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 1963°C, 3565°F, 2236 K 
Period Boiling point 3695°C, 6683°F, 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


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
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.
A hard, shiny, silvery metal.
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.
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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.

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) 2.10 Covalent radius (Å) 1.34
Electron affinity (kJ mol−1) 109.704 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 5, 4, 3, 2, 1, 0
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  103Rh 102.906 100


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)
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
x 10-6
0.000571 0.0217 0.422 4.41
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Listen to Rhodium Podcast
Transcript :

Chemistry in its element: rhodium


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.


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 :
Assessment for Learning is an effective way of actively involving students in their learning. This is a series of plans based around chemistry topics.
Description :
When concentrated hydrochloric acid is added to a very dilute solution of copper sulfate, the pale blue solution slowly turns yellow-green on the formation of a copper chloride complex. When concentr...
Description :
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 compou...
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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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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