Chemical element · Atomic number 34
Selenium
Selenium in the periodic table: atomic number 34, electron configuration, atomic mass, physical data, oxidation states, media credit and visible sources.
Nonmetal
solid
78.97 u
Documented element sampleA documented sample of elemental selenium, whose color ranges from gray through red to black depending on the allotrope.
Image credit: James St. John
Auto-oriented, limited to 1600 × 1200 pixels and re-encoded as WebP; the subject was not altered.
Atomic classification
Shell occupancy
Selenium in the Bohr shell model
This shows the electron distribution of the neutral atom in a simplified shell model.
- K · n=1
- 2 electrons
- L · n=2
- 8 electrons
- M · n=3
- 18 electrons
- N · n=4
- 6 electrons
- Electron configuration
- [Ar]4s2 3d10 4p4
- Electrons per shell
- 2 · 8 · 18 · 6
- Group
- 16
- Period
- 4
- Block
- P
- Element category
- Nonmetal
Physical and chemical properties
- Atomic mass
- 78.97 u
- Standard state
- solid
- Density
- 4.809 g/cm³
- Melting point
- 493.65 K
- Boiling point
- 958 K
- Electronegativity
- 2.55 (Pauling)
- First ionisation energy
- 9.752 eV
- Oxidation states
- +6, +4, -2
- Discovery
- 1817
Safety and periodic classification
Safety
Safe handling cannot be inferred from Selenium's position in the periodic table alone. Laboratory, classroom and disposal decisions must follow the documentation for the exact material and its safety data sheet.
Position and comparison
Selenium is in period 4, group 16 and the P block. Its direct neighbours by atomic number are Arsenic and Bromine. The recorded Pauling electronegativity is 2.55. Periodic trends are compared only through the separately sourced neighbouring values.
Sources and scope
PubChem attributes element data to sources including IUPAC, NIST and IAEA. Quanta stores the referenced snapshot locally and leaves unknown values unavailable.
Where Selenium sits among the 118 elements
The table compares the reported values for Selenium with every other element that carries the same value in the snapshot. A rank is not a judgement, it is orientation: it says how unusual this value is across the periodic table.
| Value | Selenium | Position in the data set |
|---|---|---|
| Melting point | 493.65 K | Rank 79 of 103 elements with a reported value. Highest: Carbon. |
| Boiling point | 958 K | Rank 75 of 93 elements with a reported value. Highest: Rhenium. |
| Density | 4.809 g/cm³ | Rank 65 of 96 elements with a reported value. Highest: Osmium. |
| Electronegativity (Pauling) | 2.55 | Rank 11 of 95 elements with a reported value. Highest: Fluorine. |
| Ionisation energy | 9.752 eV | Rank 19 of 102 elements with a reported value. Highest: Helium. |
All values and ranks come from the PubChem Periodic Table snapshot, retrieved on 2026-07-15. Elements without a reported value are excluded, so the population differs per row.
The neighbours of Selenium in the periodic table
- Place in the table
- Group 16, period 4, P block. Classified as nonmetal, solid at standard conditions.
- Direct neighbours
- Atomic number 34. Preceded by Arsenic, followed by Bromine.
- Same group
- Same number of outer electrons, and therefore similar reactivity to Oxygen, Sulfur, Tellurium, Polonium, Livermorium.
- Discovery
- Recorded in the snapshot as: 1817. Who, where and under what circumstances is not part of that record, so this page carries the sourced year and no discovery narrative.
- Oxidation states
- +6, +4, -2. They follow from the electron configuration [Ar]4s2 3d10 4p4 and indicate which compounds to expect.
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