Chromium Element Facts


Chromium.

Chromium. Photo by Tomihahndorf.

24
Cr
52.00

Data Zone

Classification: Chromium is a transition metal
Color: silver-gray
Atomic weight: 51.996
State: solid
Melting point: 1907 oC, 2180 K
Boiling point: 2670 oC, 2943 K
Electrons: 24
Protons: 24
Neutrons in most abundant isotope: 28
Electron shells: 2,8,13,1
Electron configuration: [Ar] 3d5 4s1
Density @ 20oC: 7.19 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 7.23 cm3/mol
Structure: bcc: body-centered cubic
Hardness: 8.5 mohs
Specific heat capacity 0.45 J g-1 K-1
Heat of fusion 21.0 kJ mol-1
Heat of atomization 397 kJ mol-1
Heat of vaporization 339.5 kJ mol-1
1st ionization energy 652.8 kJ mol-1
2nd ionization energy 1592 kJ mol-1
3rd ionization energy 2987.2 kJ mol-1
Electron affinity 64.3 kJ mol-1
Minimum oxidation number -2
Min. common oxidation no. 0
Maximum oxidation number 6
Max. common oxidation no. 3
Electronegativity (Pauling Scale) 1.66
Polarizability volume 11.6 Å3
Reaction with air none
Reaction with 15 M HNO3 passivated
Reaction with 6 M HCl mild, ⇒ H2, CrCl3
Reaction with 6 M NaOH
Oxide(s) CrO2, CrO3, Cr2O3 (chromia) + non-stoich
Hydride(s) CrH
Chloride(s) CrCl2, CrCl3
Atomic radius 140 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion) 90.5 pm
Ionic radius (3+ ion) 75.5 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 93.9 W m-1 K-1
Electrical conductivity 7.9 x 106 S m-1
Freezing/Melting point: 1907 oC, 2180 K



Discovery of Chromium

Dr. Doug Stewart

Chromium was discovered in 1780 by French chemist Nicolas Louis Vauquelin in Paris. He discovered the element in a mineral sample of ‘Siberian red lead’- now known as crocoite (lead chromate).

He boiled the crushed mineral with potassium carbonate to produce lead carbonate and a yellow potassium salt solution of chromic acid.

Vauquelin was convinced by further experiments on the solution that he had found a new metal.

In 1781 he succeeded in isolating the metal. Initially he removed the lead from the mineral sample by precipitation with hydrochloric acid. Vauquelin then obtained the oxide by evaporation and finally isolated chromium by heating the oxide in a charcoal oven. (1),(2).

Vauquelin also identified small amounts of chromium in ruby and emerald stones.

Vauquelin went on to discover Beryllium in 1798.

Chromium was named from the Greek word ‘chroma’, meaning color because it forms a variety of colorful compounds.

Transition metals are well-known for their multiple colored ions. Here’s an entertaining color change from Cr +6 (orange) to Cr +3 (green). Ammonium dichromate burns and forms chromium (III) oxide.

Appearance and Characteristics

Harmful effects:

Chromium metal is an essential trace element, but hexavalent chromium (Cr(VI)) is very toxic and carcinogenic.

Characteristics:

Chromium is a silver, lustrous, very hard metal that can take a high mirror polish. It is also odorless, tasteless, and malleable.

The metal forms a thin protective oxide coating in air, and burns when heated to form green chromium oxide(Cr2O3).

Uses of Chromium

Chromium is used in stainless steel, and other alloys. Chromium plating, for example on cars and bicycles, produces a smooth, silver finish that is highly resistant to corrosion.

The metal is also widely used as a catalyst.

Chromium compounds are valued as pigments for their vivid green, yellow, red and orange colors.

Abundance and Isotopes

Abundance earth’s crust: 102 parts per million by weight, 40 parts per million by moles

Abundance solar system: 20 parts per million by weight, 0.4 parts per million by moles

Cost, pure: $32 per 100g

Cost, bulk: $0.28 per 100g

Source: Chromium is not found as a free element in nature but is found in the form of ores. The main ore of chromium is chromite (FeCr2O4).

To isolate the metal commercially, chromite ore is oxidized to chromium(III) oxide (Cr2O3). The metal is then obtained by heating the oxide in the presence of aluminum or silicon.

Isotopes: Chromium has 21 isotopes whose half-lives are known, with mass numbers 42 to 63. Naturally occurring chromium is a mixture of four isotopes and they are found in the percentages shown: 50Cr (4.3%), 52Cr (83.8%), 53Cr (9.5%) and 54Cr (2.4%).

References

1. Mary Elvira Weeks, Discovery of the Elements V., Journal of Chemical Education, March 1932 p 470.
2. Per Enghag, Encyclopedia of the elements: technical data, history, processing, applications.,p 577- 578, John Wiley and Sons, 2004

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Comments

  1. I learned something thanks it helped me with my science work !

  2. Im doing a powerpoint on chromium this information is very helpful

  3. Anonomyous says:

    Awesome presentation! Thanks for the help you gave me with this! I really needed it in science.