d-Block and f-Block Elements for JEE: Transition Metals Complete Guide

Cr has configuration [Ar]3d⁵4s¹ instead of [Ar]3d⁴4s². Half-filled d⁵ subshell has extra stability due to exchange energy and symmetry. Similarly, Cu has [Ar]3d¹⁰4s¹ (fully-filled d¹⁰).

Transition metal compounds are colored because of d-d transitions. When visible light is absorbed, electrons jump between d-orbital energy levels. The unabsorbed light gives the color: Cu²⁺ absorbs red-orange, appears blue.

Lanthanide contraction is the steady decrease in ionic radius from La³⁺ to Lu³⁺. Due to poor shielding by 4f electrons, effective nuclear charge increases. This makes lanthanides chemically similar and hard to separate.

Transition metals are good catalysts because: (1) variable oxidation states allow them to form intermediate compounds, (2) they can adsorb reactants on their surface, (3) d-orbital availability for bond formation. Examples: Fe (Haber process), Ni (hydrogenation), V₂O₅ (contact process).

Transition metals form complexes due to: (1) small size and high charge density, (2) presence of vacant d-orbitals that accept lone pairs from ligands, (3) ability to exhibit variable oxidation states, (4) high polarizing power.

Color is due to d-d transitions. Common colors: Ti³⁺ (purple), V³⁺ (green), Cr³⁺ (green), Mn²⁺ (pale pink), Fe³⁺ (yellow), Fe²⁺ (pale green), Co²⁺ (pink), Ni²⁺ (green), Cu²⁺ (blue), Zn²⁺ (colorless, d¹⁰).

Cu⁺ disproportionates: 2Cu⁺ → Cu + Cu²⁺. The +2 state is more stable in aqueous due to: (1) higher hydration energy of Cu²⁺ (higher charge), (2) Cu²⁺ has one unpaired electron that is more stable in aqueous environment.

Melting points generally increase then decrease across the series. Maximum at middle (Cr, Mo, W have highest due to strong metallic bonding with maximum number of unpaired d-electrons contributing to bonding).

Lanthanide contraction is the steady decrease in atomic/ionic radius from La³⁺ to Lu³⁺. Caused by poor shielding of 4f electrons leading to increased effective nuclear charge. Consequences: similar properties, difficult separation, post-lanthanide elements have smaller radii than expected.

Mn exhibits all oxidation states from +2 to +7. Stability: Mn²⁺ (d⁵, half-filled) is most stable. MnO₄⁻ (Mn⁺⁷) is a strong oxidizing agent in acidic medium.

KMnO₄: purple solid, Mn in +7 state, strong oxidizing agent. In acidic medium: MnO₄⁻ → Mn²⁺ (pale pink). Uses: volumetric analysis (titrations), disinfectant, bleaching agent, water treatment.

K₂Cr₂O₇: orange-red crystals, Cr in +6 state, strong oxidizing agent in acidic medium: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (green). Used in: oxidation reactions, volumetric analysis, tanning leather.

Actinides are radioactive because their nuclei are large and unstable (all isotopes of elements beyond Bi are radioactive). Lanthanides are mostly stable (except Promethium). Actinides also show more variable oxidation states due to smaller energy gap between 5f and 6d orbitals.