Selenium (Se)
Selenium is element 34, a reactive nonmetal in period 4, group 16 of the periodic table. At room temperature it is a solid.
Electron configuration [Ar] 3d10 4s2 4p4
Electrons per shell 2, 8, 18, 6
Selenium data
| Atomic number | 34 |
|---|---|
| Symbol | Se |
| Atomic mass | 78.96 u |
| Category | Reactive nonmetal |
| Group | 16 |
| Period | 4 |
| Block | p |
| State at 25 °C | solid |
| Melting point | 494 K (220.9 °C) |
| Boiling point | 958 K (684.9 °C) |
| Density | 4.819 g/cm³ |
| Electronegativity (Pauling) | 2.55 |
| Atomic radius | 116 pm |
| 1st ionization energy | 941 kJ/mol |
| Electron affinity | -195 kJ/mol |
| Oxidation states | -2, +2, +4, +6 |
| Common ion charge | 2− |
| Discovered | 1817 |
Orbital diagram
Each box is one orbital holding up to two electrons of opposite spin. Following Hund’s rule, electrons spread out singly across a subshell before pairing.
Work through selenium’s configuration and its ions →
Where selenium sits
Selenium belongs to the reactive nonmetals. Its Pauling electronegativity of 2.55 is mid-range; see the electronegativity chart and periodic trends for context. To use its mass in a calculation, try the molar mass calculator.
Selenium FAQ
What is the electron configuration of selenium?
Selenium (Z = 34) has the configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4. In noble-gas shorthand that is [Ar] 3d10 4s2 4p4.
How many valence electrons does selenium have?
6 (outer shell (n = 4)).
What is the atomic mass of selenium?
The standard atomic weight of selenium is 78.96 u, so one mole weighs 78.96 g.
Is selenium a metal?
No. Selenium is a reactive nonmetal.
What is the charge of a selenium ion?
Selenium most commonly forms Se2−.