Selenium (Se)
Reactive nonmetal
Selenium is a nonmetal whose electrical conductivity changes with light — a property that powered early photocells and photocopiers. In trace amounts it is an essential antioxidant nutrient; in larger doses it is toxic.
Position on the periodic table
Atomic properties
| Atomic number | 34 |
|---|---|
| Atomic mass | 78.9718 amu |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4 |
| Noble gas shorthand | [Ar] 3d10 4s2 4p4 |
| Electrons per shell | 2, 8, 18, 6 |
| Valence electrons (outer shell) | 6 |
| Common oxidation states | -2, +4, +6 |
| Electronegativity (Pauling) | 2.55 |
| Covalent radius (approx.) | 120 pm |
| First ionization energy | 941 kJ/mol |
| Electron affinity | 195 kJ/mol |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Density | 4.81 g/cm³ |
| Melting point | 220.85 °C (494 K) |
| Boiling point | 684.85 °C (958 K) |
| Appearance | black, red, and gray (not pictured) allotropes |
| Radioactive | No |
| Origin | Occurs naturally |
Electron configuration of selenium
Selenium's ground-state electron configuration is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4, usually shortened to [Ar] 3d10 4s2 4p4. Its electrons occupy 4 shells (2, 8, 18, 6), placing it in period 4 of the p-block. The 6 outer-shell electrons drive its bonding behaviour. Explore it interactively in the electron configuration calculator.
Uses of selenium
- Glass decolourising and red glass pigment
- Photocells and light meters
- Anti-dandruff shampoos (selenium sulfide)
- Dietary supplement (in trace amounts)
Biological role: Essential trace element — selenium enzymes protect cells from oxidative damage and support thyroid function.
Occurrence: Recovered mainly from the sludge of copper refining.
Common compounds of selenium
SeO2
Selenium dioxide
H2Se
Hydrogen selenide
History and discovery
Discovered: 1817 — Jöns Jakob Berzelius. Name origin: From Greek selene, the Moon — a companion to tellurium, named after the Earth.
Jöns Jacob Berzelius discovered selenium in 1817 in the residue of a sulfuric acid plant.
Safety notes
The gap between the needed dose and a toxic dose of selenium is narrow — never exceed supplement guidelines.
Educational context only — always follow your school's laboratory rules and never handle chemicals without proper supervision. See our disclaimer.
Practice questions
Quick practice: Selenium
1. What is the chemical symbol of Selenium?
2. What is the atomic number of Selenium?
3. Which category does Selenium belong to?
4. What is the state of Selenium at room temperature?
5. Which period of the periodic table is Selenium in?
Selenium FAQs
What is the atomic number of selenium?
Selenium's atomic number is 34 — every selenium atom has 34 protons in its nucleus.
What is the symbol for selenium?
Se. From Greek selene, the Moon — a companion to tellurium, named after the Earth.
Is selenium a metal, nonmetal, or metalloid?
Selenium is classified as a reactive nonmetal.
What state is selenium at room temperature?
At about 25 °C, selenium is a solid. It melts at 220.85 °C.
How many valence electrons does selenium have?
Selenium has 6 electrons in its outer shell (shell pattern: 2, 8, 18, 6).
What is selenium used for?
Key uses include: glass decolourising and red glass pigment; photocells and light meters; anti-dandruff shampoos (selenium sulfide).
Related elements
Keep working with this element
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