Selenium
Atomic Data
| Atomic Number | 34 |
| Symbol | Se |
| Atomic Weight | 78.971 u |
| Density (STP) | 4.819 g/cm³ |
| Melting Point | 220.85 °C (494 K) |
| Boiling Point | 684.85 °C (958 K) |
| Electronegativity | 2.55 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4 |
| Oxidation States | −2, +2, +4, +6 |
| Phase at STP | Solid |
| Category | Reactive Nonmetal |
| Period / Group | 4 / 16 |
| CAS Number | 7782-49-2 |
Electron Configuration
[Ar] 3d10 4s2 4p4
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 4 | 34 |
| Total | 34 | 34 | ||
Isotopes of Selenium
Selenium has six naturally occurring stable isotopes. The most abundant is ⁸⁰Se, comprising 49.61% of all naturally occurring Selenium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Selenium-74 | ⁷⁴Se | 34 | 40 | 0.89 | Stable |
| Selenium-76 | ⁷⁶Se | 34 | 42 | 9.37 | Stable |
| Selenium-77 | ⁷⁷Se | 34 | 43 | 7.63 | Stable |
| Selenium-78 | ⁷⁸Se | 34 | 44 | 23.77 | Stable |
| Selenium-80 | ⁸⁰Se | 34 | 46 | 49.61 | Stable |
| Selenium-82 | ⁸²Se | 34 | 48 | 8.73 | Stable |
Abundance & Occurrence
Selenium is present in Earth's crust at approximately 0.05 ppm by mass and at approximately 3 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Inhalation toxicity: Selenium dust and selenium dioxide fumes are toxic by inhalation; exposure causes a garlic odour on the breath, nasal irritation, bronchospasm, and pulmonary oedema at higher concentrations.
- Hydrogen selenide (H2Se): Hydrogen selenide is one of the most toxic of all gases of its class: more toxic than hydrogen sulfide; even brief exposures at low ppm concentrations cause severe pulmonary damage.
- Systemic toxicity: Chronic selenium toxicity (selenosis) causes hair loss, nail brittleness, fatigue, and neurological symptoms; the margin between essential (trace) and toxic intake is narrow.
- Skin and eye contact: Selenium compounds cause skin and eye irritation; selenium dioxide solutions are corrosive, producing selenious acid on contact with moisture.
Selenium in the Real World
Real-World Uses
- Thin-film solar cells: Copper indium gallium selenide (CIGS) is a high-efficiency thin-film photovoltaic absorber; selenium is an essential component of this semiconductor used in flexible and building-integrated solar panels.
- Glassmaking and pigments: Selenium compounds impart red, orange, and pink colours to glass and ceramics; selenium dioxide is a glass fining agent, and selenium ruby glass has been used in traffic lights and decorative applications.
- Photocopiers and laser printers: Amorphous selenium drums were the photoconductor in early photocopiers and laser printers (Xerox machines); the drum charges, is selectively discharged by light, then attracts toner to form the image.
- Anti-dandruff treatment: Selenium sulfide (SeS₂) disrupts the growth of the Malassezia fungi responsible for dandruff and seborrhoeic dermatitis, used in medicated shampoos at 1–2.5% concentration.
- Nutritional supplement: Selenium is an essential trace element in human nutrition, incorporated into selenoproteins including glutathione peroxidase (antioxidant defence) and thioredoxin reductase; deficiency causes Keshan disease.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is selenium used for?
Selenium's main uses are in glass manufacturing (to decolourise glass or produce red/orange colours), as a photoelectric material in photocopiers and solar cells, and in pigments (cadmium selenide gives vivid red and orange colours). Selenium is an essential trace nutrient for humans and animals, required for the function of antioxidant enzymes, and is used in dietary supplements and animal feeds.
Is selenium toxic?
Selenium has a narrow window between essential and toxic amounts. It is an essential nutrient at trace levels (55 micrograms per day recommended for adults), but selenosis (selenium poisoning) occurs at intakes above about 400 micrograms per day, causing symptoms including brittle nails, hair loss, fatigue, and nerve damage. Selenium dioxide and other selenium compounds are acutely toxic.
How was selenium discovered?
Selenium was discovered in 1817 by Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn as a byproduct of sulfuric acid production. Berzelius initially thought it was tellurium, but recognised it as a new element. He named it after Selene, the Greek goddess of the Moon, because it was found alongside tellurium, which had been named after Earth (Tellus).
What role does selenium play in human health?
Selenium is incorporated into selenoproteins, a family of proteins that include several important antioxidant enzymes such as glutathione peroxidases and thioredoxin reductases. These enzymes protect cells from oxidative damage. Selenium is also required for normal thyroid hormone metabolism. Dietary selenium comes from grains, seafood, meat, and nuts: Brazil nuts are especially rich sources.