34 Se Selenium 78.96
Nonmetal p-block Period 4 Group 16

Selenium

Se · Element 34 · Chalcogens

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

STATE AT 20°C Solid
ATOMIC MASS 78.96 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 453 K 24%
Boiling point 958 K 23%
Specific heat 0.321 J/g·K
Thermal conductivity 2.04 W/m·K 18%

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 91%

Occurrence

Abundance in crust 0.05 mg/kg 40%

Identity

SymbolSe
Atomic number34
Atomic mass78.96 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.

Size, to scale

How big is a selenium atom?

Radius 120 pm — that is 0.12 nm, so about 4167 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Selenium is liquid over a 505 K window, from 453 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 is the remarkable metalloid that protects life from oxidative damage while powering modern photocopiers and solar panels. This fascinating element exists in multiple forms with completely different properties - it can be a red powder, a gray metal-like solid, or a black glassy material. Named after the Greek word "selene" meaning moon, selenium has gone from being considered a toxic waste product to an essential nutrient and high-tech material. Discovered in 1817 by Swedish chemist Jöns Jacob Berzelius, selenium was initially mistaken for tellurium in the residue from sulfuric acid production. Berzelius noticed this residue had a distinctive smell when burned - like radishes - and realized it was a new element. For decades, selenium was considered purely toxic with no beneficial uses, earning a reputation as an industrial poison. What makes selenium absolutely crucial today is its unique photoelectric properties. When light hits selenium, it generates electricity (photovoltaic effect) or changes its electrical conductivity (photoconductive effect). This discovery revolutionized technology, leading to photocopiers, light meters, solar cells, and photographic equipment. Before digital cameras, selenium exposure meters were essential for photography. But here's the remarkable biological twist: selenium turned out to be an essential trace element for all life. It's a key component of antioxidant enzymes like glutathione peroxidase that protect cells from damage by free radicals. Selenium deficiency can cause heart disease, weakened immunity, and reproductive problems. Brazil nuts are incredibly rich in selenium - just two nuts provide your entire daily requirement. Selenium also has fascinating optical properties. Different forms of selenium can be red, gray, or black, and it becomes electrically conductive when exposed to light. This photoconductivity made selenium crucial for early television cameras and copying machines.

The discovery of selenium

The Tellurium Impurity

Jöns Jakob Berzelius discovered selenium in 1817 while investigating residues from sulfuric acid production. Initially believing he had found tellurium contamination, Berzelius's careful analysis revealed a new element with similar properties but distinct characteristics, which he named selenium from the Greek "selene" meaning moon.

Applications

What selenium is used for

Selenium: The Photoelectric Pioneer

Selenium's unique photoelectric properties revolutionized electronics and continue to enable modern photovoltaic technology, while its biological importance makes it essential for human health and agricultural productivity.

Photovoltaic Applications

Copper indium gallium selenide (CIGS) thin-film solar cells achieve efficiencies exceeding 23% through selenium's optimal bandgap properties for solar energy conversion. These flexible solar cells enable building-integrated photovoltaics and space applications where traditional silicon panels prove impractical.

Glass Manufacturing

Selenium decolorizing removes green tints from glass caused by iron impurities, creating crystal-clear optical glass for lenses, windows, and laboratory glassware. Ruby glass production uses selenium to create brilliant red colors prized for decorative and artistic applications.

Electronic Components

Photoconductor drums in laser printers and photocopiers utilize selenium's photosensitive properties to create latent electrostatic images. Rectifier diodes historically used selenium's semiconductor properties before silicon dominance, though specialized selenium rectifiers still serve niche applications.

Selenium: Hidden Essential

  • Solar Energy Systems
    • Thin-film solar panels for residential and commercial use
    • Building-integrated photovoltaic systems
    • Flexible solar cells for curved surfaces
    • Space-grade solar panels for satellites
  • Health & Nutrition
    • Dietary supplements and multivitamins
    • Animal feed supplements (livestock and poultry)
    • Selenium-enriched foods and beverages
    • Agricultural soil amendments
  • Manufacturing & Industry
    • Glass decolorizing and tinting processes
    • Rubber vulcanization and plastic additives
    • Metal alloy production and processing
    • Chemical catalyst applications
  • Electronics & Printing
    • Laser printer and photocopier drums
    • Photographic equipment and processes
    • Electronic rectifiers and switches
    • Light-sensitive devices and sensors

Where it comes from

Natural occurrence

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

Selenium: Rare but Widespread

Selenium forms through neutron capture processes in stellar cores, with its abundance reflecting the element's position in the iron peak of nuclear binding energy. Terrestrial selenium derives from both primordial condensation and later cosmic ray interactions in the upper atmosphere.

Terrestrial Distribution

Earth's crust contains approximately 0.09 parts per million selenium, making it one of the rarer stable elements. Selenium rarely forms independent minerals, instead occurring as trace substituents in sulfur-bearing minerals and as selenide phases in precious metal deposits.

Industrial Recovery

Most commercial selenium comes as a byproduct of copper refining, where selenium concentrates in anode slimes during electrolytic copper purification. These slimes contain 5-25% selenium, recovered through roasting, acid leaching, and chemical reduction processes.

Biological Concentration

Certain plants, particularly selenium accumulator species like Astragalus and Stanleya, concentrate selenium from soils, sometimes reaching toxic levels. These "selenium indicator plants" help locate selenium-rich geological formations and can cause selenosis in grazing livestock.

Handling

Safety

Selenium Safety: Essential but Toxic

Selenium exhibits a narrow margin between essential nutrition and toxicity. While required for human health, excessive selenium exposure causes selenosis with hair loss, nail brittleness, and neurological symptoms.

Occupational Limits

  • OSHA PEL: 0.2 mg/m³ (8-hour TWA) for selenium compounds
  • NIOSH REL: 0.2 mg/m³ (10-hour TWA)
  • Hydrogen selenide: 0.05 ppm (extremely toxic gas)

Safety Measures

Respiratory protection and local exhaust ventilation prevent selenium dust and vapor exposure. Hydrogen selenide gas detection systems are essential in selenium processing facilities due to its extreme toxicity.

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 holding 2, 8, 18, 6 electrons respectively. 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 453 K (179.9 °C) and boils at 958 K (684.9 °C).

What is the atomic mass of Selenium?

The standard atomic weight of Selenium is 78.96 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

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. It is named after 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.

← Previous As Arsenic Element 33 · Metalloid Next → Br Bromine Element 35 · Halogen