Selenium (Se)
Selenium (symbol Se) is the chemical element with atomic number 34: a nonmetal, solid at 0 °C, with an atomic mass of 78.971 u. Discovered in 1817 (Jöns Jacob Berzelius).
Selenium is a nonmetal our bodies can't do without: they build it into enzymes that protect our cells from oxidation. But the margin between a helpful dose and a toxic one is narrow. Because it conducts electricity better in light, it became the heart of the first photocopiers.
Key facts
- Atomic number
- 34
- Symbol
- Se
- French name
- Sélénium
- Atomic mass
- 78.971 u
- Molar mass
- 78.971 g/mol
- Family
- Nonmetals
- Position
- period 4, group 16, p-block
- State at 0 °C
- Solid
- Melting point
- 220.9 °C
- Boiling point
- 684.9 °C
- Density
- 4.81 g/cm³
- Electronegativity (Pauling)
- 2.55
- First ionization energy
- 941 kJ/mol
- Atomic radius
- 115 pm
- Electron configuration
- [Ar] 3d¹⁰ 4s² 4p⁴
- Abundance in Earth's crust
- 0.05 ppm (69th most abundant element)
- Indicative price
- 21 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1817, Jöns Jacob Berzelius
Origin of the name
From the Greek selēnē, the Moon.
What does selenium look like?
It comes in several forms: a gray solid with a metallic sheen (the most stable), a red powder, or a black glass. It looks like a darker version of its neighbor, sulfur.
Where is selenium found?
Rare, it makes up about 0.05 ppm of Earth's crust. It stands in for sulfur in metal sulfides, especially copper ones. Soils are very unevenly supplied: some regions of China have almost none, while others contain too much.
Natural sources
- Clausthalite
- Crookesite
- Naumannite
- Chalcopyrite (traces)
How is selenium produced?
Selenium is recovered from the “anode slimes” that settle out during the electrolytic refining of copper. World output is close to 3,000 tonnes a year.
Top producers
- China
- Japan
- Germany
- Belgium
- Russia
What is selenium used for?
- Electrolytic production of manganese.
- Glassmaking: decolorizing glass or tinting it ruby red.
- Thin-film solar cells (CIGS).
- Anti-dandruff shampoos (selenium sulfide).
- Dietary supplements and animal feed.
In everyday life
- Anti-dandruff shampoo
- Brazil nuts
- Red-tinted glass
- Old photocopier drum
Role in living things
Essential: it is part of some twenty “selenoproteins,” including glutathione peroxidase, which neutralizes oxidizing molecules. An adult needs about 55 µg a day; a single Brazil nut can contain more than that. Deficiency caused Keshan disease, a heart condition, in China.
Hazards of selenium
Above roughly 400 µg a day over a long period, it becomes toxic (selenosis): hair and nail loss, garlicky breath, nerve problems. Hydrogen selenide, a gas, is very dangerous.
Cosmic origin
- Supernovae: Forged in the cores of massive stars, then scattered when they explode.
History of selenium
In 1817, Swede Jöns Jacob Berzelius found a reddish residue in a sulfuric acid factory. He first thought it was related to tellurium, named after the Earth (tellus), so he named the new element after the Moon, Selene in Greek.
Did you know?
- Some selenium “accumulator” plants make livestock sick; Marco Polo's horses, which lost their hooves in China, are thought to have grazed on them.
- Photocopying (xerography), invented by Chester Carlson, used drums coated with light-sensitive selenium.
- Too much selenium gives you garlic breath.
Isotopes of selenium
Five stable isotopes, plus selenium-82, radioactive but with a half-life of nearly a hundred billion billion years. Selenium-75 is used to X-ray welds.
- selenium-74: 0.89% of natural selenium, stable
- selenium-75: radioactive, half-life 120 days. Gamma source for inspecting welds by industrial radiography.
- selenium-76: 9.37% of natural selenium, stable
- selenium-77: 7.63% of natural selenium, stable
- selenium-78: 23.77% of natural selenium, stable
- selenium-79: radioactive, half-life 327,000 years. Long-lived fission product in nuclear waste.
- selenium-80: 49.61% of natural selenium, stable
- selenium-82: 8.73% of natural selenium, radioactive, half-life 8.7 × 10¹⁹ years. Studied for double beta decay.
Related elements
Same family (nonmetals): Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Phosphorus (P), Sulfur (S).
Same column (group 16): Oxygen (O), Sulfur (S), Tellurium (Te), Polonium (Po), Livermorium (Lv).