Learning Objectives
- Understand general characteristics of transition metals
- Study variable oxidation states, colour, and magnetic properties
- Learn about important compounds: KMnO₄ and K₂Cr₂O₇
- Understand lanthanoid and actinoid contraction
- Study catalytic activity and complex formation
Key Concepts
d-Block Elements (Transition Metals)
Groups 3-12. Configuration: (n-1)d¹⁻¹⁰ ns⁰⁻². Transition metals: incomplete d-subshell in any common oxidation state.
Zn, Cd, Hg (d¹⁰) are d-block but NOT transition metals (fully filled d in all common states).
General Properties
Variable oxidation states: Due to small energy difference between (n-1)d and ns. Minimum = +2 (ns electrons). Maximum increases then decreases across series. Mn shows +2 to +7.
Coloured compounds: Due to d-d transitions. Partially filled d-orbitals absorb visible light. Cu²⁺ (blue), Fe³⁺ (yellow), Cr³⁺ (green), Mn²⁺ (pink).
Magnetic properties: Paramagnetic when unpaired electrons present. μ = √(n(n+2)) BM, where n = number of unpaired electrons.
Catalytic activity: Due to variable oxidation states and ability to form intermediates. Examples: Fe in Haber, V₂O₅ in Contact, Ni in hydrogenation, Pt in Ostwald.
Complex formation: Due to small size, high charge, and availability of d-orbitals.
Interstitial compounds: Small atoms (H, C, N, B) occupy voids. Very hard, high MP, chemically inert.
Alloy formation: Similar atomic sizes allow substitution in crystal lattice. Brass (Cu+Zn), Bronze (Cu+Sn), Steel (Fe+C+others).
Important Compounds
KMnO₄ (Potassium permanganate): Strong oxidising agent. Purple colour. In acidic medium: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.51 V). In neutral: MnO₄⁻ → MnO₂. In basic: MnO₄⁻ → MnO₄²⁻.
K₂Cr₂O₇ (Potassium dichromate): Orange in acidic, yellow (CrO₄²⁻) in basic. Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. Used in breathalyser test (orange → green with alcohol).
Chromate-dichromate equilibrium: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O.
f-Block Elements
Lanthanoids (4f): Ce to Lu. Common oxidation state: +3. Some show +4 (Ce) or +2 (Eu, Yb) due to empty/half-filled/fully-filled 4f.
Lanthanoid contraction: Gradual decrease in atomic/ionic radii across the series due to poor shielding by 4f electrons. Consequences: similar properties of 2nd and 3rd row transition metals (Zr/Hf, Nb/Ta, Mo/W).
Actinoids (5f): Th to Lr. Show wider range of oxidation states (+3 to +7). Many are radioactive.
Actinoid contraction: Similar to lanthanoid contraction due to poor shielding by 5f electrons.
Summary
Transition metals have partially filled d-orbitals leading to variable oxidation states, coloured compounds, paramagnetism, catalytic activity, and complex formation. KMnO₄ and K₂Cr₂O₇ are important oxidising agents. Lanthanoids show gradual contraction across the series. Actinoids show wider oxidation state range and are mostly radioactive.
Important Terms
- Transition Metal: Element with incomplete d-subshell in any oxidation state
- d-d Transition: Electron jumping between d-orbitals causing colour
- Lanthanoid Contraction: Gradual size decrease across 4f series
- Interstitial Compound: Small atoms in voids of metal lattice
- Magnetic Moment: μ = √(n(n+2)) BM
Quick Revision
- d-block: (n-1)d¹⁻¹⁰ ns⁰⁻²; Transition metals: partially filled d
- Coloured: d-d transitions; Magnetic: μ = √(n(n+2)) BM
- KMnO₄: +7 → +2 (acidic), +4 (neutral), +6 (basic)
- K₂Cr₂O₇: +6 → +3 (acidic); orange → green
- Lanthanoid contraction: poor shielding by 4f electrons
- Zn, Cd, Hg: d-block but NOT transition metals