| Discovery date | 1817 |
| Discovered by | Jöns Jacob Berzelius |
| Origin of the name | The name is derived from 'selene', the Greek name for the Moon. |
| Allotropes | Red Se, Grey Se, Black Se |
Se
Selenium
34
78.971
| Group | 16 | Melting point | 220.8°C, 429.4°F, 494 K |
| Period | 4 | Boiling point | 685°C, 1265°F, 958 K |
| Block | p | Density (g cm−3) | 4.809 |
| Atomic number | 34 | Relative atomic mass | 78.971 |
| State at 20°C | Solid | Key isotopes | 80Se |
| Electron configuration | [Ar] 3d104s24p4 | CAS number | 7782-49-2 |
| ChemSpider ID | 4885617 | ChemSpider is a free chemical structure database | |
Image explanation
The element is named after Selene, the Greek goddess of the moon. The image is of a crescent moon against a cratered surface.
Appearance
A semi-metal that can exist in two forms: as a silvery metal or as a red powder.
Uses
The biggest use of selenium is as an additive to glass. Some selenium compounds decolourise glass, while others give a deep red colour. Selenium can also be used to reduce the transmission of sunlight in architectural glass, giving it a bronze tint. Selenium is used to make pigments for ceramics, paint and plastics.
Selenium has both a photovoltaic action (converts light to electricity) and a photoconductive action (electrical resistance decreases with increased illumination). It is therefore useful in photocells, solar cells and photocopiers. It can also convert AC electricity to DC electricity, so is extensively used in rectifiers.
Selenium is toxic to the scalp fungus that causes dandruff so it is used in some anti-dandruff shampoos. Selenium is also used as an additive to make stainless steel.
Biological role
Selenium is an essential trace element for some species, including humans. Our bodies contain about 14 milligrams, and every cell in a human body contains more than a million selenium atoms.
Too little selenium can cause health problems, but too much is also dangerous. In excess it is carcinogenic and teratogenic (disturbs the development of an embryo or foetus).
Natural abundance
Selenium is found in a few rare minerals. Most of the world’s selenium is obtained from the anode muds produced during the electrolytic refining of copper. These muds are either roasted with sodium carbonate or sulfuric acid, or smelted with sodium carbonate to release the selenium.
Selenium was discovered by Jöns Jacob Berzelius at Stockholm in 1817. He had shares in a sulfuric acid works and he was intrigued by a red-brown sediment which collected at the bottom of the chambers in which the acid was made.
At first he thought it was the element tellurium because it gave off a strong smell of radishes when heated, but he eventually realised that it was in fact a new element. He also noted that it was like sulfur and indeed had properties intermediate between sulfur and tellurium. Berzelius found that selenium was present in samples of tellurium and gave that element its characteristic smell. He also became affected by it personally – it can be absorbed through the skin – and it caused him to experience the bad breath associated with those who work with this element.
| Atomic radius, non-bonded (Å) | 1.90 | Covalent radius (Å) | 1.18 |
| Electron affinity (kJ mol−1) | 194.965 |
Electronegativity (Pauling scale) |
2.55 |
|
Ionisation energies (kJ mol−1) |
1st
940.963
2nd
2044.52
3rd
2973.717
4th
4143.563
5th
6589.9
6th
7882.9
7th
14993.8
8th
-
|
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|
| Common oxidation states | 6, 4, -2 | ||||
| Isotopes | Isotope | Atomic mass | Natural abundance (%) | Half life | Mode of decay |
| 74Se | 73.922 | 0.89 | > 5.5 x 1018 y | EC-EC | |
| 76Se | 75.919 | 9.37 | - | - | |
| 77Se | 76.920 | 7.63 | - | - | |
| 78Se | 77.917 | 23.77 | - | - | |
| 80Se | 79.917 | 49.61 | - | - | |
| 82Se | 81.917 | 8.73 | > 9.5 x 1019 y | β-β- | |
|
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|
Specific heat capacity (J kg−1 K−1) |
321 | Young's modulus (GPa) | Unknown | |||||||||||
| Shear modulus (GPa) | Unknown | Bulk modulus (GPa) | 8.3 | |||||||||||
| Vapour pressure | ||||||||||||||
| Temperature (K) |
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| Pressure (Pa) |
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| Listen to Selenium Podcast |
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Transcript :
Chemistry in its element: selenium(Promo) You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry. (End promo) Chris Smith Hello, this week flaky scalps, skunks, dead polo ponies and an element that makes you stink of garlic. Yum! But it's not all bad news. Bernie Bulkin We know selenium is there, right under sulfur, in the periodic table, but it doesn't get much attention. The inorganic chemistry textbooks that I studied from talk extensively about sulphur and, where appropriate, say things like 'selenium also forms similar acids', or 'selenium also has many allotropic forms'. How slighted is this important element! When I was in my early 20s I developed a dry scalp condition for a few years, probably a result of anxiety over research grants I was trying to obtain. The treatment for this was a shampoo containing selenium sulphide, surprising to me because I thought that selenium was highly toxic. In fact a little investigation showed me that it was perfectly safe in small amounts. Selenium is one of those all too common elements that is essential to life in small quantities, and very toxic in larger quantities. 400 micrograms per day is set as the safe upper intake level in humans. But we require it as part of various enzymes, such as glutathione peroxidase, as well as in the thyroid. It is widespread, and accumulated in various foods, such as nuts, tuna, and lobster, so it is rare for humans to have a selenium deficiency. But for horses, with their more limited diet, selenium deficiency is common and often corrected with dietary supplements. Again, this requires great care. Recently 21 polo horses died from selenium overdose in Florida, the result of a veterinary pharmacist overdoing it in mixing the drugs. It was Berzelius who discovered selenium in 1817, as an impurity in sulphuric acid. Tellurium had already been discovered, and named after the Greek word for earth, so he named selenium using the Greek word for moon, selene. It occurs in various minerals, together with sulphur as you would expect. We know its evolution in plants goes back a long ways, because we find selenium compounds in coals, and much of what is released into the atmosphere today comes from coal burning. Indeed, the toxicity level of selenium to humans was established only 20 years ago by studies of Chinese victims of selenium poisoning, selenosis, who grew corn on selenium rich coal rocks. Selenosis has some lovely symptoms: a garlic odor on the breath, hair loss, sloughing of nails, fatigue, irritability, and eventually cirrhosis of the liver and death. It is the selenates and selenites that are the most toxic, since the elemental selenium is not readily incorporated into biological processes. While some of the allotropic forms of selenium resemble those that we know well from study of sulphur, there are others that are different. Most important, so called gray selenium consists of long chains of atoms forming extended helical structures. While selenium is not a metallic element, gray selenium is a good photoconductor, and was used in early photocells. Subsequently, selenium and various selenium compounds have been used in a variety of photoconductor and photovoltaic applications. Indeed, the newest and most promising class of mass produced solar cells are copper indium gallium selenide. At one time virtually all copying machines used selenium ; this has now been largely replaced by organic photoconductors. But the diversity of uses of selenium does not stop with shampoo and horse food supplements and photovoltaics. Selenium is added to synthetic rubber to improve resistance to abrasion, it has been added to brass, along with bismuth, to replace lead in pipes, and it is used, as sodium selenate, as an insecticide to stop attacks on flowering plants such as chrysanthemums and carnations. Selenium in its allotropic red form is added to glass to give it a scarlet color, but it also can be used to remove the greenish tint sometimes found in glass due to iron compounds. There have been numerous studies, none of them very conclusive, about the possible role of selenium in cancer prevention, and in increasing the efficacy of chemotherapy. Most of these seem to indicate that if it is effective at all, it works somehow in conjunction with vitamin E, which, like selenium, plays an antioxidant role in the body. Also intriguing to me was a recent study indicating that selenium deficient soils may play a role in susceptibility to HIV/AIDS in Africa. The rationale is that low selenium levels are associated with weakened immune systems, since with lack of antioxidant capacity there is stress on the immune system. But I save the best occurrence of selenium in nature for last. Butyl seleno mercaptan is the essential ingredient of skunk smell, and is certainly a contender for the title of the worst smelling compound. Once you have smelled it you will never forget it, nor underestimate the impact that this interesting element can have. Chris Smith So it can clear up an itchy scalp but it might make you stink in the process. That was Cambridge University's Bernie Bulkin with the story of Selenium. Next week we're visiting the element that Superman made famous. Angelos Michaelides Krypton is a fictional planet in the DC Comics universe, and the native world of the super-heroes Superman, Supergirl, and Krypto the "super dog". Krypton has been portrayed consistently as having been destroyed just after Superman's flight from the planet, with exact details of its destruction varying by time period, writers and franchise. So much for trying to do a "wikipedia" search for this "hidden" element! Chris Smith And you can catch the facts about Krypton, rather than the fiction with Angleos Michaelides at next week's Chemistry in its Element. I'm Chris Smith, thank you for listening and goodbye. (Promo) Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements. (End promo)
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Visual Elements images and videos
© Murray Robertson 1998-2017.
W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.
John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.
Derived in part from material provided by the British Geological Survey © NERC.
Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.
Produced by The Naked Scientists.
Created by video journalist Brady Haran working with chemists at The University of Nottingham.
© Murray Robertson 1998-2017.
Data
W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J. J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions (version 4.1), 2015, National Institute of Standards and Technology, Gaithersburg, MD, accessed November 2016.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.
Uses and properties
John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.
Supply risk data
Derived in part from material provided by the British Geological Survey © NERC.
History text
Elements 1-112, 114, 116 and 117 © John Emsley 2012. Elements 113, 115, 117 and 118 © Royal Society of Chemistry 2017.
Podcasts
Produced by The Naked Scientists.
Periodic Table of Videos
Created by video journalist Brady Haran working with chemists at The University of Nottingham.
