About

Selenium is a nonmetal with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium. Its electrical conductivity changes with light exposure (photoconductivity), a property used in photocells, early photocopiers, and light meters. It is also an essential trace nutrient for animals, functioning in antioxidant enzymes.

Atomic Properties

Atomic Number34

Atomic Weight78.971 u

Neutrons (common)45

Electron Configuration[Ar] 3d¹⁰ 4s² 4p⁴

Full Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴

Electrons per Shell2, 8, 18, 6

Electronegativity2.55

Electron Affinity195 kJ/mol

Oxidation States+6, +4, +2, -2

Covalent Radius116 pm

Van der Waals Radius190 pm

Ionic Radius198 pm

Periodic Table Position

Group16

Period4

BlockP

Chemical SeriesOther nonmetals

Physical Properties

State at STPsolid

Appearanceblack, red, and gray allotropes

Crystal Structurehexagonal

Magnetic Orderingdiamagnetic

Speed of Sound3,350 m/s

Poisson Ratio0.33

Thermal Properties

Melting Point494 K

Boiling Point958 K

Thermal Expansion0 1/K

Thermal Conductivity0.519 W/(m·K)

Specific Heat0.321 J/(g·K)

Molar Heat25.363 J/(mol·K)

Heat of Fusion6.69 kJ/mol

Heat of Vaporization95.48 kJ/mol

Abundance

In Earth's Crust0.05 ppm

In Ocean0 ppm

In Solar System0.034 ppm

In Human Body0.2 ppm

In Universe0.03 ppm

Discovery

Discovered byJöns Jacob Berzelius & Johan Gottlieb Gahn

Discovery Year1,817

Occurrenceprimordial

CAS Number7782-49-2

Electron Configuration

Energy

↿⇂

↿

Isotopes of Selenium

Symbol ▲	Mass (u) ▲	Abundance ▲	Half-life ▲	Decay Mode ▲
⁷⁴Se	73.922470	0.89%	Stable	Stable
⁷⁵Se	74.922523	—	119.78 d	Electron Capture
⁷⁶Se	75.919210	9.37%	Stable	Stable
⁷⁷Se	76.919910	7.63%	Stable	Stable
⁷⁸Se	77.917310	23.77%	Stable	Stable
⁸⁰Se	79.916520	49.61%	Stable	Stable
⁸²Se	81.916700	8.73%	87.6 Ey	Double Beta Minus Decay

Interactive Visualization

Explore the 3D Bohr model of Selenium in our interactive viewer.

View 3D Model of Selenium