Atomic number 34
Selenium
Nonmetal
Quick facts
- Atomic mass
- 78.971
- Group
- 16
- Period
- 4
- State (STP)
- solid
- Electronegativity
- 2.55
- Common ox. states
- -2, +4, +6
Properties
Electron configuration
[Ar] 3d¹⁰ 4s² 4p⁴
- Ionization energy
- 941.0 kJ/mol
- Electron affinity
- 195.0 kJ/mol
- Melting point
- 494.00 K (220.9 °C)
- Boiling point
- 958.00 K (684.9 °C)
How it bonds
Selenium below sulfur: 2 bonds + 2 lone pairs in H₂Se, expanded-octet +4 and +6 in SeO₂ and SeO₃. Essential trace element (selenocysteine in the enzyme glutathione peroxidase) but toxic in larger doses.