ElementsWhat is selenium?
Selenium can be a red powder or a grey solid. It conducts electricity better in light than in the dark. The body needs a very small amount, but too much is harmful.
What is selenium used for?
Glass making, pigments, feed and food supplements, and solar cells.
Where the name comes from
From the Greek word selene, the Moon.
Key facts
- Symbol
- Se
- Atomic number
- 34
- Atomic mass
- 78.97 u
- Family
- Nonmetal
- Period
- 4
- Group
- 16
- Block
- p
- State at room temperature
- Solid
- Melting point
- 220.5 °C (493.65 K, 429 °F)
- Boiling point
- 684.9 °C (958 K, 1,265 °F)
- Density
- 4.81 g/cm³
- Electrons per shell
- 2, 8, 18, 6
- Electron configuration
- [Ar]4s2 3d10 4p4
- Common oxidation states
- +6, +4, -2
- Electronegativity (Pauling)
- 2.55
- First ionisation energy
- 9.752 eV
- Electron affinity
- 2.021 eV
- Atomic radius (van der Waals)
- 190 pm
- Discovered
- 1817
Place in the periodic table
Selenium is element number 34. It sits in period 4 (row 4) of the periodic table, in group 16 (column 16), and belongs to the p-block. Just before it comes arsenic (33), and just after it comes bromine (35). In the same column, sulfur is above it and tellurium is below it. Elements in one column tend to behave alike.
Family: Nonmetals.
Physical properties
At room temperature selenium is a solid. It melts at 220.5 °C and boils at 684.9 °C, so it stays liquid over a range of 464 degrees. Its density is 4.81 grams per cubic centimetre, so a one-litre block would weigh about 4.81 kilograms. By density it ranks 65th among the 96 elements with a measured value.
Inside the atom
An atom of selenium has 34 protons in its nucleus and, when neutral, 34 electrons around it. The electrons are arranged in 4 shells: 2, 8, 18, 6. The outermost shell holds 6 electrons. Written in full, the electron configuration is [Ar]4s2 3d10 4p4.
Its electronegativity on the Pauling scale is 2.55. This number shows how strongly an atom pulls shared electrons towards itself: fluorine, the strongest, is 3.98. Removing one electron from an atom takes 9.752 electronvolts (about 941 kilojoules per mole). One mole of selenium weighs 78.97 grams, so a single gram contains about 7.63 × 1021 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains selenium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.