Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

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 (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

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

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

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.

Atoms of the same element with different numbers of 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.

Fact box

Group 18  Melting point −157.37°C, −251.27°F, 115.78 K 
Period Boiling point −153.415°C, −244.147°F, 119.735 K 
Block Density (g cm−3) 0.003425 
Atomic number 36  Relative atomic mass 83.798  
State at 20°C Gas  Key isotopes 84Kr 
Electron configuration [Ar] 3d104s24p6  CAS number 7439-90-9 
ChemSpider ID 5223 ChemSpider is a free chemical structure database


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.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
There are many different isotopes of krypton. This symbol represents the isotope krypton-86.
Krypton is a gas with no colour or smell. It does not react with anything except fluorine gas.
Krypton is used commercially as a filling gas for energy-saving fluorescent lights. It is also used in some flash lamps used for high-speed photography.

Unlike the lighter gases in its group, it is reactive enough to form some chemical compounds. For example, krypton will react with fluorine to form krypton fluoride. Krypton fluoride is used in some lasers.

Radioactive krypton was used during the Cold War to estimate Soviet nuclear production. The gas is a product of all nuclear reactors, so the Russian share was found by subtracting the amount that came from Western reactors from the total in the air.

From 1960 to 1983 the isotope krypton-86 was used to define the standard measure of length. One metre was defined as exactly 1,650,763.73 wavelengths of a line in the atomic spectrum of the isotope.
Biological role
Krypton has no known biological role.
Natural abundance
Krypton is one of the rarest gases in the Earth’s atmosphere. It makes up just 1 part per million by volume. It is extracted by distillation of air that has been cooled until it is a liquid.
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Having discovered the noble gas argon, extracted from air, William Ramsay and Morris William Travers of University College, London, were convinced this must be one of a new group of elements of the periodic table. They decided others were likely to be hidden in the argon and by a process of liquefaction and evaporation they hoped it might leave behind a heavier component, and it did. It yielded krypton in the afternoon of 30th May 1898, and they were able to isolate about 25 cm3 of the new gas. This they immediately tested in a spectrometer, and saw from its atomic spectrum that it was a new element.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 2.02 Covalent radius (Å) 1.16
Electron affinity (kJ mol−1) Not stable Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)


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. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

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

Oxidation states and isotopes

Common oxidation states 2
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  78Kr 77.920 0.355 > 1.5 x 1021 y
  80Kr 79.916 2.286
  82Kr 81.913 11.593
  83Kr 82.914 11.5
  84Kr 83.911 56.987
  86Kr 85.911 17.279


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk Unknown
Crustal abundance (ppm) 0.0001
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) Unknown
Reserve distribution (%) Unknown
Top 3 producers
  • Unknown
Top 3 reserve holders
  • Unknown
Political stability of top producer Unknown
Political stability of top reserve holder Unknown


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

A measure of the stiffness of a substance. 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

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A 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)
248 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) Unknown
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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Listen to Krypton Podcast
Transcript :

Chemistry in its element: krypton


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 Superman makes an appearance and we're not talking about the rather tacky 1980s dance either, we're talking Krypton. Here's UCL's Angelos Michaelides.

Angelos Michaelides

Krypton is a fictional planet in the DC Comics universe, and the native world of the super-heroes Superman and, in some tellings, 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!

The story of its discovery, however, reveals a Victorian man of Science who, in his own way, qualifies as a superhero. Born in Glasgow in 1852, William Ramsay was already established as one of the foremost chemists of his day when he took up his appointment at University College London in 1887. The chair to which he succeeded had been occupied by leaders of scientific progress and, almost immediately after entering on his new duties, he was elected as a Fellow of The Royal Society. Great things were therefore believed of him, but nobody could have foreseen the discoveries which came so rapidly.

Ramsay's colleagues of this period describe him as "charming, witty, and generous" - traits which no doubt made him an easy man with whom to collaborate. Lord Rayleigh, himself an eminent physicist, was therefore lucky in more ways than one that Ramsay responded to his letter to Nature in September 1892. In it, Lord Rayleigh had expressed puzzlement as to why atmospheric nitrogen was of greater density than nitrogen derived from chemical sources, and wondered if any chemist would like to turn his mind to this anomaly. It does not appear that anyone except Professor Ramsay attempted to attack the question experimentally.

Correspondence between the two men reveals the enthusiasm with which Ramsay set to the task and details painstaking and meticulous work first to isolate sufficient atmospheric nitrogen and then to test it, using fractional distillation, for impurities, - anything, basically, that wasn't nitrogen. In this way, Ramsay wrote to Rayleigh : "We may discover a new element". In fact, they discovered Argon, and Ramsay went on to discover an entirely new class of gases. In 1904, he was awarded the Nobel Prize for Chemistry for the discovery of argon, neon, xenon and, of course, krypton.

Like its fellows, krypton is a colourless, odourless, tasteless, noble gas that occurs in trace amounts in the atmosphere. Like the other noble gases, it too is useful in lighting and photography, and its high light output in plasmas allows it to play an important role in many high-powered lasers. Unlike its lighter fellows it is reactive enough to form chemical compounds: krypton fluoride being the main example, which has led to the development of the krypton flouride laser. A laser of invisible light developed in the 1980's by the Los Alamos National Laboratory, which has found uses in fusion research and lithography. The heaviest stable krypton isotope, krypton 86, rose to prominence in the second half of the last century with a tad over one and a half million wavelengths of its orange-red spectral line being used as the official distance of a metre.

But the potential applications and practical uses of krypton are perhaps irrelevant in the story of its discovery. The point of Ramsay's work was not to put his knowledge to some utilitarian purpose - the point was to discover. Scientific endeavour is perhaps too often judged by whether or not its results are "useful". But discovery and knowledge are sometimes an end in themselves. The purist knows the joy of discovering that which was hitherto unknown.

Sir William Ramsay was a purist - a man with an insatiable appetite to better understand the world. He travelled to Canada, the United States, Finland, India, and Turkey with his wife, Lady Ramsay. He was a man open to new ideas, always endeavouring on his travels to learn local languages and customs and always alive to new experiences. One anecdote, related by a travelling companion to Iceland, describes him standing on the site of a geyser with a small glass jar, capturing gases as they erupt from underfoot. The image is unmistakably one of a childlike fascination with nature, in a man whose dedication to research knew no limits.

In his 1918 biography of Ramsay, Sir William Tilden describes him as a man "ever filled with that divine curiosity which impels the discoverer forward" who enjoyed the satisfaction of knowing that he was achieving something. Indeed, in a memorial lecture, for his late friend Henri Moissan in 1912, Ramsay quoted the following words:

"But what I cannot convey in the following pages is the keen pleasure I have experienced in the pursuit of these discoveries. To plough a new furrow; to have full scope to follow my own inclination; to see on all sides new subjects of study bursting upon me, that awakens a true joy which only those can experience who have themselves tasted the delights of research"

What's left, then, is the joy of finding what is hidden, a fact reflected in the very name of this element, Krypton, taken from "krypto", Greek for hidden. And nothing to do with a SuperDog.

Chris Smith

The hidden element that Lord Raleigh suspected might be there and William Ramsay actually uncovered. Thank you very much to Angelos Michaelides. He's based at University College London. Next week to one of those elements, the chemical symbol of which appears to bear absolutely no relationship to the name of the substance itself. Why?

Katherine Holt

Many centuries ago mid-European tin smelters observed that when a certain mineral was present in the tin ore, their yield of tin was much reduced. They called this mineral 'wolfs foam' because, they said, it devoured the tin much like a wolf would devour a sheep!

Chris Smith

And Katherine Holt will be telling us the tale behind tungsten's letter W on the periodic table in next week's Chemistry in its Element, hope you can join us. 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 There's more information and other episodes of Chemistry in its element on our website at

(End promo)
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  Help Text


Description :
C5e Demonstrate that dissolving, mixing and change of state are reversible.
Description :
Education in Chemistry
Description :
The reaction between aluminium and iodine is catalysed by water. This is a spectacular demonstration as clouds of purple iodine vapour are produced.
Description :
FunKids radio, in collaboration with the RSC, has produced a set of short chemistry snippets introducing children to chemistry- the what, why and how.
Description :
This resource is designed to provide strategies for dealing with some of the misconceptions that students have in the form of ready-to-use classroom resources.
Description :
Find out more about the career of Christopher Page, a scientific associate in nuclear magnetic resonance spectroscopy. 

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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.


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.



Produced by The Naked Scientists.


Periodic Table of Videos

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