Periodic Table > Chromium
 

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 1907 oC, 3465 oF, 2180 K 
Period Boiling point 2671 oC, 4840 oF, 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
 

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

The image reflects the toxic nature of the metal and its ‘mirror shine’ when polished.

Appearance
A hard, silvery metal with a blue tinge.
Uses

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.

 
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) 2.06 Covalent radius (Å) 1.30
Electron affinity (kJ mol-1) 64.259 Electronegativity
(Pauling scale)
1.66
Ionisation energies
(kJ mol-1)
 
1st
652.869
2nd
1590.628
3rd
2987.19
4th
4743.22
5th
6701.87
6th
8744.939
7th
15455.02
8th
17820.8
 

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 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
 

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, 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
 

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)
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
7.59
x 10-5
0.0239 1.8 52.1 774 - -
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History

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

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Podcasts

Listen to Chromium Podcast
Transcript :

Chemistry in its element - chromium


(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 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. 

(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.