34
Se
Selenium
78.971
Selenium (Se)
NonmetalGroup 16Period 4p-block-block
Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.
Latin name: Selenium
Atomic Number
34
Atomic Mass
78.971 g/mol
Phase (STP)
Solid
Electron Config.
[Ar] 3d¹⁰ 4s² 4p⁴
General
SymbolSe
Atomic Number34
Atomic Mass78.971 g/mol
CategoryNonmetal
Group16
Period4
Blockp-block
Electron Configuration[Ar] 3d¹⁰ 4s² 4p⁴
Valence Electrons6
Colourgray metallic or red (allotropic)
Appearanceblack, red, and gray (not pictured) allotropes
CAS NumberCAS7782-49-2
Physical Properties
Phase (STP)Solid
Density4.81 g/cm³
Melting Point220.85°C · 429.53°F · 494.00 K
Boiling Point684.85°C · 1264.73°F · 958.00 K
Atomic Properties
Atomic Radius103 pm
Electronegativity2.55 Pauling
Ionization Energy941 kJ/mol
Oxidation States+6, +4, -2
Ion ChargeSe2-
Thermodynamic
Molar Heat25.363 J/(mol·K)
Crystal Structure
Crystal Structurehexagonal
Mechanical Properties
Mohs Hardness2
Abundance
In Earth's Crust0.0050%
Discovery & History
Discovery Year1817
Discovered ByJöns Jakob Berzelius
Country of DiscoverySweden
Isotopes of Selenium
| Isotope | Abundance | Half-life | Decay |
|---|---|---|---|
| Se-74 | 0.89% | Stable | — |
| Se-76 | 9.37% | Stable | — |
| Se-77 | 7.63% | Stable | — |
| Se-78 | 23.77% | Stable | — |
| Se-79 | — | 3.27 × 10⁵ years | β⁻ |
| Se-80 | 49.61% | Stable | — |
| Se-82 | 8.73% | 9.2 × 10¹⁹ years | β⁻β⁻ |