Selenium (Se): electron configuration, atomic mass and facts
Selenium (Se) is element 34 of the periodic table, a reactive nonmetal in period 4, group 16. Its electron configuration is [Ar] 3d10 4s2 4p4, with electrons arranged in shells 2, 8, 18, 6, and its standard atomic weight is 78.971 u.
▶ Watch Selenium’s electrons orbit in the interactive periodic tableSelenium at a glance
| Symbol | Se |
|---|---|
| Atomic number | 34 |
| Atomic mass | 78.971 u |
| Category | Reactive nonmetal |
| Group | 16 |
| Period | 4 |
| Block | p-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4 |
| Noble-gas shorthand | [Ar] 3d10 4s2 4p4 |
| Electrons per shell | 2, 8, 18, 6 |
| Outer-shell electrons | 6 |
| Electronegativity (Pauling) | 2.55 |
| State at 20 °C | solid |
Where is Selenium on the periodic table?
Selenium is in period 4, group 16, in the p-block. The period number matches its 4 electron shells, and the block names the subshell its last electrons fill. In the same group, Sulfur sits directly above it and Tellurium sits directly below it. Elements in one group have the same number of outer-shell electrons (6 here), which is why they react in similar ways.
How to work out the electron configuration of Selenium
Put 34 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁴ (34). In shorthand, [Ar] stands for the 18 electrons of argon, giving [Ar] 3d¹⁰ 4s² 4p⁴.
How electronegative is Selenium?
Selenium has a Pauling electronegativity of 2.55, ranking 10 of the 100 elements with a known value (fluorine is highest at 3.98). That is high: it pulls shared electrons strongly towards itself in a bond.
Frequently asked questions
What is the electron configuration of Selenium?
The electron configuration of Selenium (Se) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4. In noble-gas shorthand it is written [Ar] 3d10 4s2 4p4.
How many electrons does Selenium have in each shell?
Selenium has 34 electrons arranged in shells as 2, 8, 18, 6. It has 6 electrons in its outer shell.
What is the atomic mass of Selenium?
The standard atomic weight of Selenium is 78.971 u, so one mole of Selenium atoms has a mass of about 78.971 g.
Is Selenium a metal?
No. Selenium is a reactive nonmetal.
Explore nearby elements
More reactive nonmetals: Hydrogen, Carbon, Nitrogen, Oxygen, Phosphorus, Sulfur. See all elements on the periodic table.