Selenium (Se)
Element 34 · Nonmetal · atomic mass 78.971 u · group 16, period 4
Electron shells: 2, 8, 18, 6
Selenium's electrical resistance drops sharply in light, which made photocopying possible. It is an essential nutrient with an unusually narrow safe range.
Xerox photocopying was built on selenium's habit of conducting only when light hits it.
- Atomic number
- 34
- Atomic mass
- 78.971 u
- Electron configuration
- [Ar] 4s² 3d¹⁰ 4p⁴
- Group · period · block
- 16 · 4 · p
- Oxidation states
- −2, +4, +6
- Electronegativity
- 2.55 (Pauling)
- Melting point
- 221 °C
- Boiling point
- 685 °C
- Density
- 4.809 g/cm³
- State at 20 °C
- Solid
- Appearance
- Grey metallic or red powder
- Discovered
- Jöns Jacob Berzelius, 1817
34 protons, and 34 electrons when neutral
IUPAC standard atomic weight
Chalcogens
What Selenium is used for
- Glass decolourising and red glass
- Photocopier and laser-printer drums
- Anti-dandruff shampoos
- Animal-feed supplements
Atomic structure of Selenium
A neutral selenium atom has 34 protons and 34 electrons. Its most common isotope carries about 45 neutrons, which is where the mass number 79 comes from. Change the neutron count and you have a different isotope of selenium; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 18, 6. In orbital notation that is [Ar] 4s² 3d¹⁰ 4p⁴, leaving 6 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Selenium's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Selenium sits in the periodic table
Poor conductors that gain or share electrons rather than losing them. They make up almost all the mass of living things.
Discovery and naming
Selenium was discovered by Jöns Jacob Berzelius in 1817. Its name comes from Greek selene, "moon".
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Selenium?
The standard atomic weight of Selenium is 78.971 u, as published by IUPAC's Commission on Isotopic Abundances and Atomic Weights.
How many protons, neutrons and electrons does Selenium have?
Selenium has 34 protons, because the atomic number is the proton count and that is what makes an atom Selenium at all. A neutral Selenium atom has the same number of electrons, 34. The neutron count varies by isotope; the most common one has about 45, since 79 − 34 = 45.
What is the electron configuration of Selenium?
The electron configuration of Selenium is [Ar] 4s² 3d¹⁰ 4p⁴. Its electrons are arranged 2, 8, 18, 6 across 4 shells.
Is Selenium a metal?
No. Selenium is classified as a nonmetal, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Selenium in?
Selenium is in group 16 (the chalcogens), period 4, in the p-block. The group is its column and the period is its row.
What is the charge of Selenium?
Selenium most commonly forms a 2− ion. It is 2 electrons short of a full outer shell, so it gains them rather than losing 6. Across all its compounds it shows oxidation states of −2, +4, +6.
What is the melting point of Selenium?
Selenium melts at 221 °C and boils at 685 °C. That puts it in the solids at everyday temperatures.
What is Selenium used for?
Glass decolourising and red glass, Photocopier and laser-printer drums, Anti-dandruff shampoos, Animal-feed supplements. Selenium's electrical resistance drops sharply in light, which made photocopying possible.
Who discovered Selenium, and where does its name come from?
Selenium (Se) was discovered by Jöns Jacob Berzelius in 1817. The name comes from Greek selene, "moon".