34SeSelenium78.971 u
Selenium (Se)
Selenium is a nonmetal with atomic number 34, in period 4 and group 16 of the periodic table. Its atomic mass is 78.971 u and its electron configuration is [Ar] 3d¹⁰ 4s² 4p⁴. Solid at 25 °C.
Key facts
| Atomic number | 34 |
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| Atomic mass | 78.971 u |
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| Category | Nonmetal |
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| Period | 4 |
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| Group | 16 |
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| Block | p |
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| Electron configuration | [Ar] 3d¹⁰ 4s² 4p⁴ |
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| Electrons per shell | 2, 8, 18, 6 |
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| Electronegativity (Pauling) | 2.55 |
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| Oxidation states | −2, −1, +1, +2, +3, +4, +5, +6common in bold |
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| State at 25 °C | Solid |
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| Melting point | 494 K220.85 °C |
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| Boiling point | 958 K684.85 °C |
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| Density | 4.819 g/cm³ |
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Atomic properties
| First ionization energy | 9.752 eV |
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| Electron affinity | 195 kJ/mol |
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| Atomic radius | 140 pm |
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| Covalent radius | 120 pm |
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| Van der Waals radius | 190 pm |
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| Common ions | Se⁴⁻ |
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| Protons, neutrons, electrons | 34, 45, 34 |
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Physical and thermal properties
| Molar volume | 16.42 cm³/mol |
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| Heat of fusion | 5.4 kJ/mol |
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| Heat of vaporization | 26 kJ/mol |
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| Specific heat | 321.2 J/(kg·K) |
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| Thermal conductivity | 0.52 W/(m·K) |
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| Thermal expansion | --- |
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| Mohs hardness | 2 |
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| Young’s modulus | 10 GPa |
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| Crystal structure | Simple monoclinic |
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| Magnetic type | Diamagnetic |
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Identification and history
| CAS number | 7782-49-2 |
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| Latin name | Selenium |
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| Discovered by | J. Berzelius · 1817 |
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| Named by | --- |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 74Se | 73.922476 | 0.86 % |
| 76Se | 75.919214 | 9.23 % |
| 77Se | 76.919914 | 7.6 % |
| 78Se | 77.917309 | 23.69 % |
| 80Se | 79.916522 | 49.8 % |
| 82Se | 81.9167 | 8.82 % |
Data checked against PubChem (NIH), IUPAC, the NIST Atomic Spectra Database and the IAEA (NUBASE and AME), the same data you see in the app.
A dash (---) means there is no reliable published measurement. The app never makes one up.
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Periodic Table