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 1907°C, 3465°F, 2180 K 
Period Boiling point 2671°C, 4840°F, 2944 K 
Block Density (g cm−3) 7.15 
Atomic number 24  Relative atomic mass 51.996  
State at 20°C Solid  Key isotopes 52Cr 
Electron configuration [Ar] 3d54s1  CAS number 7440-47-3 
ChemSpider ID 22412 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
The image reflects the toxic nature of the metal and its ‘mirror shine’ when polished.
A hard, silvery metal with a blue tinge.
Chromium is used to harden steel, to manufacture stainless steel (named as it won’t rust) and to produce several alloys.

Chromium plating can be used to give a polished mirror finish to steel. Chromium-plated car and lorry parts, such as bumpers, were once very common. It is also possible to chromium plate plastics, which are often used in bathroom fittings.

About 90% of all leather is tanned using chrome. However, the waste effluent is toxic so alternatives are being investigated.

Chromium compounds are used as industrial catalysts and pigments (in bright green, yellow, red and orange colours). Rubies get their red colour from chromium, and glass treated with chromium has an emerald green colour.
Biological role
Chromium is an essential trace element for humans because it helps us to use glucose. However, it is poisonous in excess. We take in about 1 milligram a day. Foods such as brewer’s yeast, wheat germ and kidney are rich in chromium.
Natural abundance
Chromium is found mainly in chromite. This ore is found in many places including South Africa, India, Kazakhstan and Turkey. Chromium metal is usually produced by reducing chromite with carbon in an electric-arc furnace, or reducing chromium(III) oxide with aluminium or silicon.
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Chromium was discovered by the French chemist Nicholas Louis Vauquelin at Paris in1798. He was intrigued by a bright red mineral that had been discovered in a Siberian gold mine in 1766 and was referred to as Siberian red lead. It is now known as crocoite and is a form of lead chromate. Vauquelin analysed it and confirmed that it was a lead mineral. Then he dissolved it in acid, precipitated the lead, filtered this off, and focused his attention on the remaining liquor from which he succeeded in isolating chromium. Intrigued by the range of colours that it could produce in solution, he named it chromium from the Greek word chroma meaning colour. He then discovered that the green colouration of emeralds was also due to chromium

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) 2.06 Covalent radius (Å) 1.30
Electron affinity (kJ mol−1) 64.259 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, 3, 2, 0
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  50Cr 49.946 4.345 > 1.3 x 1018 β+EC 
  52Cr 51.941 83.789
  53Cr 52.941 9.501
  54Cr 53.939 2.365


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 6.2
Crustal abundance (ppm) 135
Recycling rate (%) >30
Substitutability High
Production concentration (%) 37
Reserve distribution (%) 46
Top 3 producers
  • 1) South Africa
  • 2) Kazakhstan
  • 3) India
Top 3 reserve holders
  • 1) Kazakhstan
  • 2) South Africa
  • 3) India
Political stability of top producer 44.3
Political stability of top reserve holder 61.8


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)
449 Young's modulus (GPa) 279.1
Shear modulus (GPa) 115.4 Bulk modulus (GPa) 160.1
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - 2.45
x 10-8
x 10-5
0.0239 1.8 52.1 774 - -
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Listen to Chromium Podcast
Transcript :

Chemistry in its element: chromium


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

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Meera Senthilingam

This week an element that adds sparkle and value to minerals, through the colourful characteristics of its compounds.

Christopher Blanford

In the Western world, the colourful history of chromium begins, suitably enough, at the far end of the visible spectrum with a red-orange mineral that was named "Siberian red lead" by its discoverer, the 18th-century geologist Johann Lehmann. Although Mendeleev's periodic table was still almost a century away at this time, scientists around the world were rapidly discovering new elements - 30% of the naturally occurring elements were first isolated between 1775 and 1825. It was in the middle of this surge of discovery, over 35 years after Siberian red lead was first found that the French chemist Louis Vauquelin showed that this mineral, now known as crocoite, contained a previously unknown chemical element.

It took Vauquelin several steps to isolate chromium. First he mixed the crocoite solution with potassium carbonate to precipitate out the lead. Then he decomposed the lemon yellow chromate intermediate in acid, and finally removed the compounded oxygen by heating with carbon - leaving behind elemental chromium.

The name for this new element was debated among his friends, who suggested "chrome" from the Greek word for colour because of the colouration of its compounds. Although he objected to this name at first because the metal itself had no characteristic colour, his friends' views won out.

When Vauquelin exhibited his pale grey metal to the French Academy of Sciences, he commented on the metal's brittleness, resistance to acids and incapability of being melted. He thought these properties made it overly difficult to work with and thus limited its applications as a metal. He did suggest, however, that chromium's compounds would be widely used as beautiful, brilliantly coloured pigments. A browse through images of chromium compounds on Wikipedia shows a whole spectrum of colours: dark red chromium(VI) oxide, orange-red lead chromate, bright yellow sodium chromate, brilliant chrome green (that's chrome(III) oxide), light blue chromium(II) chloride, and violet anhydrous chromium(III) chloride. The last of these compounds shows an amazing property when hydrated. Its colour changes between pale green, dark green and violet depending on how many of the chromium ion's six coordination sites are occupied by chloride rather than water.

Of all these pigments, one of them stands out. I'm a chemist who was born, raised and schooled in the Midwestern United States, so the iconic yellow school buses in North America were familiar sights. Chrome yellow, also known as "school bus yellow", was adopted in 1939 for all U.S. school buses to provide high contrast and visibility in twilight hours. However, the presence of both toxic lead and hexavalent chromium of Erin Brockovitch fame has led to it being largely replaced by a family of azo dyes, known as Pigment Yellows, though chrome yellow is still used in some marine and industrial applications.

Of all chromium's natural occurrences, my favourites are gemstones, where a trace of the element adds a blaze of colour. As corundum, beryl, and crysoberyl, these metal oxides are colourless and obscure minerals. But add a dash of chromium, and they become ruby, emerald and alexandrite.

The chemist's tool of crystal-field theory, which models the electronic structure of transition metal complexes, provides a surprisingly accurate way of describing and predicting the source and variability of colour in chromium's compounds. In ruby - which is aluminium oxide with a few parts per thousand of the aluminium ions are replaced by chromium(III) ions - the chromium atoms are surrounded by six oxygen atoms. This means that the chromium atoms strongly absorb light in the violet and yellow-green regions. We see this as mainly red with some blue, giving, in the best cases, the characteristic pigeon-blood colour of the finest rubies.

The Cr3+ ion is about 26% bigger than the Al3+ ion it replaces. So, when more chromium is added to aluminium oxide, the octahedral environment around the chromium becomes distorted and the two bands of absorption shift towards the red. In aluminium oxide in which 20 to 40% of the atoms of aluminium have swapped to chromium, the absorbed and transmitted colours swap and we see this complex as green, transforming a synthetic ruby into a green sapphire.

My next gem, the emerald, in an oxide of silicon, aluminium and beryllium. It has the same substitution of a chromium ion for an aluminium ion and a similar distorted octahedral arrangement of oxygen around chromium, giving emeralds their characteristic green colour, like that from green sapphires.

Of the chromium gemstones, alexandrite is the most fascinating to me. Its stones are strongly pleochroic. That is, they absorb different wavelengths depending on the direction and polarisation of the light that's hitting them. So, depending on a gem's orientation, alexandrite's colour ranges from red-orange to yellow and emerald green. Its colour also changes depending on whether it is viewed in daylight or under the warm red tones of candlelight. When moved from daylight to candlelight, the best specimens turn from a brilliant green to a fiery red. Lesser gems turn from dull green to a turbid blood red.

Outside this rainbow of chromium compounds, chromium helps prevent a particularly undesirable colour: rust brown. In corrosion-resistant, or "stainless", steels, at least 11% of its mass is chromium. The alloyed chromium reacts with oxygen to form a transparent nanoscopic layer of oxide that forms a barrier to further oxygen penetration and so prevents the ruddy, flaky products of iron oxidation.

Given these widespread uses of chromium complexes, it should come as no surprise when I tell you that under one-half of a per cent of chromium produced is chromium in its elemental form. So, to some extent, Vauquelin's prediction from two centuries ago about the limited usefulness of elemental chromium was spot on. On the other hand, the first picture in my mind for chromium (after gemstones, of course) is when it is in its metallic form, such as for the mirrored corrosion and wear-resistant "chrome" surfaces of ball bearings and the shiny silvery trim on car parts.

Meera Senthilingam

So it's shiny and colourful as well as corrosion and wear resistant. I don't think I would say chromium had limited uses, would you? That was Oxford University's Christopher Blanford with the complex and colourful chemistry of chromium. Next week, a planetary element.

Brian Clegg

We're so familiar with uranium and plutonium that it's easy to miss that they are named after the seventh and ninth planets of the solar system. (At least, Pluto was the ninth planet until it was stripped of its status in 2006.) Between those planets sits Neptune, and the gap between the two elements leaves a space for their relatively unsung cousin, neptunium - element number 93 in the periodic table. In June 1940, American physicists Edwin McMillan and Philip Abelson, working at the Berkeley Radiation Laboratory, wrote a paper describing a reaction of uranium that had been discovered when bombarding it with neutrons using a cyclotron particle accelerator. Remarkably, the openly published Berkeley paper would show the first step to overcoming one of the biggest obstacles to building an atomic bomb.

Meera Senthilingam

And Brian Clegg will reveal how this obstacle was overcome in next week's Chemistry in its Element. Until then, I'm Meera Senthilingam 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.