Se
Selenium
Element 34 · Reactive nonmetal
Selenium (Se)
Selenium is a nonmetal whose electrical conductivity rises in light. It is an essential trace nutrient but toxic in excess.
- Reactive nonmetal
- Period 4
- Group 16
- p-block
- solid at room temperature
Key properties
- Atomic mass
- 78.97
- u
- Electron configuration
- [Ar] 3d10 4s2 4p4
- Melting point
- 220.5 °C
- 493.65 K
- Boiling point
- 684.9 °C
- 958 K
- Density
- 4.809 g/cm³
- Electronegativity
- 2.55
- Pauling
- Atomic radius
- 190 pm
- van der Waals
- 1st ionisation energy
- 9.752 eV
Compare with every element:Electronegativity chartAtomic radius chartIonisation energy chartDensity chart of the elementsMelting point chartElectron configuration chart
Did you know?
Notable facts about selenium
- 01
Selenium’s light sensitivity made it key to early photocopiers and light meters.
- 02
Brazil nuts are so rich in selenium that a couple a day meets adult needs.
- 03
Selenium is added to glass to cancel the green tint from iron or to make it red.
Applications
What is selenium used for?
- Glass decolourising and colouring
- Dietary supplements
- Solar cells (CIGS)
- Anti-dandruff shampoo
History
Discovery & name origin
Discovered by
Year
Where
Sweden
Name origin
Greek selene, “moon” — named as a partner to tellurium, “earth”.
Atomic structure
Atomic properties
Atomic structure
Atomic properties
- Atomic number
- 34
- Atomic mass
- 78.97 u
- Group
- 16
- Period
- 4
- Block
- p-block
- Electron configuration
- [Ar] 3d10 4s2 4p4
- Full configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4
- Electrons per shell
- 2, 8, 18, 6
- Electronegativity (Pauling)
- 2.55
- Electron affinity
- 2.021 eV
- Atomic radius (van der Waals)
- 190 pm
- Stable isotopes
- 5
Oxidation states
Highlighted: common oxidation states (+6, +4, -2).
Successive ionisation energies
Energy to remove each electron in turn (kJ/mol). Big jumps show where a new, inner shell begins.
- 1st941
- 2nd2,045
- 3rd2,973.7
- 4th4,144
- 5th6,590
- 6th7,880
- 7th14,990
Physical
Physical properties & colour
Physical
Physical properties & colour
- State at 20 °C
- Solid
- Melting point
- 493.65 K · 220.5 °C · 428.9 °F
- Boiling point
- 958 K · 684.9 °C · 1,264.7 °F
- Density
- 4.809 g/cm³
- Molar heat capacity
- 25.363 J/(mol·K)
- Appearance
- Black, red, and grey (not pictured) allotropes
- CPK model colour
- Copied
The CPK colour is the convention used for this element’s atoms in molecular models.
FAQ
Selenium questions
What is Se on the periodic table?
Se is the symbol for selenium, element 34 on the periodic table. It sits in group 16, period 4, in the reactive nonmetal category, and has an atomic mass of 78.97.
What is the atomic number of selenium?
Selenium has atomic number 34, so every selenium atom has 34 protons in its nucleus. Its symbol is Se and its atomic mass is 78.97.
Who discovered selenium?
Selenium was discovered by Jöns Jacob Berzelius in 1817 (Sweden). The name comes from Greek selene, “moon” — named as a partner to tellurium, “earth”.
What is selenium used for?
Selenium is used for: glass decolourising and colouring; dietary supplements; solar cells (CIGS); anti-dandruff shampoo.
Is selenium a metal?
No. Selenium is classed as a reactive nonmetal.
What is the electron configuration of selenium?
The electron configuration of selenium is [Ar] 3d10 4s2 4p4. Its 34 electrons are arranged in shells as 2, 8, 18, 6.