Glossary
Allotropes
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.
| Discovery date | 1894 |
| Discovered by | Lord Rayleigh and Sir William Ramsay |
| Origin of the name | The name is derived from the Greek, 'argos', meaning idle. |
| Allotropes |
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Glossary
Group
A vertical column in the periodic table. Members of a group
typically have similar properties and electron configurations in their
outer shell.
Period
A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.
Block
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.
Sublimation
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.
Isotopes
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.
| Group | 18 | Melting point | −189.34°C, −308.81°F, 83.81 K |
| Period | 3 | Boiling point | −185.848°C, −302.526°F, 87.302 K |
| Block | p | 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 | |
Glossary
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.
Appearance
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.
History
History
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.
Glossary
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.
Isotopes
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 | |
Glossary
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.
Substitutability
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.
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Glossary
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.
Podcasts
Podcasts
| Listen to Argon Podcast |
|
Transcript :
Chemistry in its element: argon (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 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. (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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Video
Video
Resources
Resources
Terms & Conditions
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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.
