Chromium (24)

Chromium is steel-gray, lustrous, hard, metallic, and takes a high polish. Its compounds are toxic. It is found as chromite ore. Siberian red lead (crocoite, PrCrO4) is a chromium ore prized as a red pigment for oil paints.

Emerald is a form of beryl (a beryllium aluminum silicate) which is green because of the inclusion of a little chromium into the beryl crystal lattice in place of some of the aluminum ions. Similarly, traces of chromium incorporated into the crystal lattice of corundum (crystalline aluminum oxide, Al2O3) as a replacement for some of the Al3+ ions results in another highly colored gem stone, in this case the red ruby.

•Name: Chromium
•Symbol: Cr
•Atomic number: 24
•Atomic weight: 51.9961
•Standard state: solid at 298 K
•CAS Registry ID: 7440-47-3
•Group in periodic table: 6
•Period in periodic table: 4
•Block in periodic table: d-block
•Color: silvery metallic (surprise surprise)
•Classification: Metallic

Historical information

Chromium was discovered by Louis-Nicholas Vauquelin at 1797 in France. Origin of name is from the Greek word "chroma" meaning "color", named for the many colored compounds known for chromium. In the mid-18th century analysis of Siberian "red lead" (PbCrO4, crocoite) from Siberia showed that it contained quite a lot of lead, but also a further material. This was eventually identified as chromium oxide. Chromium oxide was discovered in 1797 by Louis-Nicholas Vauquelin, who prepared the metal itself in the following year. Starting from crocoite the procedure was to powder the mineral and to precipitate the lead out through its reaction with hydrochloric acid (HCl in water). The residue was chromium oxide, CrO3. Heating this oxide in an oven in the presence of charcoal as a reducing agent gave the metal itself.

Vauquelin also analyzed an emerald from Peru and discovered that its green color is because of the presence of the new element, chromium. In fact, the name chromium is from the Greek word "chroma" meaning "color", so named because of the many different colored compounds displayed by chromium.

A year or two after Vauquelin's discovery, a German chemist named Tassaert working in Paris found chromium in an ore now called chromite. This ore, Fe(CrO2)2, is now an important source of chromium.

Physical properties

•Melting point: 2180 [or 1907 °C (3465 °F)] K
•Boiling point: 2944 [or 2671 °C (4840 °F)] K
•Density of solid: 7140 kg m-3

Orbital properties

•Ground state electron configuration: [Ar].3d5.4s1
•Shell structure: 2.8.13.1
•Term symbol: 7S3

Isolation

It is not normally necessary to make chromium in the laboratory as it is so readily available commercially. The most useful source of chromium commercially is the ore chromite, FeCr2O4. Oxidation of this ore by air in molten alkali gives sodium chromate, Na2CrO4 in which the chromium is in the +6 oxidation state. This is converted to the Cr(III) oxide Cr2O3 by extraction into water, precipitation, and reduction with carbon. The oxide is then further reduced with aluminum or silicon to form chromium metal.

Cr2O3 + 2Al → 2Cr + Al2O3

2Cr2O3 + 3Si → 4Cr + 3SiO2

Another kind of isolation is by electroplating processes. This involves the dissolution of Cr2O3 (chromium (III) oxide) in sulphuric acid to give an electrolyte used for chromium electroplating.

Interesting Facts:

1. Chromium (English) Chrome (French) Chrom (Deutsch) Cromo (Italian) Cromo (Spanish) Krom (Swedish)

2. Chromium is a hard, lustrous, steel-gray metal.

3. Stainless steel is hard and resists corrosion due to the addition of chromium.

4. Chromium is the only element which shows antiferromagnetic ordering in its solid state at and below room temperature. Chromium becomes paramagnetic above 38°C.

5. Trace amounts of trivalent chromium are needed for lipid and sugar metabolism. Hexavalent chromium and its compounds are extremely toxic. The +1, +4 and +5 oxidation states also occur, although they are less common.

6. Chromium occurs naturally as a mix of three stable isotopes. 19 radioisotopes have been characterized.

7. Chromium is used to prepare pigments (including yellow, red and green), color glass green, color rubies red and emeralds green, in some tanning processes, as a decorative and protective metal coating and as a catalyst.

8. Chromium in air is passivized by oxygen, forming a protective layer that is essentially a spinel that is a few atoms thick.

9. Chromium is the 21st most abundant element in the Earth's crust. It is present at a concentration of approximately 100 ppm.

10. Most chromium is obtained by mining the mineral chromite. Although it is rare, native chromium also exists. It may be found in kimberlite pipe, where the reducing atmosphere favors the formation of diamond in addition to elemental chromium.