Selenium (Se)
[Ar] 3d¹⁰ 4s² 4p⁴. 2 unpaired electrons, period 4, group 16, p block.
Element or ion
Type a symbol, a name or an atomic number, add a charge such as Fe3+ or Cl-, or tap an element in the table. Orbitals fill from m = −l up, spin +½ first.
SeSelenium, Z = 34, 78.971 u
[Ar] 3d10 4s2 4p4
In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴
- Blockp block
- Period4
- Group16
- Valence6
- Unpaired2
- MagneticPara
Last electron 4p
- n4shell
- l1p subshell
- m−1orbital
- s−½spin down
Step by step
- Selenium has 34 electrons.
- Fill in (n + l) order: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.
- Write the subshells in order of n: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴.
- Replace the argon core with [Ar]: [Ar] 3d¹⁰ 4s² 4p⁴.
Selenium (Se): the numbers
| Atomic number | 34 |
|---|---|
| Atomic mass | 78.971 u |
| Period | 4 |
| Group | 16 |
| Block | p block |
| Valence electrons | 6 |
| Unpaired electrons | 2 |
| Magnetic | paramagnetic |
| Last electron | 4p: n = 4, l = 1, m = −1, s = −1/2 |
Selenium (Se): quick answers
What is the electronic configuration of selenium?
Selenium has 34 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴, or [Ar] 3d¹⁰ 4s² 4p⁴ in short.
How many unpaired electrons does selenium have?
Selenium has 2 unpaired electrons, so it is paramagnetic. Put one electron in each box of a subshell before pairing (Hund’s rule) to see them.
What are the quantum numbers of the last electron of selenium?
The last electron goes into 4p: n = 4, l = 1, m = −1, s = −1/2. Orbitals fill from m = −l up, spin +½ first.