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 Melting point 2446°C, 4435°F, 2719 K 
Period Boiling point 4428°C, 8002°F, 4701 K 
Block Density (g cm−3) 22.5622 
Atomic number 77  Relative atomic mass 192.217  
State at 20°C Solid  Key isotopes 193Ir 
Electron configuration [Xe] 4f145d76s2  CAS number 7439-88-5 
ChemSpider ID 22367 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
Iridium salts are highly coloured. The iridescent wings of the dragonfly represent both the origin of the element’s name and its strongly coloured salts.
Iridium is a hard, silvery metal. It is almost as unreactive as gold. It has a very high density and melting point.
Iridium is the most corrosion-resistant material known. It is used in special alloys and forms an alloy with osmium, which is used for pen tips and compass bearings. It was used in making the standard metre bar, which is an alloy of 90% platinum and 10% iridium. It is also used for the contacts in spark plugs because of its high melting point and low reactivity.
Biological role
Iridium has no known biological role, and has low toxicity.
Natural abundance
Iridium is one of the rarest elements on Earth. It is found uncombined in nature in sediments that were deposited by rivers. It is commercially recovered as a by-product of nickel refining.

A very thin layer of iridium exists in the Earth’s crust. It is thought that this was caused by a large meteor or asteroid hitting the Earth. Meteors and asteroids contain higher levels of iridium than the Earth’s crust. The impact would have caused a huge dust cloud depositing the iridium all over the world. Some scientists think that this could be the same meteor or asteroid impact that wiped out the dinosaurs.
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Iridium was discovered together with osmium in1803 by Smithson Tennant in London. When crude platinum was dissolved in dilute aqua regia, which is a mixture of nitric and hydrochloric acids, it left behind a black residue thought to be graphite. Tennant thought otherwise, and by treating it alternately with alkalis and acids he was able to separate it into two new elements. These he announced at the Royal Institution in London, naming one iridium, because its salts were so colourful and the other osmium because it had a curious odour (see osmium).

Despite its seeming intractability, a group of chemists, including the great Humphry Davy, demonstrated in 1813 that iridium would indeed melt like other metals. To achieve this they exposed it to the powerful current generated by a large array of batteries.

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) 2.13 Covalent radius (Å) 1.32
Electron affinity (kJ mol−1) 150.884 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 6, 4, 3, 2, 1, 0, -1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  191Ir 190.961 37.3
  193Ir 192.963 62.7


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 7.6
Crustal abundance (ppm) 0.000037
Recycling rate (%) >30
Substitutability High
Production concentration (%) 60
Reserve distribution (%) 95
Top 3 producers
  • 1) South Africa
  • 2) Russia
  • 3) Zimbabwe
Top 3 reserve holders
  • 1) South Africa
  • 2) Russia
  • 3) USA
Political stability of top producer 44.3
Political stability of top reserve holder 44.3


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)
131 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)
- - - - - - 1.48
x 10-9
x 10-7
x 10-5
0.00112 0.0225
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Listen to Iridium Podcast
Transcript :

Chemistry in its element: iridium


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Meera Senthilingam

This week a rare, sexy, superhero of an element whose name is a little bit deceiving. Here's Brian Clegg.

Brian Clegg

There are many reasons to single out an element - in the case of iridium it has to be because it has the sexiest name. It's the sort of name a science fiction writer would give to a new substance that was strong yet beautiful. It's a name that belongs to a superhero of the elements.

So how does the real thing live up to the name? It's hard, certainly, a dense silver-white transition metal of the platinum group, looking a bit like polished steel, but not quite as flashy as the name sounds. It's not iridescent itself. Yet its name derives from the same source.

When Smithson Tennant, later professor of chemistry at Cambridge, gave it the name in 1804, he was referring to Iris, the Greek rainbow goddess. He said 'I should incline to call this metal iridium, from the striking variety of colours which it gives, while dissolving in marine acid.' (Marine acid is a variant of muriatic acid, one of the old names for hydrochloric acid.)

Iridium was originally found as a contaminant (with the element osmium) in platinum, and it was from the solid remnants left when platinum was dissolved in a mix of sulphuric and hydrochloric acids that Tennant made his discovery of both elements. He might equally well have named iridium after its weight - it's more than twice as dense as lead, and with osmium it's one of the two densest of all the elements (there is some dispute over which is the heaviest, though osmium usually gets the laurels). Alternatively, Tennant could have reflected on its extremely high melting point, of nearly 2,500 degrees Celsius.

That 'superhero' feel also comes through in iridium's resistance to corrosion. We're used to gold and platinum as the exemplars of metals that stay pure, but iridium fights off corrosion better than either. It was partly for this reason - and the metal's sheer hardness - that iridium was first put to use in alloys to make the tips of fountain pens. Set in gold, these nibs shook off the worst ink and pressure could put on them. To this day you will see fountain pens claiming to have iridium nibs, though in practice it has been replaced by cheaper materials like tungsten.

There was only ever a small percentage of iridium in these pens, which is just as well. It's a rare material that makes platinum seem commonplace. There are only about 3 tonnes of iridium produced each year. These days it is more likely to turn up in the central electrode of spark plugs, where its resistance to corrosion and hardness are equally valuable. You'll also find it in specialist parts of industrial machinery.

Iridium, with atomic number 77 and two stable isotopes, 191 and 193, turns up in an alloy with platinum in the standard bar and weight used for many years to define the metre and the kilogramme. The metre was originally one 10 millionth of the distance from the North Pole to the Equator in a great circle running through Paris, but this wasn't a practical measure, so a metal bar was set up to define the length, first in pure platinum, and then from 1889 in the platinum/iridium alloy. Now, though, the distance is defined from the speed of light, permanently fixed in 1983 as 299,792,458 metres per second. As the second is accurately defined by an atomic clock, the metre falls out of the calculation.

The kilogramme, surprisingly, is still based on the mass of a particular block of platinum/iridium alloy kept in a vault in France, although there is a move for this too to be linked to a more reliable measurement of a natural quantity, such as a fixed number of known atoms. Iridium has also found its way into space, both as a secure container for the plutonium fuel of the nuclear electric generators on long range probes and as a coating on the X-ray mirrors of telescopes like the Chandra X-ray Observatory.

But perhaps iridium's best-known claim to fame is as a clue in a piece of 65 million-year-old Crime Scene Investigation. The concentration of iridium in meteorites is considerably higher than in rocks on the Earth, as most of the Earth's iridium is in the molten core. One class of meteorite, called chondritic (meaning they have a granular structure) still has the original levels of iridium that were present when the solar system was formed.

In 1980, a team led by physicist Luis Alvarez was investigating the layer of sedimentary clay that was produced around 65 million years ago, a time of particular interest because this so-called K/T boundary between the Cretaceous and Tertiary periods marks the point at which the majority of dinosaurs became extinct. This layer contains considerably more iridium that would normally be expected, suggesting that there may have been a large meteor or asteroid strike on the Earth at this time.

There is so much iridium present that the asteroid would have to have been around 10 kilometres across - sizeable enough to devastate global weather patterns, bringing about changes in climate that could have wiped out the dinosaurs. It was iridium that provides the principle clue as to why we now believe that so many species were wiped out, leaving the way clear for mammals to take the fore.

In one small way, iridium disappoints. Unlike its oxides, the element itself doesn't display the rainbow hues that its name suggests. But that apart, this is a true superhero of an element: tough, practically incorruptible and, yes, extremely dense.

Meera Senthilingam

So, a rare metal that not only has uses varying from fountain pens to telescopes but also helped us understand the extinction of the dinosaurs. That was Brian Clegg brightening up the Periodic Table with the iridescent tale of Iridium. Now next week a colourful element that likes to shed a tear

Claire Carmalt

Indium is a soft, malleable metal with a brilliant lustre. The name indium originates from the indigo blue it shows in a spectroscope. Indium has a low melting point for metals and above its melting point it ignites burning with a violet flame. Bizarrely, the pure metal of indium is described as giving a high-pitched "cry" when bent. This is similar to the sound made by tin or the "tin cry", however, neither of them is really much like a cry!

Meera Senthilingam

And join UCL's Claire Carmalt to find out what tricks, other than crying, indium has up its sleeve in next week's Chemistry in its element. Until then I'm Meera Senthilingam from the and thank you for listening.


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.



Produced by The Naked Scientists.


Periodic Table of Videos

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