Category: Nonmetal
Atomic Number: 34
Period: 4 | Group: 16
Block: p-block
The element your eyes need to work and photocopiers needed to exist.
| Property | Value |
|---|---|
| Phase at STP | Solid |
| Density | N/A |
| Melting Point | 494.00 K (220.85 °C) |
| Boiling Point | 958.00 K (684.85 °C) |
| Heat Capacity | N/A |
| Property | Value |
|---|---|
| Atomic Mass | 78.9710 u |
| Electronegativity (Pauling) | N/A |
| Oxidation States | -2, +2, +4, +6 |
| Electron Configuration |
| Discovered By | Unknown |
|---|---|
| Discovery Year | 1817 |
| Named After | Greek selene, the Moon |
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.
| 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 | 46 | 49.80% | Stable | - |
| Se-82 | 48 | 8.82% | 9.6 × 10¹⁹ yrs | 2β⁻ |
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.
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.
| Earth's Crust | N/A |
|---|---|
| Universe | N/A |