d- and f-Block Elements
'Transition metals are the WORKHORSES of chemistry — they catalyse reactions, form coloured compounds, and display multiple oxidation states.'
1. Chapter Overview
The d-block elements (Groups 3 to 12) are known as TRANSITION METALS — they have incompletely filled d-orbitals in the ground state or common oxidation states. The f-block elements (LANTHANOIDS and ACTINOIDS) have incompletely filled f-orbitals. Topics include: ELECTRONIC CONFIGURATION of d- and f-block elements, PERIODIC TRENDS (atomic radii, ionisation enthalpy, oxidation states), MAGNETIC PROPERTIES, FORMATION OF COLOURED IONS, CATALYTIC PROPERTIES, the preparation and properties of POTASSIUM DICHROMATE (K₂Cr₂O₇) and POTASSIUM PERMANGANATE (KMnO₄), and the LANTHANOID CONTRACTION.
2. Electronic Configuration
d-Block
- General: (n−1)d¹⁻¹⁰ ns¹⁻² (except for anomalies in Cr, Cu, etc.).
- Anomalies: Cr (3d⁵4s¹ — HALF-FILLED d subshell), Cu (3d¹⁰4s¹ — FULLY FILLED d subshell). 'Half-filled and fully-filled d subshells provide EXTRA STABILITY.'
f-Block
- Lanthanoids (4f): Ce to Lu (4f¹⁻¹⁴5d⁰⁻¹6s²).
- Actinoids (5f): Th to Lr (5f⁰⁻¹⁴6d⁰⁻²7s²). All are RADIOACTIVE.
3. General Properties of Transition Metals
Physical Properties
- HIGH melting and boiling points (due to strong metallic bonding via d-electrons).
- HARD and DENSE.
- GOOD conductors of heat and electricity.
Variable Oxidation States
- 'Transition metals show MULTIPLE oxidation states because the (n−1)d and ns orbitals have SIMILAR energies — electrons can be removed from both.'
- Examples: Mn (+2 to +7), Cr (+2 to +6), Fe (+2, +3).
- Stability trends: Higher oxidation states are more stable with O, F (e.g., Mn₂O₇ exists, MnF₇ exists); lower oxidation states are more stable in aqueous solution.
Formation of Coloured Ions
- 'The colour of transition metal ions is due to d−d TRANSITIONS — electrons absorb visible light and jump between d-orbitals.'
- The colour depends on: (1) the metal, (2) its oxidation state, (3) the ligands attached.
- Ti³⁺ (violet), Cu²⁺ (blue), Ni²⁺ (green), Fe³⁺ (yellow/brown).
4. Magnetic Properties
| Type | Behaviour | Examples |
|---|---|---|
| Paramagnetic | ATTRACTED by magnetic field (unpaired electrons) | Cu²⁺ (1 unpaired), Fe²⁺ (4 unpaired) |
| Diamagnetic | WEAKLY REPELLED (no unpaired electrons) | Zn²⁺, Sc³⁺ |
| Ferromagnetic | STRONGLY ATTRACTED (permanent magnetic moment) | Fe, Co, Ni |
5. Catalytic Properties
- Transition metals are EXCELLENT catalysts because they:
- Have VARIABLE oxidation states — can accept and donate electrons easily.
- Provide SURFACE for adsorption of reactants.
- Examples: Fe (Haber process — NH₃ synthesis). Ni (hydrogenation of oils). Pt (catalytic converters). V₂O₅ (Contact process — H₂SO₄ manufacture).
6. Important Compounds
Potassium Dichromate (K₂Cr₂O₇)
- Preparation: Chromite ore (FeCr₂O₄) → Na₂CrO₄ → Na₂Cr₂O₇ → K₂Cr₂O₇.
- Properties: ORANGE crystalline solid. STRONG OXIDISING AGENT in acidic medium: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
- Colour change: Orange (Cr₂O₇²⁻) → Green (Cr³⁺).
- Tests: Oxidises Fe²⁺ to Fe³⁺, SO₂ to SO₄²⁻, I⁻ to I₂.
Potassium Permanganate (KMnO₄)
- Preparation: Pyrolusite (MnO₂) → K₂MnO₄ → KMnO₄ (electrolytic oxidation).
- Properties: DEEP PURPLE crystalline solid. STRONG OXIDISING AGENT.
- Oxidation in different media:
| Medium | Half-Reaction | Colour Change |
|---|---|---|
| Acidic | MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O | Purple → Colourless/Pale pink |
| Neutral | MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ | Purple → Brown (MnO₂) |
| Alkaline | MnO₄⁻ + e⁻ → MnO₄²⁻ | Purple → Green (manganate) |
7. Lanthanoid Contraction
- 'The STEADY DECREASE in atomic and ionic radii across the lanthanoid series (from La to Lu).'
- Cause: The 4f electrons have POOR SHIELDING EFFECT — as nuclear charge increases, the outer electrons are pulled INWARDS.
- Consequences:
- Similar chemical properties across the series.
- Difficult SEPARATION of lanthanoids.
- Post-lanthanoid elements (Zr, Hf; Nb, Ta) have ALMOST IDENTICAL radii — extremely similar properties.
Comparison: Lanthanoids vs Actinoids
| Property | Lanthanoids (4f) | Actinoids (5f) |
|---|---|---|
| Electronic configuration | 4f¹⁻¹⁴5d⁰⁻¹6s² | 5f⁰⁻¹⁴6d⁰⁻²7s² |
| Radioactivity | Mostly non-radioactive | ALL radioactive |
| Oxidation states | Mainly +3 (some +2, +4) | +3 to +7 (more VARIABLE) |
| Colour | Many have characteristic colours | Coloured |
| Separation | Very DIFFICULT (similar properties) | Slightly easier (more variation) |
8. Common Mistakes
- Cr and Cu electronic configuration exceptions: Cr is NOT 3d⁴4s². It is 3d⁵4s¹ (half-filled stability). Cu is 3d¹⁰4s¹ (fully filled d).
- Oxidation states: Not ALL transition metals show the same range. Sc only shows +3. Zn only shows +2.
- Colour in transition metals: Colour is due to d-d transitions — requires at LEAST ONE unpaired d-electron. Sc³⁺ and Zn²⁺ are COLOURLESS (d⁰ and d¹⁰).
- KMnO₄ titrations: In ACIDIC medium, MnO₄⁻ is REDUCED to Mn²⁺ (pale pink, almost colourless). In NEUTRAL, it forms MnO₂ (brown precipitate).
9. CBSE Exam Focus
- Electronic configuration of d-block elements — general and exceptions (Cr, Cu)
- General properties — oxidation states, colour, magnetic, catalytic
- K₂Cr₂O₇ — preparation, properties as oxidising agent
- KMnO₄ — preparation, oxidising action in acidic/neutral/alkaline media
- Lanthanoid contraction — cause and consequences
- Comparison of lanthanoids and actinoids
10. Self-Test
Q1: Write the electronic configuration of Cr (Z=24) and Cu (Z=29). A1: Cr: [Ar]3d⁵4s¹ (NOT 3d⁴4s²). Cu: [Ar]3d¹⁰4s¹ (NOT 3d⁹4s²).
Q2: Why are transition metal ions coloured? A2: Due to d-d transitions involving unpaired d-electrons. Electrons in lower d-orbitals absorb visible light and get excited to higher d-orbitals. The COMPLEMENTARY colour is transmitted/reflected, giving the ion its colour.
Q3: Balance the following in acidic medium: Cr₂O₇²⁻ + Fe²⁺ → Cr³⁺ + Fe³⁺. A3: Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺.
Q4: Why is Zn not considered a transition metal? A4: Zn has electronic configuration [Ar]3d¹⁰4s². Its d-orbitals are FULLY FILLED (d¹⁰) in the ground state and common oxidation state (+2). It does NOT show variable oxidation states or form coloured ions. 'A transition metal MUST have incompletely filled d-orbitals.'
Q5: What is lanthanoid contraction? Write two consequences. A5: The gradual DECREASE in atomic/ionic radii from La to Lu due to poor shielding by 4f electrons. Consequences: (1) Lanthanoids have VERY SIMILAR chemical properties — difficult to separate. (2) Post-lanthanoid pairs (Zr/Hf, Nb/Ta) have ALMOST IDENTICAL atomic radii.
11. Conclusion
Transition metals are the MOST VERSATILE elements in the periodic table:
- ELECTRONS: 'The d-orbitals give them variable oxidation states, colour, and magnetic properties.'
- CATALYSIS: 'They can accept and donate electrons with ease — the hallmark of a good catalyst.'
- COMPOUNDS: 'K₂Cr₂O₇ and KMnO₄ are POWERFUL oxidising agents — workhorses of analytical chemistry.'
- f-BLOCK: 'Lanthanoids and actinoids — the inner transition series — with unique properties of their own.'
'd- and f-block elements are the colour, magnetism, and catalytic POWER of the periodic table.'
