Selenium
Selenium is a nonmetal that is an essential trace nutrient but can be toxic in excessive amounts.
Selenium connects the chemistry of glass with trace nutrients in biology.
Explore Selenium's atom- Atomic number
- 34
- Standard atomic weight
- 78.971(8)
- Family
- Nonmetal
- Period
- 4
- Group
- 16
- Standard state
- Solid
In everyday life
Selenium compounds can modify glass colour; the body needs small amounts of selenium.
Something to remember
Its name refers to the Moon, echoing tellurium's name for Earth.
Meet its electrons
[Ar] 4s2 3d10 4p4
Published neutral-atom ground-state configuration; excited atoms and ions can differ.
Electrons per shell: 2 · 8 · 18 · 6.
The playground shows an illustrative shell model. Real electrons are described by quantum orbitals, rather than tiny planets traveling along fixed paths.
A connection to explore
Selenium and tellurium belong to the same group and take their names from the Moon and Earth.
Meet Tellurium (Te)Properties and conditions
- Standard atomic weight
- 78.971(8)CIAAW 2024 standard atomic weight (dimensionless). Parentheses give uncertainty in the last quoted digits. Source flags: r.CIAAW standard atomic weights ↗ · Retrieved 2026-10-15
- Standard state
- SolidPubChem classification, including explicit predictions where stated. Reference temperature and pressure are not supplied in the table.PubChem periodic table ↗ · Retrieved 2026-10-15
- First ionisation energy
- 9.752 eVEnergy to remove the first electron from an isolated neutral gas-phase atom. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electronegativity
- 2.55 Pauling scaleA dimensionless relative measure; values depend on the chosen scale. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Melting point
- 493.65 KPubChem reported value. Pressure, allotrope and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Boiling point
- 958 KPubChem reported value. Pressure and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Density
- 4.809 g/cm³PubChem reported value. Temperature, pressure, allotrope and measurement uncertainty are not specified in the table; gas densities are especially condition-dependent.PubChem periodic table ↗ · Retrieved 2026-10-15
- Oxidation states
- +6, +4, -2Selected PubChem oxidation states; formal electron bookkeeping, not a list of stable free-ion charges or every possible compound.PubChem periodic table ↗ · Retrieved 2026-10-15
- Discovery
- 1817PubChem discovery attribution. Ancient knowledge and competing priority claims cannot always be represented by a single year.PubChem periodic table ↗ · Retrieved 2026-10-15
- Atomic radius
- 190 pmPubChem tabulated radius. Radius definitions depend on bonding and convention; measurement definition and uncertainty are not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electron affinity
- 2.021 eVPubChem reported gas-phase electron affinity. Sign conventions and electronic states matter; uncertainty is not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
Isotopes, abundance and decay
Isotopes share a proton count but have different neutron counts. This selection includes naturally tabulated compositions and a reference isotope for other elements. It is not a list of every known isotope.
Explore 6 isotopes of Se
| Isotope | Relative atomic mass | Natural fraction | Half-life | Decay |
|---|---|---|---|---|
| Se-74 | 73.922475934(15) | 0.0089(4) | No decay observed | ;2β+ ? |
| Se-76 | 75.919213704(17) | 0.0937(29) | No decay observed | No decay tabulated |
| Se-77 | 76.919914154(67) | 0.0763(16) | No decay observed | No decay tabulated |
| Se-78 | 77.91730928(20) | 0.2377(28) | No decay observed | No decay tabulated |
| Se-80 | 79.9165218(13) | 0.4961(41) | No decay observed | ;2β− ? |
| Se-82 | 81.9166995(15) | 0.0873(22) | 87.6 Ey (uncertainty 1.5) | ;2β−=100 |
Fractions use NIST’s terrestrial isotopic compositions, which can vary by sample. Parentheses denote uncertainty in the last digits. Half-lives and decay branches use NUBASE2020; # marks estimates, >/< mark limits. “No decay observed” follows that evaluation and does not prove permanent stability. Decay branch intensities are percentages; SF means spontaneous fission, EC electron capture, IT an isomeric transition.
Follow the science
Use these original references to explore the data and context. Atomic weights can vary with isotope composition; values for elements without stable isotopes may describe a representative isotope rather than a standard atomic weight.