Selenium – facts and properties
- Symbol
- Se
- Atomic number
- 34
- Atomic mass
- 78.971 u
- Period
- 4
- Group
- 16
- Category
- polyatomic nonmetal
- Electron configuration
- [Ar] 3d¹⁰ 4s² 4p⁴
- Electronegativity
- 2.55
- Atomic radius
- 103 pm
- Valence electrons
- 6
- State at standard conditions
- solid
- Melting point
- 220.5 °C
- Boiling point
- 685 °C
- Density
- 4.809 g/cm³
- Discovered
- 1817
Selenium is a nonmetal with atomic number 34 that sits in the oxygen group below sulfur. Its properties lie between those of sulfur and tellurium. It is an element with two faces: the body needs it, yet the harmful amount lies only a little above the daily requirement. In technology its reaction to light made it a material for photocells and early photocopiers.
Position in the periodic table
Occurrence and extraction
Selenium is rare and usually occurs in small amounts together with sulfur, for example in sulfide ores of copper, lead and zinc. Selenium minerals of its own exist but are unimportant. The element is very unevenly distributed: some soils contain a lot of selenium and others almost none, which decides how much ends up in the plants that grow there.
Selenium is obtained as a by-product of copper refining. During electrolysis a sludge settles out, the anode slime. Besides precious metals it contains selenium, which is separated by roasting and reduced to the element. Large producers include Japan, China, Germany and Russia.
The element was discovered in 1817 by the Swedish chemist Jöns Jakob Berzelius. In a red deposit from a sulfuric acid factory he first suspected tellurium but then found something new. Tellurium is named after the Earth (Latin Tellus), so he gave the new element the name of the Moon: selenium comes from Selene, the Greek moon goddess.
Properties and reactions
Selenium is in period 4 and group 16 and has the electron configuration [Ar] 3d¹⁰ 4s² 4p⁴. It has 6 valence electrons. Its electronegativity is 2.55. The most stable form has a density of 4.809 g/cm³, melts at 220.5 °C and boils at 685 °C.
Like sulfur, selenium exists in several modifications. Grey selenium consists of long chains, is shiny and conducts electricity poorly. Red selenium consists of rings of eight atoms and forms a red powder; when heated it turns black and glassy. Grey selenium is a semiconductor whose electrical conductivity rises sharply when it is illuminated. This property is called photoconductivity. It was discovered in 1873 while testing telegraph cables.
Chemically selenium resembles sulfur. It burns with a blue flame to give selenium dioxide, and a smell reminiscent of rotten radish is produced. In compounds it appears as selenide (minus two), selenite (plus four) and selenate (plus six). Hydrogen selenide, H₂Se, is a colourless, foul-smelling and toxic gas.
Uses in everyday life and technology
Its sensitivity to light made selenium an early building block of technology. Light sensors and exposure meters for cameras were made of it. Chester Carlson invented xerography, the copying process, in 1938. The first copiers used plates and drums coated with a layer of glassy selenium: where light fell on the layer it lost its charge, and toner powder stuck only to the dark areas. Today organic materials mostly replace selenium. Early rectifiers in power supplies were made of selenium too.
In the glass industry selenium works in two ways: it removes the green tint caused by iron, and together with cadmium it colours glass red. Thin-film solar cells contain copper indium gallium selenide. In metalworking, selenium improves the machinability of some steels. Selenium sulfide is an active ingredient in anti-dandruff products.
For humans and animals, selenium is an essential trace element. It is found in selenocysteine, a special amino acid located in enzymes such as glutathione peroxidase. These enzymes protect cells from aggressive oxygen compounds, and other selenoproteins help control thyroid hormones. The range between deficiency and excess is narrow. Deficiency occurs in regions with selenium-poor soil, for example parts of China, where it favoured Keshan disease, a disorder of the heart muscle. Too much selenium causes hair loss, brittle nails and garlic-smelling breath. Finland, whose soils are poor in selenium, has added it to fertiliser since 1984. In Germany a balanced diet with fish, eggs, meat and wholegrain is usually enough. Brazil nuts vary widely in selenium, so it is better not to eat large amounts. High-dose supplements are not advisable.
Molecules containing this element
- Selenium disulfide SeS2
- Selenous acid H2SeO3
- Sodium selenite Na2SeO3
Quick question
What is the atomic number of selenium (Se)?
FAQ
Why is selenium both essential and toxic?
The body needs selenium for enzymes that protect cells from damage by oxygen compounds, but the amount required is tiny. In excess, selenium compounds react with sulfur groups in proteins and generate aggressive oxygen compounds, exactly the damage that selenium enzymes normally protect against. Only a small gap separates the requirement from a harmful dose.
What was selenium used for in photocopiers?
Glassy selenium is a semiconductor that holds charge in the dark and becomes conductive in light. A selenium layer on a drum was charged, exposed and dusted with toner, which stuck only to the charged, dark areas. Today organic photoconductors mostly do this job.
Which foods contain selenium?
Fish, meat, eggs, offal and wholegrain products. The content of plants depends on the soil. Brazil nuts can contain a lot of selenium but vary strongly. People who eat a varied diet usually meet their needs without supplements.