Periodic table › Selenium
1. Identity and classification
Non-conducting gas or brittle molecular/network solid
- Name and symbol
- Selenium (Selenium) · Se
- Atomic number
- 34
- Relative atomic weight / mass number
- 78.971 ± 0.008
- Group · period · block
- 16 · 4 · p
- Category
- Reactive Nonmetal
- State near room temperature
- Solid
- Stable isotopes
- At least one stable isotope
- CAS number
- 7782-49-2
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 34 · 34
- Neutrons in reference isotope
- 46 (80Se)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴- Abbreviated configuration
[Ar]4s² 3d¹⁰ 4p⁴- Electrons per shell
- 2, 8, 18, 6
- Listed oxidation states
- +6, +4, -2
- Electronegativity
- 2.55 (Pauling)
- First ionization energy
- 940.925 kJ/mol · 9.752 eV
- Electron affinity
- 194.997 kJ/mol
- Covalent radius
- 120 pm
- van der Waals radius
- 190 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 4.809 g/cm³
- Melting point
- 493.65 K · 220.5 °C · 428.9 °F
- Boiling point
- 958 K · 684.85 °C · 1264.73 °F
- Crystal structure
- Hexagonal
- Specific heat
- 0.321 J/(g·K)
- Thermal conductivity
- 0.519 W/(m·K)
- Enthalpy of fusion
- 6.69 kJ/mol
- Enthalpy of vaporization
- 95.48 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- 2
- Speed of sound
- 3350 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
The listed electron configuration and oxidation states describe atomic structure; compound properties depend on bonding and composition.
Selected compounds: SeO / Se₂O₃ (Oxide) · SeClₙ (Chloride) · SeFₙ (Fluoride)
Reaction reference
Oxidation behavior depends on temperature, surface passivation and the chemical environment.
Direct reaction with water is element- and condition-dependent.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 74Se | Stable | 0.86% | Stable |
| 76Se | Stable | 9.23% | Stable |
| 77Se | Stable | 7.60% | Stable |
| 78Se | Stable | 23.69% | Stable |
| 79Se | Radioactive | — | 3.27E+5 Y |
| 80Se | Stable | 49.80% | Stable |
| 82Se | Radioactive | 8.82% | 9.6E+19 Y |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1817 CE
- Scientists associated with discovery
- Jöns Jacob Berzelius & Johan Gottlieb Gahn
- Location
- Sweden (Stockholm)
- Origin
- Natural
- Listed crustal abundance
- Not established
Minerals and naming reference
Selenide impurities in copper sulfide anode slimes, Crooksite
Named from scientific/historical roots associated with Selenium (Se), discovered or characterized by Jöns Jacob Berzelius & Johan Gottlieb Gahn.
7. Applications
Antioxidant glutathione peroxidase enzyme in humans, glass decolorizing & ruby-red glass, CIGS thin-film solar cells, anti-dandruff shampoos
Detailed applications by field
- everyday
- Antioxidant glutathione peroxidase enzyme in humans, glass decolorizing & ruby-red glass, CIGS thin-film solar cells, anti-dandruff shampoos
- industry
- Used in reactive nonmetal processing, specialized alloys, and chemical manufacturing (Antioxidant glutathione peroxidase enzyme in humans, glass decolorizing & ruby-red glass, CIGS thin-film solar cells, anti-dandruff shampoos).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Selenium's electronic structure ([Ar] 3d¹⁰ 4s² 4p⁴) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Selenium.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Selenium.
- agriculture
- Soil geochemistry analysis, trace micronutrient research, or analytical environmental monitoring.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 4 p-block elements.
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Classified as essential in human and biological systems.