3478.971SeSelenium
Selenium (Se) — element 34reactive nonmetal · group 16, period 4
Selenium is a chemical element with the symbol Se and atomic number 34. It is a reactive nonmetal in group 16, period 4 of the periodic table (the p-block), with a standard atomic mass of 78.971 u. At room temperature (25 °C) selenium is a solid.
💡 Known for: Used in glassmaking and solar cells, and an essential trace nutrient in small amounts.
# Selenium properties
| Symbol | Se |
|---|---|
| Atomic number | 34 |
| Atomic mass | 78.971 u |
| Category | reactive nonmetal |
| Group | 16 |
| Period | 4 |
| Block | p-block |
| Electron configuration | [Ar] 3d¹⁰ 4s² 4p⁴ |
| Full configuration | 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴ |
| Electrons per shell | 2, 8, 18, 6 |
| Protons / electrons | 34 / 34 |
| State at 25 °C | solid |
| Melting point | 493.65 K (220.5 °C, 428.9 °F) |
| Boiling point | 958 K (684.9 °C, 1264.7 °F) |
| Density | 4.809 g/cm³ |
| Electronegativity (Pauling) | 2.55 |
| First ionization energy | 9.752 eV |
| Atomic radius | 103 pm |
| Discovered | 1817 |
# Electron configuration of selenium
The electron configuration of selenium is [Ar] 3d¹⁰ 4s² 4p⁴, written out in full as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴. Its 34 electrons fill 4 shells as 2, 8, 18, 6, so the outermost shell holds 6 electrons. As a group 16 element, selenium has 6 valence electrons.
# Related elements
AsArsenicprevious elementBrBrominenext elementOOxygengroup 16SSulfurgroup 16TeTelluriumgroup 16PoPoloniumgroup 16LvLivermoriumgroup 16HHydrogenreactive nonmetalCCarbonreactive nonmetalNNitrogenreactive nonmetalPPhosphorusreactive nonmetal
# Selenium FAQ
What is the atomic number of selenium?
The atomic number of selenium is 34: every selenium atom has 34 protons in its nucleus. Its symbol is Se.
What is the atomic mass of selenium?
The standard atomic mass of selenium is 78.971 u.
What is the electron configuration of selenium?
Selenium's electron configuration is [Ar] 3d¹⁰ 4s² 4p⁴ (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴), with 2, 8, 18, 6 electrons per shell.
Is selenium a metal?
No. Selenium is a reactive nonmetal, a nonmetal.
What is the melting point of selenium?
Selenium melts at 493.65 K (220.5 °C, 428.9 °F) and boils at 958 K (684.9 °C, 1264.7 °F).
What is selenium used for / known for?
Used in glassmaking and solar cells, and an essential trace nutrient in small amounts.