Class 12 — d and f Block Elements

Chapter Overview & Weightage

The “d and f Block Elements” chapter is a pure-memorisation goldmine in Class 12 Chemistry. Once you’ve learned the trends and a handful of named compounds, you can score 5+ marks consistently.

CBSE Class 12 Board — d & f Block Weightage

Year	Marks	Question Type
2024	5	1-mark MCQ + 3-mark trends + 1-mark name
2023	4	2-mark properties + 2-mark KMnO₄
2022	6	5-mark long on transition element trends
2021	5	3-mark + 2-mark (lanthanoid contraction)
2020	4	1-mark + 3-mark short answer

About 6–8% of the paper consistently. Easy marks if you’ve revised the property tables.

Key Concepts You Must Know

Ranked by board frequency:

General properties of transition metals — variable oxidation states, coloured ions, complex formation, magnetic behaviour, catalytic activity.
Why are transition metals coloured? — d-d transitions in partially filled d-orbitals.
Why do they form complexes? — small size, high charge, vacant d-orbitals for ligand donation.
Lanthanoid contraction and its consequences — similar size of 4d and 5d elements (Zr-Hf, Mo-W).
Preparation and properties of KMnO₄ and K₂Cr₂O₇ — appears every 2-3 years as a 3-mark question.
Comparison: 3d vs 4d vs 5d series — sizes, melting points, oxidation state stability.
Inner transition elements (lanthanides and actinides) — general properties and contraction.

Important Formulas

Spin-only magnetic moment: $\mu = \sqrt{n(n+2)}$ B.M., where $n$ = number of unpaired electrons.

Common transition-metal oxidation states to remember:

Sc: +3 (only)
Ti: +2, +3, +4
V: +2, +3, +4, +5
Cr: +2, +3, +6
Mn: +2 to +7 (most variable)
Fe: +2, +3
Co: +2, +3
Ni: +2 (most stable), +3
Cu: +1, +2
Zn: +2 (only)

Solved Previous Year Questions

PYQ 1 — Magnetic moment (CBSE 2023, 2 marks)

Q. Calculate the spin-only magnetic moment of $Fe^{3+}$ .

Solution. $Fe$ : [Ar] 3d⁶ 4s². $Fe^{3+}$ : [Ar] 3d⁵ (lose 4s² and one 3d electron). Five unpaired d-electrons.

\mu = \sqrt{5 \times 7} = \sqrt{35} \approx 5.92 \text{ B.M.}

PYQ 2 — Lanthanoid contraction (CBSE 2022, 3 marks)

Q. What is lanthanoid contraction? Mention two consequences.

Solution. Lanthanoid contraction is the steady decrease in atomic and ionic radii from La to Lu, despite increasing nuclear charge. The cause: poor shielding by 4f electrons.

Consequences: (1) The 5d transition metals (Hf, Ta, W) have nearly the same size as their 4d counterparts (Zr, Nb, Mo), making them chemically similar and hard to separate. (2) Densities of post-lanthanoid 5d elements are very high.

PYQ 3 — KMnO₄ preparation (CBSE 2024, 3 marks)

Q. How is KMnO₄ prepared from pyrolusite (MnO₂)?

Solution. Two-step process:

(1) Pyrolusite is fused with KOH in air or with KNO₃: $2MnO_2 + 4KOH + O_2 \to 2K_2MnO_4 + 2H_2O$ . Product: green potassium manganate.

(2) Manganate is oxidised electrolytically (or by Cl₂/O₃) in alkaline solution to permanganate: $2K_2MnO_4 + Cl_2 \to 2KMnO_4 + 2KCl$ . Product: purple permanganate.

Difficulty Distribution

Easy (35%): Identifying oxidation states, simple property questions, definitions.
Medium (50%): Magnetic moments, comparisons across series, lanthanoid contraction questions.
Hard (15%): Reasoning questions like “why is Cu²⁺ blue but Cu⁺ colourless?” or “why does Zn show only +2?”

Expert Strategy

Toppers’ approach:

Make a property-table flashcard. Atomic number, configuration, common OS, magnetic moment, colour. Revise once a week.
Standard reasoning questions. Memorise these:
- “Why coloured?” — d-d transitions.
- “Why complex formation?” — small size + vacant d-orbitals.
- “Why catalytic activity?” — variable OS allows redox cycling.
- “Why magnetic?” — unpaired electrons.
KMnO₄ and K₂Cr₂O₇ are guaranteed. Learn preparation, properties, and three reactions of each.
Lanthanoid contraction in one line. “Steady decrease in radius from La to Lu due to poor 4f shielding.”

Common Traps

Trap 1 — $Zn$ is not a transition element strictly. Zn has full d¹⁰ configuration in both atom and Zn²⁺. By definition, transition elements have partially filled d-orbitals. Some board markers reject Zn answers — but most accept it as a “metal of the d-block.”

Trap 2 — Number of unpaired electrons. Be careful with $Co^{3+}$ (3d⁶, but in strong field becomes diamagnetic). Spin-only formula assumes high-spin unless told otherwise. CBSE usually gives high-spin assumption.

Trap 3 — KMnO₄ vs MnO₂. Don’t confuse the starting material (MnO₂, pyrolusite) with the product (KMnO₄). The fusion oxidises Mn from +4 to +6, then electrolysis raises to +7.

Trap 4 — Actinoid contraction is more pronounced than lanthanoid. Because 5f is even more poorly shielded than 4f. Comparison questions sometimes ask which is greater — actinoid wins.