PeriodicElementTable.com
34Se78.971
Nonmetal

Selenium

Selenium (Se) is element 34, a nonmetal in group 16, period 4, with an atomic mass of 78.971 u. It is a solid at room temperature and melts at 220.5 °C. Its electron configuration is [Ar] 4s² 3d¹⁰ 4p⁴.

Selenium is a grayish nonmetal that sits directly below sulfur on the periodic table and shares some of its chemistry, but with a distinctive talent all its own: its electrical conductivity changes dramatically when light shines on it. It's also one of the few elements that's both essential to human health in trace amounts and toxic in excess.

Group · Period
16 · 4
At room temp
solid
Melts at
493.65 K
Density
4.809 g/cm³
Discovered
1817

Key facts about selenium

Quick answers, built from the sourced data in the property tables.

What is the electron configuration of selenium?
The electron configuration of selenium is [Ar] 4s² 3d¹⁰ 4p⁴. Written out in full, that is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.
What is the density of selenium?
The density of selenium is 4.809 g/cm³.
What are the melting and boiling points of selenium?
Selenium melts at 493.65 K (220.5 °C, 428.9 °F) and boils at 958 K (684.85 °C, 1264.73 °F).
What is the electron affinity of selenium?
The electron affinity of selenium is 2.021 eV, which is about 195 kJ/mol.
How many stable isotopes does selenium have?
Selenium has 5 stable isotopes: Se-74 (0.86%), Se-76 (9.23%), Se-77 (7.6%), Se-78 (23.69%), and Se-80 (49.8%). Percentages are natural abundances.

Uses

Selenium’s photoconductivity — its ability to conduct electricity much more readily when exposed to light — made it central to the technology behind early photocopiers and light-sensitive exposure meters, where a selenium-coated surface converts patterns of light into an electrical signal. It’s also used to decolorize glass by counteracting the greenish tint that iron impurities naturally give it, and, in higher concentrations, to produce a deep ruby-red glass and certain ceramic glazes.

In nutrition, selenium is an essential trace mineral that the body uses in antioxidant enzymes and thyroid-related processes, which is why small amounts appear in multivitamins and are naturally present in foods like Brazil nuts, seafood and grains. Selenium compounds are also used in some anti-dandruff shampoos and in agriculture as a soil supplement in regions where local soils are naturally deficient in it.

History

Selenium was discovered in 1817 by Swedish chemists Jöns Jacob Berzelius and Johan Gottlieb Gahn while they were investigating reddish sediment left behind during sulfuric acid production. Berzelius initially suspected the material was tellurium, an element discovered decades earlier, but careful analysis showed it was something new that simply behaved similarly. Because tellurium had been named after the Earth, using the Latin word tellus, Berzelius named the new element selenium after Selene, the Greek goddess of the moon, treating the pairing as a kind of chemical companionship between the two elements.

Fun facts

  • Selenium was named after Selene, the Greek goddess of the moon, because it was found alongside tellurium, which had already been named after the Earth.
  • Its conductivity increases sharply when exposed to light, a property called photoconductivity that made it central to early photocopiers and light meters.
  • Selenium is an essential trace nutrient for humans, but the gap between a healthy dose and a toxic one is unusually narrow for an element.

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Frequently asked questions

Is selenium good for you or dangerous?

Both, depending on the amount. Selenium is an essential trace element that the body needs in small quantities for antioxidant enzymes and thyroid function, which is why it appears in multivitamins and some foods naturally. But because the useful dose range is narrow, consuming too much — through supplements or contaminated food or water — can cause selenium toxicity.

Why is selenium named after the moon?

Its discoverers, Jöns Jacob Berzelius and Johan Gottlieb Gahn, found selenium in 1817 in residue left over from sulfuric acid production, mixed in with tellurium, an element already named after the Earth via the Latin word tellus. Naming the new, closely related element after the moon, via the Greek word selene, continued that theme.

What was selenium's role in old photocopiers and cameras?

Selenium is photoconductive, meaning it barely conducts electricity in the dark but conducts much more readily when light hits it. Older photocopiers and light meters exploited this to translate patterns of light and shadow into an electrical signal or a static charge that could be developed into an image.

Compounds

4 notable compounds containing Se

  • H2Segas

    Hydrogen selenide

    A colorless, extremely toxic gas with a foul odor reminiscent of rotten radishes, the selenium analog of hydrogen sulfide and a byproduct of some metal refining processes.

    Used for: Dopant source gas in semiconductor and solar-cell manufacturing

  • SeO2oxide

    Selenium dioxide

    A white to pale yellow solid and the most common laboratory selenium compound, readily formed by burning selenium in air and dissolving in water to give selenous acid.

    Used for: Oxidizing agent in organic synthesis and catalyst for selective allylic oxidation

  • H2SeO3acid

    Selenous acid

    A weak, water-soluble acid formed when selenium dioxide dissolves in water, serving as the parent compound for selenite salts.

    Used for: Precursor for selenite compounds and a bluing agent in gun-metal finishing

  • Na2SeO3salt

    Sodium selenite

    A white crystalline salt that is the most common source of dietary selenium supplementation, dissolving readily to deliver selenium in a bioavailable form.

    Used for: Selenium supplement in animal feed and multivitamins

Isotopes

33 known isotopes

Swipe to see all columns →

IsotopeNatural abundanceHalf-lifeDecay modeMass excessBinding energy / nucleonDecay widthSpin/parityMagnetic momentQuadrupole moment
Se-74 stable 0.86% Stable — -72,213.21 keV 8,687.72 keV — 0+ 1.7 μN -0.36 b
Se-76 stable 9.23% Stable — -75,251.96 keV 8,711.48 keV — 0+ 0.61 μN —
Se-77 stable 7.6% Stable — -74,599.5 keV 8,694.69 keV — 1/2- 0.53356 μN 0.76 b
Se-78 stable 23.69% Stable — -77,025.95 keV 8,717.81 keV — 0+ 0.6 μN -0.26 b
Se-80 stable 49.8% Stable — -77,759.49 keV 8,710.81 keV — 0+ 2.3 μN —
Se-82 8.82% 9.6E+19 Y Double beta-minus decay (100%) -77,593.9 keV 8,693.2 keV 1.51 × 10-43 eV 0+ — —

Spin/parity describes the nucleus's intrinsic angular momentum and symmetry. Magnetic moment (in nuclear magnetons, μN) and quadrupole moment (in barns, b) describe the shape of the nucleus's charge distribution — a nonzero quadrupole moment means the nucleus isn't perfectly spherical. These are only measured for a subset of isotopes, mostly stable or long-lived ones, so many rows show "—" rather than a value.