Selenium
Element 34 · Nonmetal
Selenium is a nonmetal best known for photoconductivity — its electrical conductivity increases dramatically when struck by light. This property made it the heart of early photocopiers and light meters, and it still serves in solar cells and rectifiers.
Biologically, selenium is a double-edged sword: it is an essential trace nutrient built into antioxidant enzymes, but the safe window is narrow and excess causes selenosis. The gray metallic-looking allotrope is the most stable form at room temperature.
Key facts
| Atomic number | 34 |
|---|---|
| Atomic weight | 78.971 |
| Category | Nonmetal |
| Group / Period | 16 / 4 |
| Block | p |
| Electron configuration | [Ar] 3d10 4s2 4p4 |
| Common oxidation states | +6, +4, -2 |
| State at STP | Solid |
| Electronegativity | 2.55 |
| Atomic radius | 103 pm |
| Density | 4.81 g/cm³ |
| Melting point | 221 °C |
| Boiling point | 685 °C |
Occurrence
Selenium is recovered mainly as a byproduct of copper refining, where it collects in anode sludges. It occurs in sulfide ores and is never found free in nature. Berzelius discovered it in 1817 and named it after Selene, the Greek moon goddess, as a companion to tellurium (named for Earth).
Applications
Thin-film solar cells use copper indium gallium selenide absorbers
Anti-dandruff shampoos contain selenium sulfide
Glassmakers add selenium to decolorize or tint glass red
Dietary supplements provide selenium for antioxidant enzymes
Photocopiers historically used selenium's light-sensitive conductivity
Common compounds
- selenium dioxide
- selenic acid
- sodium selenite
- hydrogen selenide
Eating too many Brazil nuts can give you selenium poisoning — each nut holds a day's worth.
Data source: IUPAC standard atomic weights 2021