Chapter 8 — The d- and f-Block Elements — covers transition metals (3d, 4d, 5d series) and inner transition metals (lanthanoids and actinoids). These elements are known for their coloured compounds, variable oxidation states, and catalytic properties. This chapter carries 5-7 marks in Board exams. Focus on properties of 3d series and preparation of KMnO₄ and K₂Cr₂O₇.
Key Concepts
d-Block Elements (Transition Elements)
General electronic configuration: (n−1)d¹⁻¹⁰ ns¹⁻²
Characteristics of Transition Elements
| Property | Explanation |
|---|---|
| Variable oxidation states | Due to involvement of both (n−1)d and ns electrons in bonding |
| Coloured compounds | Due to d-d transitions (unpaired d-electrons absorb visible light) |
| Complex formation | Small size + high charge + vacant d-orbitals accept lone pairs from ligands |
| Catalytic activity | Variable oxidation states + ability to form intermediates |
| Magnetic properties | Unpaired d-electrons → paramagnetic; all paired → diamagnetic |
| Alloy formation | Similar atomic sizes → atoms can substitute in crystal lattice |
| Interstitial compounds | Small atoms (H, C, N) fit in voids of metal lattice |
Electronic Configurations — Anomalies
| Element | Expected | Actual | Reason |
|---|---|---|---|
| Cr (Z=24) | [Ar] 3d⁴ 4s² | [Ar] 3d⁵ 4s¹ | Half-filled d⁵ is extra stable |
| Cu (Z=29) | [Ar] 3d⁹ 4s² | [Ar] 3d¹⁰ 4s¹ | Fully-filled d¹⁰ is extra stable |
Oxidation States of 3d Series
| Element | Common Oxidation States | Most Stable |
|---|---|---|
| Sc | +3 | +3 |
| Ti | +2, +3, +4 | +4 |
| V | +2, +3, +4, +5 | +5 |
| Cr | +2, +3, +6 | +3 |
| Mn | +2, +3, +4, +6, +7 | +2 |
| Fe | +2, +3 | +2, +3 |
| Co | +2, +3 | +2 |
| Ni | +2 | +2 |
| Cu | +1, +2 | +2 |
| Zn | +2 | +2 |
Important Compounds
Potassium Permanganate (KMnO₄)
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O (fusion with KOH)
3K₂MnO₄ + 2CO₂ → 2KMnO₄ + MnO₂ + 2K₂CO₃ (disproportionation)
Or electrolytically: MnO₄²⁻ → MnO₄⁻ + e⁻
Properties:
- Dark purple crystals, powerful oxidising agent
- In acidic medium: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (colourless)
- In neutral/alkaline: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (brown ppt)
- Used in titrations (self-indicator — purple → colourless)
- Oxidises Fe²⁺ to Fe³⁺, oxalate to CO₂, SO₃²⁻ to SO₄²⁻
Potassium Dichromate (K₂Cr₂O₇)
4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl (less soluble, crystallises)
Properties:
- Orange crystals, powerful oxidising agent in acidic medium
- In acidic medium: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (green)
- pH-dependent colour: CrO₄²⁻ (yellow, basic) ⇌ Cr₂O₇²⁻ (orange, acidic)
- Used in breathalyser test (turns green with alcohol)
2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O
Add acid → shifts right (orange) | Add base → shifts left (yellow)
f-Block Elements
Lanthanoids (4f series: Ce to Lu)
- General configuration: [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s²
- Common oxidation state: +3 (most stable). Some show +2 (Eu, Yb) and +4 (Ce, Tb)
- Lanthanoid contraction: Steady decrease in ionic radii from La³⁺ to Lu³⁺ due to poor shielding by 4f electrons
- Consequences of contraction: similar properties of 4d and 5d series elements in same group
Actinoids (5f series: Th to Lr)
- General configuration: [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s²
- Show wider range of oxidation states than lanthanoids (+2 to +7)
- Most are radioactive; only Th, Pa, U occur naturally
- Also show contraction (actinoid contraction) similar to lanthanoids
Comparison: Lanthanoids vs Actinoids
| Property | Lanthanoids | Actinoids |
|---|---|---|
| Filling orbitals | 4f | 5f |
| Oxidation states | Mainly +3 | +2 to +7 (wider range) |
| Radioactivity | Not radioactive (except Pm) | All radioactive |
| Complex formation | Less tendency | Greater tendency |
| Contraction | Lanthanoid contraction | Actinoid contraction (greater) |
Important Definitions
| Term | Definition |
|---|---|
| Transition Element | Element with partially filled d-orbitals in its ground state or common ion |
| Lanthanoid Contraction | Gradual decrease in ionic radii across lanthanoid series due to poor 4f shielding |
| Interstitial Compound | Compound formed when small atoms (H, C, N) occupy voids in metal lattice |
| d-d Transition | Transition of electron between split d-orbitals responsible for colour |
Solved Examples — NCERT Based
Example 1: Magnetic Moment
Q: Calculate the magnetic moment of Fe²⁺ and Fe³⁺. Which is more paramagnetic?
Solution:
Fe²⁺ = [Ar] 3d⁶ → 4 unpaired electrons → μ = √(4×6) = √24 = 4.90 BM
Fe³⁺ = [Ar] 3d⁵ → 5 unpaired electrons → μ = √(5×7) = √35 = 5.92 BM
Fe³⁺ is more paramagnetic (higher magnetic moment due to more unpaired electrons).
Example 2: Colour of Transition Metal Ions
Q: Why are Zn²⁺ compounds colourless while Cu²⁺ compounds are blue?
Solution:
Zn²⁺ has configuration 3d¹⁰ — all d-orbitals are fully filled. No d-d transition is possible → no visible light absorbed → colourless.
Cu²⁺ has configuration 3d⁹ — one unpaired electron. d-d transition absorbs red light → blue colour observed (complementary colour).
Example 3: KMnO₄ as Oxidising Agent
Q: Write the ionic equation for the reaction of KMnO₄ with FeSO₄ in acidic medium.
Solution:
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (reduction)
Fe²⁺ → Fe³⁺ + e⁻ (oxidation) × 5
Net: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
Purple KMnO₄ becomes colourless (Mn²⁺), and pale green Fe²⁺ becomes yellow Fe³⁺.
Example 4: Lanthanoid Contraction
Q: What is lanthanoid contraction? What are its consequences?
Solution:
Lanthanoid contraction is the gradual decrease in atomic/ionic radii from La to Lu due to poor shielding effect of 4f electrons.
Consequences:
1. Similarity in properties of 2nd and 3rd row transition metals (Zr/Hf, Nb/Ta, Mo/W have almost identical radii)
2. Difficulty in separating lanthanoids from each other
3. Basicity decreases from La(OH)₃ to Lu(OH)₃
Important Questions for Board Exams
1 Mark Questions
- Why do transition metals show variable oxidation states?
- Why is Zn not considered a transition element?
- What causes the colour of transition metal compounds?
- Calculate the magnetic moment of Mn²⁺.
2 Mark Questions
- Explain why Cr and Cu have anomalous electronic configurations.
- What is lanthanoid contraction? Give two consequences.
- Why is +2 the most stable oxidation state of Mn?
- Write the chromate-dichromate equilibrium. What happens when acid is added?
3 Mark Questions
- Describe the preparation and oxidising properties of KMnO₄ in acidic medium.
- How is K₂Cr₂O₇ prepared from chromite ore? Write its action as an oxidising agent in acidic medium.
- Compare lanthanoids and actinoids (any 5 differences).
- Why do transition metals act as good catalysts? Give three examples.
5 Mark Questions
- Discuss the general characteristics of transition elements: (a) variable oxidation states (b) colour (c) magnetic properties (d) catalytic activity (e) complex formation.
- What is KMnO₄? How is it prepared? Write its reactions in (a) acidic medium (b) alkaline medium. Why is it called a self-indicator?
Quick Revision Points
- d-block: (n-1)d¹⁻¹⁰ ns¹⁻²; Zn/Cd/Hg are NOT typical transition elements (d¹⁰)
- Anomalies: Cr = [Ar]3d⁵4s¹, Cu = [Ar]3d¹⁰4s¹
- Colour: due to d-d transitions; no colour if d⁰ or d¹⁰
- Magnetic moment: μ = √(n(n+2)) BM, where n = unpaired electrons
- KMnO₄: purple, self-indicator; acidic → Mn²⁺ (colourless); alkaline → MnO₂ (brown)
- K₂Cr₂O₇: orange in acid, yellow (CrO₄²⁻) in base
- Lanthanoid contraction: poor 4f shielding → size decreases La to Lu
- Lanthanoids: mainly +3; Actinoids: +2 to +7, all radioactive
- Max oxidation state in 3d: increases Sc(+3) to Mn(+7), then decreases
Chapter Navigation
Previous: The p-Block Elements Class 12 Notes
Next: Coordination Compounds Class 12 Notes
Related Chapters in Class 12 Chemistry
- Coordination Compounds Class 12 Notes
- The p-Block Elements Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
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