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| Selenium | |
|---|---|
| Pronunciation | / s ə ˈ l iː n i ə m / ⓘ ( sə- LEE -nee-əm ) |
| Appearance | grey metallic-looking, red, and vitreous black (not pictured) allotropes |
| Standard atomic weight A r°(Se) | |
| [ 1 ] | 78.971 ± 0.008 · 78.971 ± 0.008 ( abridged ) |
| Selenium in the periodic table | |
| Hydrogen | Helium |
| Lithium | Beryllium |
| Sodium | Magnesium |
| Potassium | Calcium |
| Rubidium | Strontium |
| Caesium | Barium |
| Francium | Radium |
| Atomic number ( Z ) | 34 |
| Group | group 16 (chalcogens) |
| Period | period 4 |
| Block | p-block |
| Electron configuration | [ Ar ] 3d10 4s2 4p4 |
| Electrons per shell | 2, 8, 18, 6 |
| Physical properties | |
| Phase at STP | solid |
| Melting point | 494 K (220.85 °C, 429.53 °F) |
| Boiling point | 958 K (685 °C, 1265 °F) |
| Density (near r.t. ) | gray: 4.81 g/cm3 / alpha: 4.39 g/cm3 / vitreous: 4.28 g/cm3 |
| when liquid (at m.p. ) | 3.99 g/cm3 |
| Critical point | 1766 K, 27.2 MPa |
| Heat of fusion | gray: 6.69 kJ/mol |
| Heat of vaporization | 95.48 kJ/mol |
| Molar heat capacity | 25.363 J/(mol · K) |
| Specific heat capacity | 321.169 J/(kg · K) |
| 500 | 552 |
| Atomic properties | |
| Oxidation states | common: −2, +2, +4, +6 / −1, 0, +1, +3, +5 |
| Electronegativity | Pauling scale: 2.55 |
| Ionization energies | 1st: 941.0 kJ/mol · 2nd: 2045 kJ/mol · 3rd: 2973.7 kJ/mol |
| Atomic radius | empirical: 120 pm |
| Covalent radius | 120±4 pm |
| Van der Waals radius | 190 pm |
| Other properties | |
| Natural occurrence | primordial |
| Crystal structure | grey: hexagonal ( hP3 ) |
| Thermal expansion | amorphous: 37 µm/(m⋅K) (at 25 °C) |
| Thermal conductivity | amorphous: 0.519 W/(m⋅K) |
| Magnetic ordering | diamagnetic |
| Molar magnetic susceptibility | −25.0 × 10−6 cm3/mol (298 K) |
| Young's modulus | 10 GPa |
| Shear modulus | 3.7 GPa |
| Bulk modulus | 8.3 GPa |
Wikipedia
Chemical element with atomic number 34 (Se)
Selenium is a chemical element; it has symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elemental state or as pure ore compounds in Earth's crust. Selenium was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium.