The d-Block Elements
The transition metals are the elements in the d-block of the periodic table (Groups 3-12). They have a unique set of properties that distinguish them from the main-group elements.
Characteristic Properties
1.Variable Oxidation States:
Unlike main-group metals that typically form only one ion (e.g., Na⁺, Mg²⁺), most transition metals can form ions with several different positive charges (oxidation states).
Reason: The energies of the valence d and s orbitals are very close. This allows the atoms to lose a variable number of electrons from both the s and d sublevels. For example, Iron (Fe) commonly forms both Fe²⁺ and Fe³⁺ ions.
2.Formation of Colored Compounds:
Many compounds and complex ions of transition metals are brightly colored (e.g., KMnO₄ is deep purple, CuSO₄ is blue).
Reason: The presence of partially filled d orbitals. The energy differences between the d orbitals are small. When visible light shines on the compound, electrons can absorb a specific wavelength (color) of light to be promoted from a lower-energy d orbital to a higher-energy one. The color we perceive is the complementary color of the light that was absorbed. Main-group metal compounds are usually white because the energy gap to promote their electrons is too large, requiring UV light.
3.Catalytic Activity:
Many transition metals and their compounds are excellent catalysts for a wide range of chemical reactions.
Reason: Their ability to exist in multiple oxidation states allows them to effectively facilitate redox reactions. They can also provide a surface (in the case of solid catalysts) for reactants to adsorb and interact. Examples include iron in the Haber process for making ammonia and platinum in catalytic converters.