Selenium (Se) – element 34
Selenium is a chemical element with the symbol Se and atomic number 34. It is a reactive nonmetal in group 16 (chalcogens), period 4 of the periodic table (p-block), and is a solid at room temperature.
Selenium (Se) has atomic number 34, atomic mass 78.971 u and electron configuration [Ar] 3d10 4s2 4p4.
Selenium facts
| Symbol | Se |
|---|---|
| Atomic number | 34 |
| Atomic mass | 78.971 u (g/mol) |
| Category | reactive nonmetal |
| Group | group 16 (chalcogens) |
| Period | 4 |
| Block | p-block |
| Phase at 20 °C | Solid |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 |
| Noble-gas notation | [Ar] 3d10 4s2 4p4 |
| Electrons per shell | 2, 8, 18, 6 |
| Valence electrons | 6 |
Bohr model and shells
Selenium has 34 electrons in 4 shells: 2, 8, 18, 6. Full configuration and noble-gas notation are in the table; work out any element or ion with the electron configuration calculator.
Physical and atomic properties
| Melting point | 494 K (220.9 °C) |
|---|---|
| Boiling point | 958 K (684.9 °C) |
| Density | 4.81 g/cm³ |
| Electronegativity (Pauling) | 2.55 |
| Electron affinity | 194.959 kJ/mol |
| Ionization energies | 941, 2045, 2973.7 kJ/mol (1st, 2nd, 3rd) |
| Molar heat capacity | 25.363 J/(mol·K) |
| Discovered by | Jöns Jakob Berzelius |
Use the atomic mass in the molar mass calculator for any formula containing Se.
Position in the periodic table
Tap any element to open its page.
Frequently asked questions
What is the atomic mass of selenium?
The atomic mass of selenium is 78.971 u, so one mole of selenium atoms weighs 78.971 g.
What is the electron configuration of selenium?
Selenium has 34 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴, or [Ar] 3d¹⁰ 4s² 4p⁴ in noble-gas notation. Electrons per shell: 2, 8, 18, 6.
How many valence electrons does selenium have?
Selenium has 6 valence electrons (group 16).
Is selenium a metal?
No. Selenium is a nonmetal (reactive nonmetal).
Where is selenium on the periodic table?
Selenium is in group 16 (chalcogens), period 4, in the p-block.
Data: Periodic-Table-JSON (CC BY-SA 3.0); standard atomic weights from IUPAC (abridged). Values in brackets are the mass number of the longest-lived isotope.