Class 12 Chemistry Chapter 8 Notes: The d and f Block Elements | Important Questions & PYQs

 Class 12 Chemistry – Chapter 8 The d and f Block Elements Notes

1. Position in the Periodic Table

• d-Block elements: Groups 3 to 12

• f-Block elements:

  • Lanthanoids (Z = 58–71) → 4f filling

  • Actinoids (Z = 90–103) → 5f filling

• Called transition elements because they form a transition between s-block and p-block

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2. Electronic Configuration

• General configuration (d-block):

  $$(n-1)d^{1\text{–}10}n{s}^{1\text{–}2}$$

• Exception examples:

  • Cr → 3d⁵ 4s¹

  • Cu → 3d¹⁰ 4s¹

• Zn, Cd, Hg have fully filled d-orbitals → not true transition elements

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3. General Properties of Transition Elements

(A) Variable Oxidation States

• Due to participation of ns and (n–1)d electrons

• Maximum oxidation states shown by Mn (+7)

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(B) Formation of Coloured Compounds

• Due to d–d electronic transitions

• Ions with d⁰ or d¹⁰ configuration are colourless
  (e.g., Sc³⁺, Zn²⁺, Cu⁺)

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(C) Magnetic Properties

• Paramagnetism due to unpaired electrons

• Spin-only magnetic moment:

$$\mu =\sqrt{n(n+2)}\text{BM}$$

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(D) Catalytic Properties

• Due to:

  • Variable oxidation states

  • Ability to form complexes

• Examples:

  • Fe → Haber process

  • V₂O₅ → Contact process

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(E) High Enthalpy of Atomisation

• Strong metallic bonding due to unpaired d-electrons

• Zn has lowest atomisation enthalpy (d¹⁰ configuration)

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(F) Formation of Complexes

• Small size + high charge density

• Example: [Fe(CN)₆]³⁻, [Cu(NH₃)₄]²⁺

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(G) Alloy Formation

• Example: Steel, brass, bronze

• Improves hardness and strength

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(H) Interstitial Compounds

• Small atoms (H, C, N, B) occupy interstitial spaces

• Increase hardness, decrease ductility

• Example: Steel (C in Fe lattice)

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4. Important Transition Metal Compounds

Potassium Dichromate (K₂Cr₂O₇)

• Prepared from chromite ore (FeCr₂O₄)

• Strong oxidising agent

• pH effect:

  • Acidic → Dichromate (orange)

  • Basic → Chromate (yellow)

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Potassium Permanganate (KMnO₄)

• Prepared from pyrolusite (MnO₂)

• Very strong oxidising agent

• Used in:

  • Redox titrations

  • Water purification

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5. Lanthanoids

General Features

• Configuration:

  $$[Xe]4f^{1\text{–}14}5d^{0\text{–}1}6s^2$$

• Common oxidation state: +3

• Soft, silvery metals

• Reactivity similar to Ca and Al

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Lanthanoid Contraction

• Gradual decrease in atomic & ionic size

• Due to poor shielding by 4f electrons

Consequences

1. Difficulty in separation

2. Decrease in basicity of hydroxides

3. Similar sizes of 4d and 5d elements (Zr ≈ Hf)

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6. Actinoids

General Features

• Configuration:

  $$[Rn]5f^{1\text{–}14}6d^{0\text{–}1}7s^2$$

• Show wide range of oxidation states (+3 to +7)

• Most are radioactive

• More reactive than lanthanoids

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Actinoid Contraction

• Greater than lanthanoid contraction

• Due to poorer shielding by 5f electrons

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7. Comparison: Lanthanoids vs Actinoids

Property	Lanthanoids	Actinoids
f-orbitals	4f	5f
Oxidation states	+3 mainly	+3 to +7
Radioactivity	Mostly non-radioactive	All radioactive
Reactivity	Less	More

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8. Stability of Oxidation States

• Half-filled (d⁵) and fully filled (d¹⁰) are most stable

• Mn²⁺ (d⁵) → very stable

• Cr²⁺ → strong reducing agent

• Mn³⁺ → strong oxidising agent

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9. Coloured vs Colourless Ions

• Coloured: Ti³⁺, V³⁺, Mn²⁺, Fe³⁺, Co²⁺

• Colourless: Sc³⁺, Zn²⁺, Cu⁺

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10. Important Exam-Favourite Points

✔ Transition elements show variable oxidation states
✔ Zn, Cd, Hg are NOT true transition elements
✔ Colour due to d–d transitions
✔ Lanthanoid contraction affects 5d elements
✔ Actinoids show maximum oxidation states

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11. Applications of d- and f-Block Elements

• Fe → construction, machinery

• Cu → electrical wiring

• Pt, Pd → catalysts

• Ce alloy (Misch metal) → jet engines

• U, Pu → nuclear fuel

• 
  

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are transition elements?
   Elements having partially filled (n−1)d orbitals in their atoms or common oxidation states.

2. Why are Zn, Cd and Hg not considered true transition elements?
   Because they have completely filled d-orbitals (d¹⁰) in their common oxidation states.

3. Why do transition elements show variable oxidation states?
   Due to involvement of both ns and (n−1)d electrons in bonding.

4. Why do transition metal ions form coloured compounds?
   Due to d–d electronic transitions.

5. Which transition metal shows the highest oxidation state?
   Manganese (+7).

6. What causes paramagnetism in transition metals?
   Presence of unpaired electrons.

7. What is lanthanoid contraction?
   Gradual decrease in atomic and ionic size of lanthanoids.

8. Why are actinoids radioactive?
   Because their nuclei are unstable.

9. Which ions are colourless: d⁰ or d¹⁰?
   Both d⁰ and d¹⁰ ions are colourless.

10. Why do transition metals form complexes?
    Due to small size and high charge density.

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📝 Long Answer Questions

1. Explain the general properties of transition elements.

Transition elements show several characteristic properties:

• Variable oxidation states: Due to participation of ns and (n−1)d electrons

• Coloured compounds: Caused by d–d transitions

• Magnetic properties: Most are paramagnetic due to unpaired electrons

• Catalytic activity: Due to variable oxidation states and complex formation

• High enthalpy of atomisation: Due to strong metallic bonding

• Complex formation: Example: [Fe(CN)₆]³⁻, [Cu(NH₃)₄]²⁺

• Alloy formation: Improves hardness and strength (steel, brass)

• Interstitial compounds: Small atoms occupy gaps, increasing hardness

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2. Explain lanthanoid contraction and its consequences.

Lanthanoid contraction is the gradual decrease in size of lanthanoids from La³⁺ to Lu³⁺.

Cause:

• Poor shielding effect of 4f electrons

Consequences:

• Difficulty in separation of lanthanoids

• Decrease in basicity of lanthanoid hydroxides

• Similar sizes of 4d and 5d elements (e.g., Zr ≈ Hf)

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3. Compare lanthanoids and actinoids.

Property	Lanthanoids	Actinoids
f-orbital filling	4f	5f
Oxidation states	+3 mainly	+3 to +7
Radioactivity	Mostly non-radioactive	All radioactive
Reactivity	Less	More
Contraction	Lanthanoid contraction	Actinoid contraction (greater)

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📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. Why are transition elements called transition metals?

2. Why are Zn, Cd and Hg not considered transition elements?

3. Which transition metal shows maximum oxidation state?

4. Why are d⁰ and d¹⁰ ions colourless?

5. What causes paramagnetism in transition metals?

6. Write the formula for spin-only magnetic moment.

7. What is lanthanoid contraction?

8. Which series shows greater contraction: lanthanoids or actinoids?

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🔹 2 Mark PYQs

1. Why do transition metals show variable oxidation states?

2. Write two reasons for high enthalpy of atomisation of transition metals.

3. Why do transition metals form coloured compounds?

4. Write two differences between lanthanoids and actinoids.

5. Why are transition metals good catalysts?

6. What are interstitial compounds? Write one example.

7. Why is Cr²⁺ a strong reducing agent?

8. Why is Mn³⁺ a strong oxidising agent?

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🔹 3 Mark PYQs

1. Explain catalytic properties of transition metals.

2. Explain lanthanoid contraction and write its two consequences.

3. Describe magnetic properties of transition elements.

4. Explain formation of alloys in transition metals.

5. Explain why actinoids show a wide range of oxidation states.

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🔹 5 Mark PYQs

1. (a) Describe general characteristics of transition elements.
   (b) Why do they form complexes?

2. (a) What is lanthanoid contraction?
   (b) Explain its causes and consequences.

3. (a) Compare lanthanoids and actinoids.
   (b) Give reasons for higher reactivity of actinoids.

4. (a) Explain colour and magnetic properties of transition metals.
   (b) Why is Zn not a transition metal?

5. (a) Describe preparation, properties and uses of KMnO₄
   OR
   (b) Describe preparation and properties of K₂Cr₂O₇

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Variable oxidation states
✔ Colour & magnetic behaviour
✔ Zn, Cd, Hg – not transition elements
✔ Lanthanoid contraction (cause + effects)
✔ Comparison: lanthanoids vs actinoids
✔ KMnO₄ & K₂Cr₂O₇

✔ Catalytic nature of transition metals ❓ FAQs (Concept Clarity)

1. Why are half-filled and fully filled d-orbitals more stable?
   Because they have symmetrical electron distribution and exchange energy.

2. Why is Zn the least atomised transition metal?
   Because of fully filled d¹⁰ configuration.

3. Why do transition metals act as good catalysts?
   They can change oxidation states and form intermediate complexes.

4. Why is actinoid contraction greater than lanthanoid contraction?
   Because 5f electrons shield nuclear charge even more poorly than 4f electrons.

5. Why are interstitial compounds hard but less ductile?
   Because small atoms distort metal lattice.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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