34 Se Selenium 78.971
Nonmetal p-block Period 4 Group 16

Selenium

Se · Element 34 · Nonmetal

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The element your eyes need to work and photocopiers needed to exist.

State at 20°C Solid
Atomic mass 78.971 u
Electron configuration[Ar] 3d¹⁰ 4s² 4p⁴
33As All 118 35Br

Structure

The selenium atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where selenium sits among all 118 elements for that property.

Physical

Density 4.81 g/cm³ 27%
Melting point 494 K 25%
Boiling point 958 K 23%
Specific heat 0.321 J/g·K
Thermal conductivity 2.04 W/m·K 17%

Atomic

Atomic radius 120 pm 14%
Covalent radius 120 pm
Van der Waals radius 190 pm

Electronic

Electronegativity 2.55 90%
Ionisation energy 940.7 kJ/mol 83%
Electron affinity 194.9 kJ/mol 89%

Occurrence

Abundance in crust 0.05 mg/kg 40%

Identity

SymbolSe
Atomic number34
Atomic mass78.971 u
CategoryNonmetal
Blockp
Crystal structurehexagonal
Oxidation states-2, +2, +4, +6
Discovered1817
Discovered byJons Jacob Berzelius

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Same element, different neutrons

Isotopes of selenium

All selenium atoms have the same number of protons, 34, and differ only in their number of neutrons: those variants are its isotopes. The most common, Se-80, has 46 neutrons and makes up 49.80% of natural selenium.

Isotopes of selenium found in nature, with their share of the natural element, and well-known radioactive ones
ISOTOPE Neutrons Natural Abundance Half-life Decay Mode
Se-74 40 0.86% Stable —
Se-75 41 — 119.78 days ε
Se-76 42 9.23% Stable —
Se-77 43 7.60% Stable —
Se-78 44 23.69% Stable —
Se-80 most common 46 49.80% Stable —
Se-82 48 8.82% 9.6 × 10¹⁹ yrs 2β⁻

The classroom shortcut, rounded atomic mass minus atomic number, gives 45 for selenium, but the most common selenium atom has 46 neutrons: the atomic mass, 78.971, is an average over all natural isotopes.

Decay modes

ε
electron capture
2β⁻
double beta-minus decay

Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.

Size, to scale

How big is a selenium atom?

Radius 120 pm — that is 0.12 nm, so about 4.17 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Selenium is liquid over a 464 K window, from 494 K to 958 K.

Where it sits

Position in the table

Selenium sits in period 4, group 16. Everything in group 16 shares the same outer-electron count, which is why they behave so similarly.

Other nonmetals

1H 6C 7N 8O 15P 16S
All nonmetals

The story

What selenium is, and how we found it

Selenium (Se) is element 34, a nonmetal in group 16, below sulfur and above tellurium. It exists in several forms. The stable gray form is a crystalline solid that conducts electricity weakly, and conducts better when light falls on it; red selenium can be amorphous or crystalline, and black selenium is glassy. Gray selenium melts at 221 °C (494 K), boils at 685 °C (958 K) and has a density of 4.81 g/cm³. Selenium is an essential trace element for people and animals, yet toxic in larger amounts.

Atomic structure and isotopes

Selenium has 34 electrons in four shells, arranged 2, 8, 18, 6, with the configuration [Ar] 3d¹⁰ 4s² 4p⁴. Like sulfur, it forms selenides containing Se²⁻ and compounds in the +4 and +6 oxidation states, such as selenium dioxide and selenic acid.

Natural selenium has six isotopes; selenium-80 is the most abundant, at about 50%. Selenium-82 is radioactive, but with a half-life of about 10²⁰ years it is effectively stable.

The discovery of selenium

A red deposit at Gripsholm (1817)

In 1817 Jöns Jacob Berzelius and Johan Gottlieb Gahn investigated a red-brown deposit in the lead chambers of a sulfuric acid works at Gripsholm, Sweden, which burned pyrite from the Falun copper mine. At first they thought it was tellurium.

A new element

By early 1818 Berzelius had concluded that the deposit contained a new element. Because tellurium had been named after the Earth (Latin tellus), he named the new element selenium, after the Moon (Greek selene).

Light-sensitive selenium (1873)

In 1873 the English engineer Willoughby Smith found that selenium's electrical resistance drops when light falls on it. This photoconductivity led to early light meters and photocells and, from the 1950s, to xerographic photocopiers.

An essential nutrient (1957)

Long known as a poison to livestock, selenium was shown in 1957 by Klaus Schwarz and Calvin Foltz to be an essential nutrient that prevents liver damage in rats.

Applications

What selenium is used for

Glass and metallurgy

  • Glass: a trace of selenium cancels the green tint that iron gives to glass, and larger amounts color glass red or bronze, as in some tinted architectural glass.
  • Manganese production: selenium dioxide is added in the electrolytic production of manganese, a major use in China.
  • Alloys: selenium improves the machinability of some steels, copper and brass.

Electronics and energy

  • Solar cells: copper indium gallium selenide (CIGS) is used in thin-film solar panels.
  • Light-sensitive devices: selenium's photoconductivity made it the light-sensitive drum coating of early photocopiers and laser printers, and it was used in rectifiers and light meters. Silicon and organic materials have replaced it in most of these.

Agriculture and health

Selenium is added to animal feed and, in some countries with selenium-poor soils such as Finland, to fertilizers. Selenium sulfide is the active ingredient of some anti-dandruff shampoos, and selenium is sold as a dietary supplement.

In everyday life

  • Diet: Brazil nuts, seafood, meat, eggs and grains supply selenium. The NIH recommends 55 micrograms a day for adults, and a single Brazil nut can contain more than that.
  • Shampoo: anti-dandruff shampoos with selenium sulfide.
  • Glass: clear glass bottles and windows often contain a trace of selenium to remove the green tint.
  • Solar panels: some thin-film panels use CIGS.

Where it comes from

Natural occurrence

0.05 mg/kg of Earth's crust · more abundant than 40% of elements

Abundance

Selenium is rare in Earth's crust, at about 0.05 parts per million. It follows sulfur in nature and occurs mainly as selenide in place of sulfide in ores of copper, lead, silver and other metals. Selenium minerals of their own, such as clausthalite (lead selenide), are uncommon.

Production

Selenium is recovered as a byproduct, mainly from the anode slimes left when copper is refined by electrolysis. China, Japan, Russia and Belgium are among the largest refiners (USGS).

In soils and food

The selenium content of soil varies widely. Soils in parts of China and Finland are naturally low, which led to selenium deficiency, while some soils in the western United States are so rich that certain plants accumulate toxic levels and poison grazing livestock.

Handling

Safety

An essential but toxic element

Selenium is an essential nutrient: it is part of selenoproteins, including enzymes that protect cells from oxidative damage and enzymes that activate thyroid hormone. The range between enough and too much is narrow. The NIH upper limit for adults is 400 micrograms a day; long-term intake above it can cause selenosis, with garlic-smelling breath, hair and nail loss and nerve damage.

Hazardous compounds

Hydrogen selenide (H₂Se) is a highly toxic gas, and selenium dioxide is toxic and corrosive. Breathing selenium dust or fumes irritates the lungs. In the United States the OSHA limit for selenium compounds in workplace air is 0.2 mg per cubic meter.

Safe handling

  • Handle powders and compounds with ventilation, gloves and eye protection, and avoid creating dust.
  • Keep selenides away from acids, which can release hydrogen selenide.
  • Do not take selenium supplements above the recommended upper limit.

Keep exploring

Use selenium in the tools

Quick answers

Selenium: common questions

What is selenium?

Selenium (symbol Se) is element 34 on the periodic table, a nonmetal in period 4, group 16. The element your eyes need to work and photocopiers needed to exist. At room temperature it is a solid.

What is the electron configuration of selenium?

Selenium's ground-state electron configuration is [Ar] 3d¹⁰ 4s² 4p⁴, giving 4 occupied shells with the distribution 2, 8, 18, 6. Its outer shell holds 6 electrons, which is what sets its bonding behaviour.

What are the melting and boiling points of selenium?

Selenium melts at 494 K (220.9 °C) and boils at 958 K (684.9 °C).

What is the atomic mass of selenium?

The standard atomic weight of selenium is 78.971 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How many neutrons does selenium have?

The most common selenium atom, Se-80, has 46 neutrons: its mass number, 80, minus its atomic number, 34. It makes up 49.80% of natural selenium. The classroom shortcut, rounded atomic mass (78.971) minus atomic number, gives 45 instead, because the atomic mass is an average over all natural isotopes of selenium.

How dense is selenium?

Selenium has a density of 4.81 g/cm³. Water is 1.0 g/cm³, so a block of selenium is about 4.8× heavier.

What is the electronegativity of selenium?

Selenium has a Pauling electronegativity of 2.55. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered selenium, and when?

Selenium was discovered in 1817 by Jons Jacob Berzelius. Origin of the name: Greek selene, the Moon.

How common is selenium on Earth?

Selenium makes up about 0.05 mg/kg of the Earth's crust — genuinely rare, which is why it is expensive.

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