Chromium (Cr), chemical element of Group 6 (VIb) of the periodic table, a hard  steel-gray metal that takes a high polish and is generally used in alloys to increase  strength and corrosion resistance. It occurs in different oxidative forms (Oliveira). 

OCCURANCE 

Chromium is a relatively abundant element in Earth’s crust; the free metal is  never found in nature. It is the seventeenth most abundant element in Earth’s  mantle. Most ores consist of the mineral chromite, the ideal formula of which is  FeCr2O4. It is widely dispersed in natural deposits, which are usually  contaminated with oxygen, magnesium, aluminum, and silica; their chromium  content varies from 42 to 56 percent. 

EXTRACTION 

To obtain pure chromium, chromite is first treated with molten alkali and oxygen,  converting all of the chromium to the alkali chromate, and the latter is dissolved  in water and eventually precipitated as sodium dichromate, Na2Cr2O7. The  dichromate is then reduced with carbon to chromium sesquioxide, Cr2O3, and  that oxide in turn is reduced with aluminum to give the chromium metal. 

FOOD SOURCES AND PROCESSING EFFECTS: 

Vegetables such as broccoli, potatoes, and green beans, whole-grain products,  beef and poultry, fruits, including apples and bananas; grape juice, milk and dairy  products, mushrooms, nuts and spices all are rich sources of chromium. One third  of the daily intake of chromium is fulfill by milk. Acidic foods in contact with  stainless steel container may receive an additional chromium. Chromium in food  may reduce due to its processing and refining (Finley & Deman).

PROPERTIES 

  • Chromium is a transition element having high melting point 1907°C and  boiling point 2671°C. 
  • Naturally occurring chromium have 3 stable isotopes 52Cr, 53Cr and 54Cr. 52Cr  is most commonly found isotope (Lunk, 2015). 

  • It exist in multiple oxidative forms ranging from Cr(0) to Cr(6). • Trivalent chromium occurs most commonly in nature usually in  combination with iron or other metal oxides (Barnhart).

Chromium in the  

periodic table 

Atomic properties 

Physical properties 

Natural abundance  (%) in 

Atomic number 24 

Molar mass  

51.9961 g mol−1 

Group 6; Period 4 

Electron configuration

Electronegativity (Pauling  scale) 1.66 

Ionization energies  

(kJ mol−1) 

1st: 652.9 

2nd: 1,590.6

Crystal structure 

Body-centered  

cubic (bcc) 

Cell parameter a:  291 pm 

Melting point 

Oceans 6 × 10−8 

Humans 3 × 10−6 

Universe 1.5 × 10−3 

Sun 2 × 10−3 

Earth’s crust 1.4 × 10−2


Chromium in the  

periodic table 

Atomic properties 

Physical properties 

Natural abundance  (%) in 

1 s2 2 s2 2p6 3 s2 3p6 3d5 4 s1

3rd: 2,987 

Atomic radius 128 pm 

Covalent radius 139 ± 5 pm 

1,907 °C 

Boiling point  

2,671 °C 

Density 7.19 g cm−3 Heat of fusion 

21.0 kJ mol−1 

Heat of  

vaporization 

347 kJ mol−1 

Molar heat capacity 23.35 J mol−1 K−1

Meteorites 3 × 10−1 Cr:Mo:W = 100:1:1 


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. It is also possible to  chromium plate plastics, which are often used in bathroom fittings. • About 90% of all leather is tanned using chromium. 
  • It is used for inhibition of water corrosion.  
  • 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.
  • Trivalent chromium is present in food.

BIOLOGICAL ROLE

     Chromium (III) is one of the trace elements, which are necessary for human and  animal vital activity. It enters the organism from digestive tract and is transported  to the tissues, where its accumulation takes place.
       The studies show chromium as a critical cofactor in the action of insulin (Kimura,  1996) (Anderson, 1997). The considerable quantities of chromium (III) reveal in  nucleic acids. It influences on their metabolism, replication and transcription. The  ion decreases the content of corticosteroids in plasma and increases the  functional activity of immune system of organism (Snitynskyi, 1999).