Transition Elements

Transition elements (also known as transition metals) are elements that have partially filled d-orbitals. IUPAC defines transition elements as an element having a d subshell that is partially filled with electrons, or an element that has the ability to form stable cations with an incompletely filled d-orbital.

In general, any element which corresponds to the d-block of the modern periodic table (which consists of groups 3-12) is considered to be a transition element. Even the f-block elements comprising the lanthanides and the actinides can be considered as transition metals.

However, since the f-block elements have incompletely filled f-orbitals, they are often referred to as inner transition elements or inner transition metals.

General Characteristics of Transition Elements

The d-block elements are known for their:

  1.             Large charge: radius ratios
  2.             High melting points and boiling points
  3.             High densities and hardness.
  4.             Formation of paramagnetic compounds
  5.            Formation of colored ions/compounds
  6.            Ability to form stable complexes
  7.           These elements also exhibit a wide variety of oxidation states and tend to form compounds that act as catalysts in many chemical processes.

Chemistry of Chromium

Chromium is one of the important transition elements. Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it transforms into a paramagnetic state. Chromium is a member of the transition metals, in Group 6. Chromium has an electronic configuration of 4s13d5, owing to the lower energy of the high spin configuration.

Symbol                                      Cr

Atomic No                                24

Atomic Mass                            51.9961 amu

Electron Configuration             [Ar] 3d5 4s1

Color                                        Silver gray

Conductivity                           Good conductor

Physical State                          Solid

Melting point                          1907 oC

Boiling point                            2672 oC

Abundance                              21st  in Earth crust

Minerals of chromium           Chromite (Fe.Cr2O3)  Chrome Orchre (Cr2O3)   Crocite (PbCrO4)

Oxidation state                      +2, +3, +6

  1. +3 Oxidation state is most stable.
  2. In +2 oxidation state they act as a good reducing agent.
  3. In +6 oxidation state it acts as a strong oxidizing agent.

Acidic Character

The acidic character of chromium oxides is increased by increasing oxidation state of chromium.

Oxides

CrO

Cr2O3

CrO3

Oxidation state of Chromium

+2

+3

+6

Nature

Basic

Amphoteric

Acidic

Chromic Compounds

The compounds in which the oxidation state of Cr is +3 are called as chromic compounds.

Examples:        Cr2O3, CrCl3

Chrome salt

Those compounds in which the oxidation state of chromium is +2 is called as chrome salt.

Examples:        CrCl2, CrSO4·n H2O.


Uses of Chromium and its compounds

1)    Chromite ore

The metallurgical grade is used primarily to produce ferrochromium alloys, which are used in the production of stainless and other special steels.

2)    Metallic chromium and chromium alloys

  • Chromium metal (pure) is used to prepare alloys with high purity specifications. Chromium is thus an important and widely used alloying element in ferrous and nonferrous alloys, including those based on nickel, iron-nickel, cobalt, aluminium, titanium and copper.

3)    Chromium [III] compounds

  • Chromic acetate is used in printing and tanning, as a textile mordant, a polymerization and oxidation catalyst, and an emulsion hardener.
  • Chromic chloride is used for the production of commercial solutions of the basic chlorides (Cr(OH)2Cl) by reaction with sodium hydroxide.
  • Chromic hydroxide has been used as a catalyst, a tanning agent, a mordant.
  • Chromic nitrate has been used as a catalyst and a corrosion inhibitor.
  • Chromic oxide (anhydrous and hydrated) is used as a pigment. A substantial portion is also used in metallurgy in the manufacture of chromium metal and aluminium-chromium master alloys and, to a lesser extent, as a catalyst, in refractory brick, and as a chemical intermediate.
  • Chromic phosphate is used in pigments, phosphate coatings and wash primers, and as a catalyst.

4)    Chromium[VI] compounds

  • Chromium trioxide is used in chromium plating, particularly in the production of automobiles. Uses in other metal-finishing operations include aluminium anodizing, particularly on military aircraft; chemical conversion coatings, which provide both decoration and corrosion protection; and the production of phosphate films on galvanized iron or steel.
  • Sodium chromate is used in inks, leather tanning, wood preservation, corrosion inhibition, as a pigment in paint, water treatment, drilling muds, textile dyeing, cutting oils, catalysts, and as a raw material for the production of other chromium compounds.
  • Sodium dichromate is the primary base material for the manufacture of chromium chemicals, which are used in leather tanning, metal treatment, drilling muds, textile dyes, catalysts, and wood and water treatment.