CBSE Class 12 Chemistry 2026 Day 3: d & f-Block Elements Important Questions,Topics, MCQs & Revision Notes
Day 3 CBSE Class 12 Chemistry 2026 revision: master d- and f-Block Elements with important questions, MCQs, trends, oxidation states, lanthanoid contraction, magnetic properties, coloured ions, KMnO₄ and K₂Cr₂O₇ reactions, lanthanoids vs actinoids comparison and smart strategy. Covers conceptual theory, short answers and high-probability questions to score in the CBSE 12th Chemistry board exam.
Day 3 Mastery: d- and f-Block Elements – Important Questions, MCQs, Trends & Smart Strategy for CBSE Class 12 Chemistry 2026
Only a few days remain before the CBSE Class 12 Chemistry Board Exam on 28 February 2026. After completing Physical Chemistry units, today’s focus shifts to Inorganic Chemistry – Unit: d- and f-Block Elements. This chapter usually contributes around 7 marks and questions are mostly conceptual — comparisons, reactions, magnetic properties, colours and preparation of compounds like KMnO₄ and K₂Cr₂O₇.
Unlike numericals, this unit rewards memory + understanding. If you revise trends, oxidation states, and exceptions properly, you can score full marks very easily. Many students ignore this chapter, but in reality it is one of the fastest scoring units in the entire Chemistry syllabus.
What the Paper Usually Asks
| Topic | Type of Question | Frequency |
| Oxidation states | Very short answer / MCQ | Very Common |
| Magnetic properties | Numerical / Conceptual | Common |
| Lanthanoid contraction | Explanation question | Very Common |
| KMnO₄ & K₂Cr₂O₇ reactions | 3 or 5 marks | High Probability |
| Lanthanoids vs Actinoids | Difference table | Repeated |
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Quick Revision Notes
d-Block (Transition Elements)
- 1. Located in groups 3–12
- 2. General configuration: (n-1)d¹⁻¹⁰ ns¹⁻²
- 3. Show variable oxidation states
- 4. Form coloured ions
- 5. Mostly paramagnetic
- 6. Act as catalysts
Important exceptions
- 1. Cr → 3d⁵ 4s¹
- 2. Cu → 3d¹⁰ 4s¹
- 3. Zn, Cd, Hg are NOT transition metals
f-Block (Inner Transition Elements)
- 1. Lanthanoids → 4f filling (Ce–Lu)
- 2. Actinoids → 5f filling (Th–Lr)
- 3. Lanthanoids commonly +3 oxidation state
- 4. Actinoids show multiple oxidation states and radioactivity
Lanthanoid Contraction
Lanthanoid contraction is the gradual decrease in atomic and ionic size from Lanthanum to Lutetium. It occurs because 4f electrons shield the nuclear charge very poorly. As nuclear charge increases, electrons are pulled closer to the nucleus.
Effects you must remember:
- 1. Zr and Hf have almost identical size
- 2. Difficult separation of lanthanoids
- 3. Similar chemical properties
- 4. Increased density across the series
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Important Compounds
Potassium Permanganate (KMnO₄)
- 1. Colour: Purple
- 2. Strong oxidising agent
- 3. Mn oxidation state: +7
Reactions:
- 1. Acidic medium → Mn²⁺ (colourless)
- 2. Neutral medium → MnO₂ (brown)
- 3. Alkaline medium → MnO₄²⁻ (green)
Potassium Dichromate (K₂Cr₂O₇)
- 1. Colour: Orange
- 2. Oxidising agent in acidic medium
- 3. Cr⁶⁺ → Cr³⁺ (green solution)
Magnetic Properties (Numerical)
Magnetic moment formula:
μ = √[n(n+2)] Bohr Magneton
Where n = number of unpaired electrons.
Example: Fe³⁺ (d⁵) → n = 5
μ = √[5(7)] ≈ 5.92 BM
25 Most Important MCQs
- 1. Zn²⁺ is diamagnetic (d¹⁰ configuration)
- 2. Mn shows highest oxidation state (+7)
- 3. KMnO₄ colour due to charge transfer
- 4. Lanthanoids mainly +3 oxidation state
- 5. Actinoids more radioactive
- 6. Cu²⁺ ion is blue
- 7. Sc³⁺ colourless (no d electron)
- 8. Uranium used as nuclear fuel
- 9. Catalytic property due to variable oxidation states
- 10. Ion exchange method used for lanthanoid separation
- 11. Cr²⁺ reducing agent
- 12. Mn³⁺ oxidising agent
- 13. Paramagnetic = unpaired electrons present
- 14. Mischmetal contains lanthanoids
- 15. Transition metals form alloys easily
- 16. Interstitial compounds are hard
- 17. Highest melting point → Chromium
- 18. Ce shows +4 oxidation state
- 19. Half-filled stability → d⁵ configuration
- 20. Zn not transition element
- 21. Lanthanide contraction due to poor shielding
- 22. Fe catalyst used in Haber process
- 23. V₂O₅ catalyst used in Contact process
- 24. Green colour of Cr³⁺
- 25. Orange colour of dichromate ion
Very Important Questions
- 1. Define transition elements
- 2. Explain variable oxidation states
- 3. Why transition metals form coloured compounds?
- 4. Why Zn, Cd and Hg are not transition metals?
- 5. Explain lanthanoid contraction
- 6. Compare lanthanoids and actinoids
- 7. Preparation of KMnO₄
- 8. Oxidising action of K₂Cr₂O₇
Lanthanoids vs Actinoids
| Lanthanoids | Actinoids |
| 4f filling | 5f filling |
| Mostly +3 oxidation | Variable oxidation states |
| Less radioactive | Highly radioactive |
| Less reactive | More reactive |
Common Mistakes Students Make
- 1. Forgetting Cr and Cu electronic configuration exceptions
- 2. Confusing KMnO₄ reactions in different media
- 3. Not writing magnetic moment formula
- 4. Writing long theory instead of point-wise answers
