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 −71°C, −96°F, 202 K 
Period Boiling point −61.7°C, −79.1°F, 211.5 K 
Block Density (g cm−3) 0.009074 
Atomic number 86  Relative atomic mass [222]  
State at 20°C Gas  Key isotopes 211Rn, 220Rn, 222Rn 
Electron configuration [Xe] 4f145d106s26p6  CAS number 10043-92-2 
ChemSpider ID 23240 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
An image based around the familiar radiation hazard symbol. The background image reflects the fact that detectable amounts of radon can build up in houses.
Radon is a colourless and odourless gas. It is chemically inert, but radioactive.
Radon decays into radioactive polonium and alpha particles. This emitted radiation made radon useful in cancer therapy. Radon was used in some hospitals to treat tumours by sealing the gas in minute tubes, and implanting these into the tumour, treating the disease in situ. Other, safer treatments are now more commonly used.

In some places, high concentrations of radon can build up indoors, escaping from the ground or from granite buildings. Home testing kits are available which can be sent away for analysis.
Biological role
Radon has no known biological role. It is, however, thought that it may have had a significant role in evolution. This is because it is responsible for much of the Earth’s background radiation that can lead to genetic modifications.
Natural abundance
Radon is produced naturally from the decay of the isotope radium-226, which is found in rocks. It was first discovered as a radioactive gas produced from radium as it decayed. There is a detectable amount in the Earth’s atmosphere.
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In 1899, Ernest Rutherford and Robert B. Owens detected a radioactive gas being released by thorium. That same year, Pierre and Marie Curie detected a radioactive gas emanating from radium. In1900, Friedrich Ernst Dorn at Halle, Germany, noted that a gas was accumulating inside ampoules of radium. They were observing radon. That from radium was the longer-lived isotope radon-222 which has a half-life 3.8 days, and was the same isotope which the Curies has observed. The radon that Rutherford detected was radon-220 with a half-life of 56 seconds.

In 1900, Rutherford devoted himself to investigating the new gas and showed that it was possible to condense it to a liquid. In 1908, William Ramsay and Robert Whytlaw-Gray at University College, London, collected enough radon to determine its properties and reported that it was the heaviest gas known.

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) 2.20 Covalent radius (Å) 1.46
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
  211Rn 210.991 - 14.6 h  β+,EC 
  220Rn 220.011 - 55.6 s  α 
  222Rn 222.018 - 3.823 d  α 


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) 0.0000000000004
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)
94 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 Radon Podcast
Transcript :

Chemistry in its element: radon


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

This week residents of Aberdeen, Edinburgh and Cornwall, watch out, radon's about.

Katherine Holt

When I bought my house recently I was intrigued by a comment in the surveyors report which stated 'higher than the actionable levels of radioactive radon gas have been found in up to 10% of dwellings in this area of the country and we recommend the property be tested for the levels of radon.' Well of course in the flurry of activity associated with moving I didn't think about this for some time but recently I started to read more about this mysterious radioactive gas which may be invading my property!

The first reports of problems associated with radon gas in domestic buildings was in the United States in 1984, when an employee at a nuclear power plant began setting off the radiation detector alarms on his way into work. The problem was eventually traced to his home, where the level of radon gas in his basement was found to be abnormally high. Radon emanates directly from the ground all over the world but especially in regions with high levels of granite or shale in the soil. Uranium, a relatively common constituent of soils, decays to form radium, which in turn decays to produce radon. In fact for most UK residents, naturally occurring radon accounts for half of their annual radiation dosage. However it only really becomes problematic when high levels are produced in confined spaces, for example the ground floor of buildings without adequate ventilation. Some homes in Cornwall, where the ground has high granite content, were found to contain worrying levels of radon. However forced ventilation methods largely remove the problem.

Radon is the product of the decay of other unstable, radioactive elements such as radium, thorium and actinium. The colourless, odourless, tasteless gas can be isolated from these sources but soon decays as it has no stable isotopes. The early pioneers in the study of radioactivity, the Curies, had noted that radium appeared to make the surrounding air radioactive. The discovery of radon is credited to a German physicist Friedrich Ernst Dorn, who traced this observed radioactivity to a gas which was given off by radium - a gas which he called 'radium emanation'. Similar 'emanations' were isolated from other elements - for example thorium, and eventually the gas was identified as the heaviest of the noble gases, named radon, and given its rightful place in the periodic table.

Not much research has been carried out on radon, due to its radioactivity, but it is largely un-reactive with few known compounds. Like the other noble gases it has been found to form compounds with fluorine. It is the densest known gas, another reason why it tends to linger in low-lying confined spaces. Below its boiling point it forms a colourless liquid and then at lower temperatures an orange-red solid which glows eerily due to the intense radiation it produces.

Radon has a fairly short half-life of only a few days so rapidly decays. Why then should we worry about radon levels in our homes? The problem is, when breathed in, it can decay to form other, longer-lasting, solid radioactive species, which can coat the lungs, leading to continual exposure. These so-called 'radon daughters' include polonium-214, polonium-218 and lead-214 - not family members you'd wish to spend a lot of time with. Prolonged radon exposure is believed to be the second most frequent cause of lung cancer after smoking. The unfortunate gentleman with the basement full of radon had a risk of consequentially developing lung cancer equivalent to smoking 135 packs of cigarettes every day!

So now that I'm comfortable in my freshly decorated new home, all that remains for me to do is to check that my surroundings are as safe as they look. Fortunately that's easily done these days with radon test kits which you can order online. You place them in the corner of a room for three months and forget about them and then send them away to be analysed. OK, so it costs £30 quid or so - but that's a small price to pay for peace of mind.

Chris Smith

But maybe don't take a deep breath before you open the results from the lab, just in case. That was UCL Chemist Katherine Holt, with the story of the radio active resident in your basement. Next week from a chemical that kills silently and slowly to an albeit more fearsome beast.

Kira Weissman

The 37-year old technician spilled only a few hundred millilitres or so in his lap during a routine palaeontology experiment. He took the normal precaution in such situations, quickly dowsing himself with water from a laboratory hose, and even plunged into a nearby swimming pool while the paramedics were en route. But a week later, doctors removed a leg, and a week after that, he was dead. The culprit: hydrofluoric acid (colloquially known as HF), and the unfortunate man was not its first victim.

Chris Smith

But what killed him, and what about the people who first isolated HF, unaware of its terrible reputation? Well you can find out what happened to them from Kira Weissman on 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 There's more information and other episodes of Chemistry in its element on our website at

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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.



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