproduct: From copper electrolytic refining
- Anode Slimes: Selenium recovered from copper refinery waste
- Sources: Copper production exclusively
- Global production: about 2,200 tons annually
Applications and Uses
Electronics
- Photoconductor in photocopiers and laser printers
- Photovoltaic cells (CIGS solar cells)
- Rectifiers (historical)
- Photocells
Glass and Ceramics
- Decolorizer for glass (removes green tint from iron)
- Red pigments for glass and ceramics
- Cadmium selenide for red pigments
Chemical Applications
- Vulcanization of rubber
- Catalyst for various reactions
- Selenium dioxide in organic synthesis
Other Uses
- Dietary supplements
- Dandruff shampoos (selenium sulfide)
- Metallurgy (improves machinability)
Biological Role
Selenium is an essential trace element for animals and humans. It is a component of selenoproteins with antioxidant functions.
In the Human Body
- About 15 mg in adult body
- Component of selenocysteine amino acid
- Essential for thyroid function
- Antioxidant (glutathione peroxidases)
- Daily requirement: 55 μg
Dietary Sources
Brazil nuts (extremely high), seafood, organ meats, and grains are good sources.
Safety and Hazards
Toxicity
Selenium is essential but toxic in excess (selenosis). Symptoms include garlic breath, hair loss, and nail brittleness. Hydrogen selenide (H₂Se) is highly toxic.
Handling Precautions
- Selenium dust is toxic if inhaled
- Hydrogen selenide is extremely poisonous
- Avoid skin contact with selenium compounds
- Selenium dioxide is irritating
- Handle in well-ventilated areas
Environmental Impact
Selenium can bioaccumulate in aquatic food chains. Agricultural runoff in selenium-rich areas can affect wildlife.
Interesting Facts
- Brazil nuts can contain 10-100 times more selenium than other foods
- Selenium-containing proteins are essential for thyroid hormone metabolism
- The selenium rectifier was an important early electronic component
- Selenium was used in early photocopiers (xerography)
- "Garlic breath" is a symptom of selenium toxicity (dimethyl selenide)