034Reactive nonmetal · Group 16 · Period 4
Selenium
Selenium (Se) is element 34, a reactive nonmetal in the p-block — group 16, period 4. It is a solid at room temperature with 4 electron shells (2, 8, 18, 6) and an atomic mass of 78.9718 u.
- Atomic mass
- 78.97 u
- Phase · STP
- solid
- Melting
- 494 K · 221 °C
- Boiling
- 958 K · 685 °C
- Electronegativity
- 2.55
- Density
- 4.81 g/cm³
Photograph — Hi-Res Images ofChemical Elements, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons, source: https://images-of-elements.com/selenium.php
Identity
- Atomic number
- 34
- Standard atomic weight
- 78.97 u
- Category
- Reactive nonmetal
- Group
- 16 · Chalcogens
- Period
- 4
- Block
- p-block
- Oxidation states
- +6, +4, -2
Physical properties
- Melting point
- 494 K · 221 °C
- Boiling point
- 958 K · 685 °C
- Density
- 4.81 g/cm³
- Molar heat capacity
- 25.363 J/(mol·K)
- Atomic radius
- 190 pm
- Electron affinity
- 194.9587 kJ/mol
Atomic structure
34protons
45neutrons
34electrons
6valence e⁻
- Noble-gas notation
- [Ar] 3d10 4s2 4p4
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4
- Electrons per shell
- 2, 8, 18, 6
Periodic trends
Where Selenium ranks across all 118 elements.
Ionization energies
Energy to remove each successive electron (kJ/mol).
9411
2.0k2
3.0k3
4.1k4
6.6k5
7.9k6
15.0k7
Successive ionization energies (kJ/mol, log scale).
Discovery
- Discovered
- 1817
- Discovered by
- Jöns Jakob Berzelius
- Named by
- —
- Appearance
- black, red, and gray (not pictured) allotropes