Periodic Table > Selenium


Some elements exist in several different structural forms, these are called allotropes.

For more information on Murray Robertson’s image see Uses and properties facts below.


Fact box terminology

Elements appear in columns or ‘groups’ in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

Elements are laid out into rows or ‘periods’ so that similar chemical behaviour is observed in columns.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp, principal, diffuse, and fundamental.

Atomic Number
The number of protons in the nucleus.

Atomic Radius/non -bonded (Å)
based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties, for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron Configuration
The arrangements of electrons above the last (closed shell) noble gas.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Melting Point (oC)
The temperature at which the solid-liquid phase change occurs.

Melting Point (K)
The temperature at which the solid-liquid phase change occurs.

Melting Point (oF)
The temperature at which the solid-liquid phase change occurs.

Boiling Point (oC)
The temperature at which the liquid-gas phase change occurs.

Boiling Point (K)
The temperature at which the liquid-gas phase change occurs.

Boiling Point (oF)
The temperature at which the liquid-gas phase change occurs.

Elements that do not possess a liquid phase at atmospheric pressure (1 atm) are described as going through a sublimation process.

Density (g cm-3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative Atomic Mass
The mass of an atom relative to that of Carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists the value given is the abundance weighted average.

Key Isotopes (% abundance)
An element must by definition have a fixed number of protons in its nucleus, and as such has a fixed atomic number, however variants of an element can exist with differing numbers of neutrons, and hence a different atomic masses (e.g. 12C has 6 protons and 6 neutrons and 13C has 6 protons and 7 neutrons).

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems (where several isotopes exist, a value is presented for the most prevalent isotope).

Fact box

Group 16  Melting point 220.8 oC, 429.4 oF, 494 K 
Period Boiling point 685 oC, 1265 oF, 958 K 
Block 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

Uses and properties terminology

Image Explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.

Natural Abundance

Where this element is most commonly found in nature.

Biological Roles

The elements role within the body of humans, animals and plants. Also functionality in medical advancements both today and years ago.


The description of the element in its natural form.

Uses and properties

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.

A semi-metal that can exist in two forms: as a silvery metal or as a red powder.


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.

Atomic data terminology

Atomic radius/non -bonded (Å)
Based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties,for more details please refer to the CRC Handbook of Chemistry and Physics.

Electron affinity (kJ mol-1)
The energy released when an additional electron is attached to the neutral atom and a negative ion is formed (where several isotopes exist, a value is presented for the most prevalent isotope). *

Electronegativity (Pauling scale)
The degree to which an atom attracts electrons towards itself, expressed on a relative scale as a function bond dissociation energies, Ed in eV. χA - χB =(eV)-1/2sqrt(Ed(AB)-[Ed(AA)+Ed(BB)]/2), with χH set as 2.2 (where several isotopes exist, a value is presented for the most prevalent isotope).

1st Ionisation energy (kJ mol-1)
The minimum energy required to remove an electron from a neutral atom in its ground state (where several isotopes exist, a value is presented for the most prevalent isotope).

Covalent radius (Å)
The size of the atom within a covalent bond, given for typical oxidation number and coordination (where several isotopes exist, a value is presented for the most prevalent isotope). ***

Atomic data

Atomic radius, non-bonded (Å) 1.90 Covalent radius (Å) 1.18
Electron affinity (kJ mol-1) 194.965 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol-1)
Bond enthalpies terminology

Covalent Bonds
The strengths of several common covalent bonds.

Bond enthalpies

Covalent bonds
H–Se  276  kJ mol -1  

Mining/Sourcing Information

Data for this section of the data page has been provided by the British Geological Survey. To review the full report please click here or please look at their website here.

Key for numbers generated

Governance indicators

1 (low) = 0 to 2

2 (medium-low) = 3 to 4

3 (medium) = 5 to 6

4 (medium-high) = 7 to 8

5 (high) = 9

Reserve distribution (%)

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

(Where data are unavailable an arbitrary score of 2 was allocated. For example, Be, As, Na, S, In, Cl, Ca and Ge are allocated a score of 2 since reserve base information is unavailable. Reserve base data are also unavailable for coal; however, reserve data for 2008 are available from the Energy Information Administration (EIA).)

Production Concentration

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

Crustal Abundance

1 (low) = 100 to 1000 ppm

2 (medium-low) =10 to 100 ppm

3 (medium) = 1 to 10 ppm

4 (medium-high) = 0.1 to 1 ppm

5 (high) = 0.1 ppm

(Where data are unavailable an arbitrary score of 2 was allocated. For example, He is allocated a score of 2 since crustal abundance data is unavailable.)

Explanations for terminology

Crustal Abundance (ppm)

The abundance of an element in the Earth's crust in parts-per-million (ppm) i.e. The number of atoms of this element per 1 million atoms of crust.


The country with the largest reserve base.

Reserve distribution (%)

This is a measure of the spread of future supplies, recording the percentage of a known resource likely to be available in the intermediate future (reserve base) located in the top three countries.

Production Concentrations

This reports the percentage of an element produced in the top three countries. The higher the value, the larger risk there is to supply.

Political stability of top producer

The World Bank produces a global percentile rank of political stability. The scoring system is given below, and the values for all three production countries were summed.

Relative Supply Risk Index

The Crustal Abundance, Reserve distribution (%), Production Concentration and Governance Factor scores are summed and then divided by 2, to provide an overall Relative Supply Risk Index.

Supply risk

Relative supply risk 7.1
Crustal abundance (ppm) 0.13
Recycling rate (%) <10
Substitutability Unknown
Production concentration (%) 35
Reserve distribution (%) 22
Top 3 producers
  • 1) Japan
  • 2) Germany
  • 3) Belgium
Top 3 reserve holders
  • 1) Russia
  • 2) Chile
  • 3) Peru
Political stability of top producer 76.9
Political stability of top reserve holder 18.4

Oxidation states and isotopes

Key for Isotopes

Half Life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture


Common Oxidation states
The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Free atoms have an oxidation state of 0, and the sum of oxidation numbers within a substance must equal the overall charge.

Important Oxidation states
The most common oxidation states of an element in its compounds.

Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

Oxidation states and isotopes

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 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 β-β- 

Pressure and temperature - advanced terminology

Specific heat capacity (J kg-1 K-1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus (GPa)

Young's modulus is a measure of the stiffness of a substance, that is, it provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus (GPa)

The shear modulus of a material is a measure of how difficult it is to deform a material, and is given by the ratio of the shear stress to the shear strain.

Bulk modulus (GPa)

The bulk modulus is a measure of how difficult to compress a substance. Given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour Pressure (Pa)

Vapour pressure is the measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

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)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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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.

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Listen to Selenium Podcast
Transcript :

Chemistry in its Element - Selenium


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. 


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements. 

(End promo)


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Images:  Visual Elements © Murray Robertson 2011
Mining and Sourcing data:  British Geological Survey – natural environment research council.
Text:  John Emsley Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, 2nd Edition, 2011.
Additional information for platinum, gold, neodymium and dysprosium obtained from Material Value Consultancy Ltd
Data: CRC Handbook of Chemistry and Physics, CRC Press, 92nd Edition, 2011.
G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants, Longman, 16th Edition, 1995.
Members of the RSC can access these books through our library.