Periodic Table > Rhodium
 

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 1963 oC, 3565 oF, 2236 K 
Period Boiling point 3695 oC, 6683 oF, 3968 K 
Block Density (g cm-3) 12.4 
Atomic number 45  Relative atomic mass 102.906  
State at 20°C Solid  Key isotopes 103Rh 
Electron configuration [Kr] 4d85s1  CAS number 7440-16-6 
ChemSpider ID 22389 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
This symbol of a rose is usually found with the motto ‘Dat Rosa Mel Apibus’ (The rose gives the bees honey). It was used by the Rosicrucians, a 17th-century secret society.
Appearance
A hard, shiny, silvery metal.
Uses
The major use of rhodium is in catalytic converters for cars (80%). It reduces nitrogen oxides in exhaust gases.

Rhodium is also used as catalysts in the chemical industry, for making nitric acid, acetic acid and hydrogenation reactions.

It is used to coat optic fibres and optical mirrors, and for crucibles, thermocouple elements and headlight reflectors. It is used as an electrical contact material as it has a low electrical resistance and is highly resistant to corrosion.
Biological role
Rhodium has no known biological role. It is a suspected carcinogen.
Natural abundance
Rhodium is the rarest of all non-radioactive metals. It occurs uncombined in nature, along with other platinum metals, in river sands in North and South America. It is also found in the copper-nickel sulfide ores of Ontario, Canada.

Rhodium is obtained commercially as a by-product of copper and nickel refining. World production is about 30 tonnes per year.
 
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) 2.10 Covalent radius (Å) 1.34
Electron affinity (kJ mol-1) 109.704 Electronegativity
(Pauling scale)
2.28
Ionisation energies
(kJ mol-1)
 
1st
719.675
2nd
1744.45
3rd
2996.83
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 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
 

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 5, 4, 3, 2, 1, 0
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  103Rh 102.906 100
 

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)
243 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)
- - - - - 1.69
x 10-8
5.99
x 10-6
0.000571 0.0217 0.422 4.41
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History

Rhodium was discovered in 1803 by William Wollaston. He collaborated with Smithson Tennant in a commercial venture, part of which was to produce pure platinum for sale. The first step in the process was to dissolve ordinary platinum in aqua regia (nitric acid + hydrochloric acid). Not all of it went into solution and it left behind a black residue. (Tennant investigated this residue and from it he eventually isolated osmium and iridium.) Wollaston concentrated on the solution of dissolved platinum which also contained palladium. He removed these metals by precipitation and was left with a beautiful red solution from which he obtained rose red crystals. These were sodium rhodium chloride, Na3RhCl6. From them he eventually produced a sample of the metal itself.

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Podcasts

Listen to Rhodium Podcast
Transcript :

Chemistry in its Element - Rhodium


(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, an element whose rarity and reluctance to react make it oh so special. Here's Lars Öhrström.

Lars Öhrström 

On an early spring day 21 years ago I walked excitedly down to the campus post office at the Royal Institute of Technology in Stockholm to fetch a small parcel, containing an even smaller plastic bottle, half filled with a purple powder. I respectfully signed for the package and solemnly carried it back with me to the laboratory, as the 50g of rhodium chloride it contained represented more than half a years earnings for a PhD student like me. 

Thus started my love story with rhodium, and although I have frequently been unfaithful since, to my disgrace with such prosaic metals as zinc and calcium, this transition metal, with atomic number 45, still has a special place in my heart. 

Rhodium chloride, that sounds much like sodium chloride, but the resemblance is only superficial. First of all, my rhodium atoms were in oxidation state three, thus requiring three chloride ions for every metal ion, and then, of course, there is the royal colour. However, the differences are much more profound as the chemistry of rhodium is much more diverse than that of sodium. 

Our rhodium chloride was to be used as starting material for new rhodium compounds that we planned to make and study as catalysts - species that make a reaction go faster without being consumed in the process. In these catalysts, rhodium is often in the oxidation state plus one or plus three. 

It would have been cheaper to buy silver-shiny rhodium metal instead. However, this would have been impractical as this noble platinum-group element is one of the least reactive metals of the periodic table. It reacts only reluctantly with the alchemist's famous aqua regia, the potent mixture of concentrated nitric and hydrochloric acids that easily dissolves gold. This was however the procedure used by English scientist William Hyde Wollaston when he first isolated rhodium from a sample of platinum ore, smuggled into Britain from present day Colombia, and purchased by Wollaston and his friend and colleague Smithson Tennant on Christmas Eve in the year 1800. 

This sample yielded not only rose coloured solutions of rhodium chloride, prompting Wollaston to give the new element the name rhodium - from the Greek word for rose - but he could also isolate palladium for the first time. Tennant also discovered the transition metals osmium and iridium in the same sample. 

While in my research group we were interested in building up organic molecules using rhodium compounds as catalysts, most people come in contact with this metal due to its ability to catalyse the breakdown of molecules in car exhaust fumes. Although 'come into contact' is a bit of an overstatement as the parts of a car that contain rhodium, the catalytic converter, is normally not accessed by the amateur mechanic. 

However, it is accessible enough on certain car models that theft of these noble metal containing devices, there is also palladium and platinum present, is becoming a problem. This is a reflection of the extreme rareness of these elements, explaining the very high price of the rhodium chloride I bought as a graduate student. They are in fact so rare that annual production is counted in kilos, not tonnes. And yes, the metals from the catalytic converters are recycled, accounting in these days for around 10 per cent of the yearly supply of rhodium, the lion's share of the rest, around 20,000 kilos, coming from mines in South Africa. 

The specific role of rhodium in catalytic converters is to break down nitrogen oxides, the so-called NOX emissions, to give oxygen and nitrogen gas, the main components of the air we breathe. 

Chemical industry is, just as my old research group, interested in using rhodium to build molecules. Rhodium was, for example, until recently the prime choice as catalysts in making one of mankind's oldest chemicals, acetic acid. It supplanted its periodic table upstairs neighbour cobalt in this process in the late 1960s in a prime example of what is now know as green chemistry making the process more energy efficient and generating less by-products. This is important as chemical plants worldwide produced some 5 million tonnes per year of acetic acid. Today, however, rhodium's downstairs neighbour iridium has largely taken over this role. 

And, if you chew gum you will most likely encounter another result of rhodium catalysis: menthol. Originally extracted from different species of mint plants, the demand for this substance with its characteristic minty scent far exceeds the natural sources, and it is now produced in several thousands tonnes a year in a process devised by Japanese Nobel prize winner Ryoji Noyori. 

So, instead of associating this metal with immense wealth, such as when the Guinness Book of Records awarded Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist in 1979, chewing gum may be what pops up in your mind the next time someone mentions rhodium. 

Meera Senthilingam 

So we have rhodium to thank when our breath is minty fresh. That was Lars Öhrström from the Chalmers tekniska högskola in Sweden, with the rare precious chemistry of rhodium. Now next week an element with a grand position in the periodic table. 

Brian Clegg 

The number 100 is a very significant one for human beings. It's partly because our number system is based on ten - so ten tens seems to have a special significance. In years, it's around the maximum lifetime of a human being, making a century more than just a useful division in the historical timeline. But in the natural world, 100 isn't quite so important. There's nothing about being element 100 that makes fermium stand out - the periodic table doesn't attach any significance to base 10. But it's hard not to think that fermium must be special in some way. 

Meera Senthilingam 

And to find out if fermium really does have any special qualities, join Brian Clegg in next week's Chemistry in its element. Until then, I'm Meera Senthilingham 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

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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.