Periodic Table > Iridium
 

Terminology


Allotropes
Some elements exist in several different structural forms, these are called allotropes.


For more information on Murray Robertson’s image see Uses and properties facts below.

 

Fact box terminology


Group
Elements appear in columns or ‘groups’ in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.


Period
Elements are laid out into rows or ‘periods’ so that similar chemical behaviour is observed in columns.


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, principal, diffuse, and fundamental.


Atomic Number
The number of protons in the nucleus.


Atomic Radius/non -bonded (Å)
based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties, for more details please refer to the CRC Handbook of Chemistry and Physics.


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


Isotopes
Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.


Melting Point (oC)
The temperature at which the solid-liquid phase change occurs.


Melting Point (K)
The temperature at which the solid-liquid phase change occurs.


Melting Point (oF)
The temperature at which the solid-liquid phase change occurs.


Boiling Point (oC)
The temperature at which the liquid-gas phase change occurs.


Boiling Point (K)
The temperature at which the liquid-gas phase change occurs.


Boiling Point (oF)
The temperature at which the liquid-gas phase change occurs.


Sublimation
Elements that do not possess a liquid phase at atmospheric pressure (1 atm) are described as going through a sublimation process.


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.


Key Isotopes (% abundance)
An element must by definition have a fixed number of protons in its nucleus, and as such has a fixed atomic number, however variants of an element can exist with differing numbers of neutrons, and hence a different atomic masses (e.g. 12C has 6 protons and 6 neutrons and 13C has 6 protons and 7 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 (where several isotopes exist, a value is presented for the most prevalent isotope).

Fact box

 
Group Melting point 2446 oC, 4434.8 oF, 2719.15 K 
Period Boiling point 4428 oC, 8002.4 oF, 4701.15 K 
Block Density (g cm-3) 22.5622 
Atomic number 77  Relative atomic mass 192.217  
State at room temperature Solid  Key isotopes 193Ir 
Electron configuration [Xe] 4f145d76s2  CAS number 7439-88-5 
ChemSpider ID 22367 ChemSpider is a free chemical structure database
 

Uses and properties terminology


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.


Natural Abundance

Where this element is most commonly found in nature.


Biological Roles

The elements role within the body of humans, animals and plants. Also functionality in medical advancements both today and years ago.


Appearance

The description of the element in its natural form.

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.
Appearance
Iridium is a hard, silvery metal. It is almost as unreactive as gold. It has a very high density and melting point.
Uses
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.

 
Atomic data terminology

Atomic radius/non -bonded (Å)
Based on Van der Waals forces (where several isotopes exist, a value is presented for the most prevalent isotope). These values were calculated using a multitude of methods including crystallographic data, gas kinetic collision cross sections, critical densities, liquid state properties,for more details please refer to the CRC Handbook of Chemistry and Physics.


Electron affinity (kJ mol-1)
The energy released when an additional electron is attached to the neutral atom and a negative ion is formed (where several isotopes exist, a value is presented for the most prevalent isotope). *


Electronegativity (Pauling scale)
The degree to which an atom attracts electrons towards itself, expressed on a relative scale as a function bond dissociation energies, Ed in eV. χA - χB =(eV)-1/2sqrt(Ed(AB)-[Ed(AA)+Ed(BB)]/2), with χH set as 2.2 (where several isotopes exist, a value is presented for the most prevalent isotope).


1st Ionisation energy (kJ mol-1)
The minimum energy required to remove an electron from a neutral atom in its ground state (where several isotopes exist, a value is presented for the most prevalent isotope).


Covalent radius (Å)
The size of the atom within a covalent bond, given for typical oxidation number and coordination (where several isotopes exist, a value is presented for the most prevalent isotope). ***

Atomic data

 
Atomic radius, non-bonded (Å) 2.13 Covalent radius (Å) 1.32
Electron affinity (kJ mol-1) 111.537 Electronegativity
(Pauling scale)
2.2
Ionisation energies
(kJ mol-1)
 
1st
865.183
2nd
-
3rd
-
4th
-
5th
-
6th
-
7th
-
8th
-
 

Mining/Sourcing Information

Data for this section of the data page has been provided by the British Geological Survey. To review the full report please click here or please look at their website here.


Key for numbers generated


Governance indicators

1 (low) = 0 to 2

2 (medium-low) = 3 to 4

3 (medium) = 5 to 6

4 (medium-high) = 7 to 8

5 (high) = 9


Reserve distribution (%)

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %

(Where data are unavailable an arbitrary score of 2 was allocated. For example, Be, As, Na, S, In, Cl, Ca and Ge are allocated a score of 2 since reserve base information is unavailable. Reserve base data are also unavailable for coal; however, reserve data for 2008 are available from the Energy Information Administration (EIA).)


Production Concentration

1 (low) = 0 to 30 %

2 (medium-low) = 30 to 45 %

3 (medium) = 45 to 60 %

4 (medium-high) = 60 to 75 %

5 (high) = 75 %


Crustal Abundance

1 (low) = 100 to 1000 ppm

2 (medium-low) =10 to 100 ppm

3 (medium) = 1 to 10 ppm

4 (medium-high) = 0.1 to 1 ppm

5 (high) = 0.1 ppm

(Where data are unavailable an arbitrary score of 2 was allocated. For example, He is allocated a score of 2 since crustal abundance data is unavailable.)


Explanations for terminology


Crustal Abundance (ppm)

The abundance of an element in the Earth's crust in parts-per-million (ppm) i.e. The number of atoms of this element per 1 million atoms of crust.


Sourced

The country with the largest reserve base.


Reserve distribution (%)

This is a measure of the spread of future supplies, recording the percentage of a known resource likely to be available in the intermediate future (reserve base) located in the top three countries.


Production Concentrations

This reports the percentage of an element produced in the top three countries. The higher the value, the larger risk there is to supply.


Political stability of top producer

The World Bank produces a global percentile rank of political stability. The scoring system is given below, and the values for all three production countries were summed.


Relative Supply Risk Index

The Crustal Abundance, Reserve distribution (%), Production Concentration and Governance Factor scores are summed and then divided by 2, to provide an overall Relative Supply Risk Index.

Supply risk

 
Relative supply risk 8.5
Crustal abundance (ppm) 3.7E-05
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) 58.8
Reserve distribution (%) 87.5
Top 3 producers
  • 1) South Africa
  • 2) Russia
  • 3) USA
Top 3 reserve holders
  • 1) South Africa
  • 2) Russia
  • 3) USA
Political stability of top producer 7
Political stability of top reserve holder Unknown
 

Oxidation states and isotopes


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

Terminology


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. Free atoms have an oxidation state of 0, and the sum of oxidation numbers within a substance must equal the overall charge.


Important Oxidation states
The most common oxidation states of an element in its compounds.


Isotopes
Elements are defined by the number of protons in its centre (nucleus), whilst the number of neutrons present can vary. The variations in the number of neutrons will create elements of different mass which are known as isotopes.

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
 

Pressure and temperature - advanced terminology


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 (GPa)

Young's modulus is a measure of the stiffness of a substance, that is, 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 (GPa)

The shear modulus of a material is a measure of how difficult it is to deform a material, and is given by the ratio of the shear stress to the shear strain.


Bulk modulus (GPa)

The bulk modulus is a measure of how difficult to compress a substance. Given by the ratio of the pressure on a body to the fractional decrease in volume.


Vapour Pressure (Pa)

Vapour pressure is the 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
3.72
x 10-7
3.06
x 10-5
1.12
x 10-3
2.25
x 10-2
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History

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.

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Podcasts

Listen to Iridium Podcast
Transcript :

Chemistry in its element - Iridium


(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) 

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 nakedscientists.com and thank you for listening. 

(Promo) 

Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists dot com. There's more information and other episodes of Chemistry in its element on our website at chemistryworld dot org forward slash elements. 

(End promo) 

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Resources

Description :
Assessment for Learning is an effective way of actively involving students in their learning. This is a series of plans based around chemistry topics.
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The purpose of this experiment is to observe and interpret some of the chemistry of three first row transition elements and to compare them with a typical s-block element.
Description :
The Periodic Table allows chemists to see similarities and trends in the properties of chemical elements. This experiment illustrates some properties of the common transition elements and their compo...
Description :
In this experiment you will be looking at a group of transition elements chromium, molybdenum and tungsten.
Description :
The purpose of this experiment is to examine some of the solution chemistry of the transition elements.
 

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References

 
Images:  Visual Elements © Murray Robertson 2011
Mining and Sourcing data:  British Geological Survey – natural environment research council.
Text:  John Emsley Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, 2nd Edition, 2011.
Additional information for platinum, gold, neodymium and dysprosium obtained from Material Value Consultancy Ltd www.matvalue.com
Data: CRC Handbook of Chemistry and Physics, CRC Press, 92nd Edition, 2011.
G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants, Longman, 16th Edition, 1995.
Members of the RSC can access these books through our library.