Chromium in Periodic Table
Chromium (Cr), chemical element, the hard, malleable, greyish white shiny metal of Group 6 (VIB) of the periodic table, widely used in making stainless steel and other alloys. In1797, chromium was discovered by French chemist Nicolas-Louis Vauquelin from Siberian mineral like PbCrO4, and the name of the element was derived from Greek latter chroma means colour. It is readily detected by borax bead test (green) and the yellow melt of chromates are formed on fusion with alkali or an oxidizing agent. Chromates produce deep blue colour with hydrogen peroxide in acid solution.
The body-centered cubic crystal lattice, chromium has chemical symbol Cr, atomic number 24, atomic weight 51.996, melting point 1907°C, boiling point 2671°C, and valence shell electron configuration [Ar] 3d5 4s1. The melting point and density of chromium are higher than those of vanadium suggest the less participation of d-electron in metallic chemical bonding. The oxidation state vs free energy diagram shows that the stable oxidation number or state of chromium is +3.
Occurrence and Production
Chromium is the 21st most abundant chemical element in the earth’s crust (0.04 percent by weight). Chromium occurs mainly as chromite (FeCr2O4), found in South Africa, India, Philippines, Turkey, and Russian countries. Crocoite (PbCrO4) and chrome ochre (Cr2O3) are the other minerals of the metal found in the earth’s environment. In gemstone, some chromium ion (Cr+3) ions are substituted by aluminum (Al+3) ions. The presence of Cr+3 and Cr+4 ions in the earth’s environment strongly depend on the pH scale and oxidative properties of the soil of location.
Naturally the chemical element, chromium contains four stable isotopes like 52Cr (83.76 percent), 53Cr (9.55 percent), 50Cr (4.31 percent), 54Cr (2.38 percent), and nineteen radioactive isotopes produced from different types of radioactive decay or nuclear reaction.
Greyish white shiny metal, chromium is extracted from chromite (Cr2O3). Powdered chromite is heated (1000°C to 1300°C) with sodium carbonate and quicklime in a revelatory furnace in presence of excess air. Metallic chromium is oxidized to form sodium chromite and iron is converted to F2O3. The sodium chromite is leached out with water and converted to sodium dichromate by adding concentrated sulfuric acid. The produced dichromate is reduced to Cr2O3 by heating with carbon. Metallic chromium is obtained by the reduction of Cr2O3 by aluminum powdered through the Thermite process (Cr2O3 + 2Al → Al2O3 + Cr). The pure form of the element may be obtained by reducing chromium (III) chloride with metallic calcium in the melting solution of CaCl2 or BaCl2.
Chemistry and Compounds
Chromium is the fourth transition metal after scandium, titanium, vanadium in the periodic table with electron configuration [Ar] 3d5 4s1. The electron configuration of Cr violates the Aufbau principle by the recording of 3d orbital to form a new electronic configuration to gain extra chemical stability by exchange energy. It dissolves slowly in dilute hydrochloric acid and sulfuric acid with evolving hydrogen but in concentrated nitric acid, it forms an impervious oxide layer. In chemistry, the common and dominated oxidation state of chromium is +3 and +6, it also forms organometallic compounds in +1, +2, +4, and +5 oxidation states.
Compounds of Chromium(II)
The metallic chromium offers a fair number of Cr(II) compounds which including oxides and dihalides. The oxides CrO, a black powdered formed by exposing chromium amalgam to air, is not well characterized. Al the four dihalides (CrF2, CrCl2, CrBr2, and CrI2) is obtained by reducing CrX3 with hydrogen or by reducing the metal and gaseous HX or iodine at 700°C to 1000°C. All the Cr(II) halides are reducing in nature, they can liberate hydrogen from water in absence of an oxidant.
Compounds of Chromium(III)
The common and most stable oxidation state of chromium is Cr(III), which forms the most stable trivalent cation in water solution and series of substituted inert metallic complexes. The oxide, Cr2O3 is formed by burning chromium in oxygen or heating ammonium dichromate. All the three halides like CrCl3 (red-violet), CrBr3 (dark green), and CrI3 (dark green) are prepared by direct combination of the metal with halides like chlorine, bromine, and iodine at various temperature region. Green colour, CrF3 also prepared by direct combination of the metal with fluorine but it may be prepared better by heating CrCl3 with HF at 500°C. The sulfide compounds can be made by direct reaction of chromium with sulfur or anhydrous CrCl3 with H2S.
Compounds of Chromium(VI)
In learning chemistry, compounds of chromium(VI) are very impotent and limited, the principal species in the +6 state are CrO3, unstable CrF6, the oxohalides CrOX4 (X = F, Cl), CrO3X2, and CrO3X–, the oxoanions, and peroxo molecule. Chromium trioxide is formed by adding concentrated sulfuric acid to a concentrated aqueous solution of potassium dichromate. The oxide is highly acidic and soluble in water solution to form H2CrO4 (pH scale = 1), HCrO4– (pH scale > 1) and Cr2O7-2. It is a powerful oxidizing agent that oxidizes most materials like paper, sugar, oxalic acid, and alcohol. Glacial acetic acid is not oxidized by it and the glacial acetic acid solution of chromium trioxide uses as an oxidizing agent in chemistry.
Chromyl chloride is the most important oxohalides of Cr(VI) formed when ionic chloride is heated with potassium dichromate and concentrated H2SO4. It is a violent oxidizing agent inflammable in phosphorus, sulfur, alcohol, etc. chromyl chloride forms another violent oxidizing agent on reaction with nitrogen pentoxide (N2O5). Sodium or potassium chromates and dichromate are also two important chemicals of chromium widely used as an oxidizing agent and analysis of redox reaction.
Uses of Chromium
About 85 percent of chromium is used in making stainless steel and other alloys in metallurgy, the reminder party used in different types of chemical, refractory, and industries. It makes steel tough and corrosion resistant, hence chromium steel (1.2 to 2 percent Cr) is widely used in cutting tools, oil tubing, automobile trim, armor plates, ball bearings making. Steels with higher chromium contact (17 to 18 percent) and 7 percent of nickel have superior corrosion resistance used in jet engines, gas turbines, and common structural supplement. Cr-vanadium steel (1 percent Cr and 0.15 percent of vanadium) and Cr-tungsten steel (3.8 percent Cr and 14 20 percent W) are used in making springs, shafts, axles, and different types of high-speed tools.
Nichrome (60 percent of Ni, 14 percent of Cr, and 25 percent of iron, and 0.2-1 percent carbon) used in resistance coil for electrical heating due to its high melting point, high electrical resistance, and low oxidation properties. Several chromium compounds used as a chemical catalyst for the production of hydrocarbon, pigments due to its bright green, yellow, red, and orange colours, preservation of wood due to the toxic nature of Cr (IV) salts, and about 90 percent of leather is tanned by using chrome alum and Cr(III) sulfate compounds. Chromium is also used in chromium plating which does not damage by sulfur compounds present in the earth’s atmosphere.