Element 34 · Reactive nonmetal
Selenium (Se) — Element 34
Greek selene — moon
Selenium (Se) is a reactive nonmetal in Period 4, Group 16 of the periodic table with atomic number 34, standard atomic weight 78.971 u, and ground-state electron configuration [Ar] 3d¹⁰ 4s² 4p⁴. At standard temperature and pressure it is classified as a solid with 6 valence electrons and common oxidation states -2, +4, +6.
Glass, photocopiers, vitamins
What are the atomic properties and physical constants of Selenium?
Reference constants compiled from IUPAC standard atomic weights, the NIST Atomic Spectra Database, and the CRC Handbook.
- Atomic number (Z)
- 34
- Standard atomic mass
- 78.971 u
- Chemical family
- Reactive nonmetal
- Ground-state configuration
- [Ar] 3d¹⁰ 4s² 4p⁴
- Electrons per shell
- 2, 8, 18, 6
- Valence electrons
- 6
- Common ion charges / oxidation states
- -2, +4, +6
- Phase at STP
- Solid
- Pauling electronegativity
- 2.55 Pauling
- First ionization energy
- 941 kJ/mol
- Atomic radius
- 103 pm
- Melting point
- 221 °C
- Boiling point
- 685 °C
- Discovery year
- 1817
- Table coordinate
- Period 4, Group 16
- Radioactivity
- Has stable isotope(s)
How are electrons arranged across Selenium’s shells?
A neutral Selenium atom contains 34 electrons distributed across 4 occupied principal energy shells ([Ar] 3d¹⁰ 4s² 4p⁴). The table below compares Selenium’s shell occupancy against each shell’s theoretical 2n² capacity.
| Principal Shell (n) | Spectroscopic Letter | Electrons in Se | Max Shell Capacity (2n²) |
|---|---|---|---|
| n = 1 | K | 2 e⁻ | 2 e⁻ |
| n = 2 | L | 8 e⁻ | 8 e⁻ |
| n = 3 | M | 18 e⁻ | 18 e⁻ |
| n = 4 | N | 6 e⁻ | 32 e⁻ |
Frequently Asked Questions about Selenium (Se)
What is the atomic mass and atomic number of Selenium (Se)?
Selenium (Se) has atomic number Z = 34 (34 protons in its nucleus) and an IUPAC standard atomic weight of 78.971 u. It sits in Period 4, Group 16 and is classified as a reactive nonmetal.
What is the ground-state electron configuration of Selenium?
The neutral ground-state electron configuration of Selenium (Z = 34) is [Ar] 3d¹⁰ 4s² 4p⁴. Across its 4 occupied principal shells (n = 1 to n = 4), the electrons per shell are 2, 8, 18, 6, with 6 valence electrons participating in bonding.
What are the common ion charges and electronegativity of Selenium?
Selenium commonly forms -2, +4, +6 oxidation states or ion charges in compounds. Its Pauling electronegativity is 2.55 and its first ionization energy is 941 kJ/mol.
When was Selenium discovered and what is it used for?
Selenium was discovered in 1817 (Greek selene — moon). Glass, photocopiers, vitamins
How does Selenium compare to neighboring elements?
Other reactive nonmetal elements: