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 −189.34°C, −308.81°F, 83.81 K 
Period Boiling point −185.848°C, −302.526°F, 87.302 K 
Block Density (g cm−3) 0.001633 
Atomic number 18  Relative atomic mass 39.948  
State at 20°C Gas  Key isotopes 40Ar 
Electron configuration [Ne] 3s23p6  CAS number 7440-37-1 
ChemSpider ID 22407 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
The image reflects the use of the element in the welding industry. Argon provides an inert atmosphere in which welded metals will not oxidise.
Argon is a colourless, odourless gas that is totally inert to other substances.
Argon is often used when an inert atmosphere is needed. It is used in this way for the production of titanium and other reactive elements. It is also used by welders to protect the weld area and in incandescent light bulbs to stop oxygen from corroding the filament.

Argon is used in fluorescent tubes and low-energy light bulbs. A low-energy light bulb often contains argon gas and mercury. When it is switched on an electric discharge passes through the gas, generating UV light. The coating on the inside surface of the bulb is activated by the UV light and it glows brightly.

Double-glazed windows use argon to fill the space between the panes. The tyres of luxury cars can contain argon to protect the rubber and reduce road noise.
Biological role
Argon has no known biological role.
Natural abundance
Argon makes up 0.94% of the Earth’s atmosphere and is the third most abundant atmospheric gas. Levels have gradually increased since the Earth was formed because radioactive potassium-40 turns into argon as it decays. Argon is obtained commercially by the distillation of liquid air.
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Although argon is abundant in the Earth’s atmosphere, it evaded discovery until 1894 when Lord Rayleigh and William Ramsay first separated it from liquid air. In fact the gas had been isolated in 1785 by Henry Cavendish who had noted that about 1% of air would not react even under the most extreme conditions. That 1% was argon.

Argon was discovered as a result of trying to explain why the density of nitrogen extracted from air differed from that obtained by the decomposition of ammonia.

Ramsay removed all the nitrogen from the gas he had extracted from air, and did this by reacting it with hot magnesium, forming the solid magnesium nitride. He was then left with a gas that would not react and when he examined its spectrum he saw new groups of red and green lines, confirming 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) 1.88 Covalent radius (Å) 1.01
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
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  36Ar 35.968 0.3336
  38Ar 37.963 0.0629
  40Ar 39.962 99.6035


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) 3.5
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)
520 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 Argon Podcast
Transcript :

Chemistry in its element: argon


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 the element that's so indolent that scientists at one time thought it wouldn't react with anything, but in the chemical world laziness can have its advantages especially if it's super quiet car tyres or a safe chemical with which to pump up your diving suit that you're after.

Here's John Emsley.

John Emsley

It's lazy, it's hard-working, it's colourless, it's colourful - it's argon!

Argon's name comes from the Greek word argos meaning lazy and indeed for more than a hundred years after its discovery chemists were unable to get it to combine with any other elements. But in the year 2000, chemists at the University of Helsinki led by Markku Räsänen announced the first ever compound: argon fluorohydride. They made it by condensing a mixture of argon and hydrogen fluoride on to caesium iodide at -265oC and exposing it to UV light. On warming above just -246oC it reverted right back to argon and hydrogen fluoride. And no other process has ever induced argon to react - [a truly lazy element].

There are 50 trillion tonnes of argon swirling around in the Earth's atmosphere and this has slowly built-up over billions of years, almost all coming from the decay of the radioactive isotope potassium-40 which has a half-life of 12.7 billion years. Although argon makes up 0.93% of the atmosphere it evaded discovery until 1894 when the physicist Lord Rayleigh and the chemist William Ramsay identified it. In 1904 Rayleigh won the Nobel Prize for Physics and Ramsay won the Nobel Prize for Chemistry for their work.

The story of its discovery started when Rayleigh found that the nitrogen extracted from the air had a higher density than that made by decomposing ammonia. The difference was small but real. Ramsay wrote to Rayleigh suggesting that he should look for a heavier gas in the nitrogen got from air, while Rayleigh should look for a lighter gas in that from ammonia. Ramsay removed all the nitrogen from his sample by repeatedly passing it over heated magnesium, with which nitrogen reacts to form magnesium nitride. He was left with one percent which would not react and found it was denser than nitrogen. Its atomic spectrum showed new red and green lines, confirming it a new element. Although in fact it contained traces of the other noble gases as well.

Argon was first isolated in 1785 in Clapham, South London, by Henry Cavendish. He had passed electric sparks through air and absorbed the gases which formed, but he was puzzled that there remained an unreactive 1%. He didn't realise that he had stumbled on a new gaseous element.

Most argon goes to making steel where it is blown through the molten iron, along with oxygen. Argon does the stirring while the oxygen removes carbon as carbon dioxide. It is also used when air must be excluded to prevent oxidation of hot metals, as in welding aluminium and the production of titanium to exclude air. Welding aluminium is done with an electric arc which requires a flow of argon of at 10-20 litres per minute. Atomic energy fuel elements are protected with an argon atmosphere during refining and reprocessing.

The ultra-fine metal powders needed to make alloys are produced by directing a jet of liquid argon at a jet of the molten metal.

Some smelters prevent toxic metal dusts from escaping to the environment by venting them through an argon plasma torch. In this, argon atoms are electrically charged to reach temperatures of 10 000 °C and the toxic dust particles passing through it are turned into to a blob of molten scrap.

For a gas that is chemically lazy argon has proved to be eminently employable. Illuminated signs glow blue if they contain argon and bright blue if a little mercury vapour is also present. Double glazing is even more efficient if the gap between the two panes of glass is filled with argon rather than just air because argon is a poorer conductor of heat. Thermal conductivity of argon at room temperature (300 K) is 17.72 mW m-1K-1 (milliWatts per metre per degree) whereas for air it is 26 mW m-1K-1.For the same reason argon is used to inflate diving suits. Old documents and other things that are susceptible to oxidation can be protected by being stored in an atmosphere of argon. Blue argon lasers are used in surgery to weld arteries, destroy tumors and correct eye defects.

The most exotic use of argon is in the tyres of luxury cars. Not only does it protect the rubber from attack by oxygen, but it ensures less tyre noise when the car is moving at speed. Laziness can prove useful in the case of this element. Its high tech uses range from double glazing and laser eye surgery to putting your name in lights.

Chris Smith

John Emsley unlocking the secrets of the heavier than air noble gas argon. Next week, would you marry this man?

Steve Mylon

It's almost never the case where the popular elements are that way because of their utility and interesting chemistry. But for gold and silver it's all so superficial. They are more popular because they're prettier. My wife for example, a non chemist, wouldn't dream of wearing a copper wedding ring. That might have something to do with the fact that copper oxide has an annoying habit of dyeing your skin green. But if she only took the time to learn about copper, to get to know it some; maybe then she would be likely to turn her back on the others and wear it with pride.

Chris Smith

Steve Mylon's back to cross your palm with copper on next week's Chemistry in its Element, I 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 :
The activity sets some critical thinking and pattern spotting tasks in the context of the noble gases. This can be used to develop skills in determining mathematical relationships between variables fr...
Description :
Estimate the percentage of ‘empty space’ in a sample of gaseous argon.
Description :
A teaching resource on the noble gases, supported by video clips from the Royal Institution Christmas Lectures® 2012.
Description :
Explore chemistry & industrial processes with 15 tutorials, all available on the Alchemy website.
Description :
An introduction to extracting gases from air
Description :
A series of short experiments and demonstrations about the chemistry of light, taken from a lecture by Peter Wothers from the University of Cambridge

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Visual Elements images and videos
© Murray Robertson 2011.



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 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
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

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

Created by video journalist Brady Haran working with chemists at The University of Nottingham.
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