d and f Block Elements Class 12 Notes (2026-27) — CBSE
Class 12 Chemistry Chapter 4 notes: transition elements, variable oxidation states, coloured ions, magnetic properties, lanthanoid contraction, KMnO4 and K2Cr2O7.
The d- and f-Block Elements — Class 12 Chemistry Notes
Chapter Snapshot
The d-block (transition) elements sit between the s- and p-blocks and have partly filled d orbitals; the f-block (lanthanoids and actinoids) have partly filled f orbitals. This chapter explains their characteristic properties — variable oxidation states, colour, magnetism, catalysis, complex and alloy formation — plus lanthanoid contraction and two important compounds, KMnO₄ and K₂Cr₂O₇.
Board relevance: reliably gives a "why/reason" question on transition-metal properties and a preparation/reaction question on permanganate or dichromate. Learn the reasons, not just the facts.
Key Concepts & Definitions
Transition element — an element whose atom, or one of its common ions, has a partly filled d subshell. (By this definition Zn, Cd, Hg are not true transition elements — they have a complete d¹⁰ configuration.)
General configuration: (n−1)d^{1–10} ns^{1–2}.
Anomalous configurations: chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹ — half-filled and fully-filled d subshells are extra stable (symmetry and exchange energy).
Characteristic Properties
1. Variable oxidation states. The (n−1)d and ns orbitals have very similar energies, so electrons from both participate in bonding — giving oxidation states differing by one unit (e.g. Fe²⁺/Fe³⁺, Mn from +2 to +7). Manganese shows the widest range because it has the most unpaired electrons (3d⁵4s²).
2. Coloured ions. Caused by d-d transitions: an electron absorbs visible light and moves between d orbitals split by the ligand field; the complementary colour is seen. Ions with d⁰ (Sc³⁺, Ti⁴⁺) or d¹⁰ (Zn²⁺, Cu⁺) are colourless — no d-d transition is possible.
3. Magnetic properties. Depend on unpaired electrons:
μ = √(n(n + 2)) BM (spin-only formula, n = number of unpaired electrons)
Unpaired electrons → paramagnetic; all paired → diamagnetic. More unpaired electrons → higher magnetic moment.
4. Catalytic activity. Because they show variable oxidation states (forming intermediates) and provide a large adsorbing surface. Examples: Fe in the Haber process, V₂O₅ in the Contact process, Ni in hydrogenation, TiCl₄ in Ziegler–Natta polymerisation.
5. Complex formation. Favoured by small size, high charge density, and vacant d orbitals to accept lone pairs from ligands.
6. Alloy formation. Similar atomic sizes let transition metals substitute for each other in the lattice (e.g. brass, bronze, steel).
7. Other trends: high melting points and densities (strong metallic bonding from unpaired d electrons); they form interstitial compounds (small atoms like H, C, N occupy lattice holes), making them hard and rigid.
Atomic radii trend: decreases at first, stays nearly constant in the middle, then increases slightly — because increasing nuclear charge is offset by increasing d-electron shielding/repulsion.
The f-Block: Lanthanoid Contraction
Lanthanoid contraction — the steady decrease in atomic and ionic radii from La to Lu.
Cause: each added electron enters the 4f subshell, which shields the nuclear charge poorly, so the effective nuclear charge felt by the outer electrons rises steadily, pulling them inwards.
Consequences:
- The elements of the second and third transition series have almost identical radii (e.g. Zr and Hf, Nb and Ta) — making them very hard to separate.
- Lanthanoids have very similar chemical properties, so their separation is difficult.
- Basic strength of the hydroxides decreases from La(OH)₃ to Lu(OH)₃.
The common oxidation state of lanthanoids is +3. Actinoids show a wider range of oxidation states and are all radioactive.
Important Compounds
Potassium permanganate, KMnO₄
- Preparation: pyrolusite (MnO₂) is fused with KOH in the presence of an oxidising agent (KNO₃ or air) to give potassium manganate (K₂MnO₄, green), which is then oxidised electrolytically (or by disproportionation in acid) to KMnO₄.
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O ; then 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
- Structure: tetrahedral MnO₄⁻; deep purple; Mn is in the +7 state.
- As an oxidising agent in acidic medium (Mn⁷⁺ → Mn²⁺, 5 electrons):
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
It oxidises Fe²⁺ → Fe³⁺, oxalate → CO₂, and I⁻ → I₂.
Potassium dichromate, K₂Cr₂O₇
- Preparation: chromite ore (FeCr₂O₄) is fused with sodium carbonate in air to give sodium chromate, which is acidified to sodium dichromate and treated with KCl to crystallise K₂Cr₂O₇.
- Chromate–dichromate equilibrium (pH dependent):
2CrO₄²⁻ (yellow) + 2H⁺ ⇌ Cr₂O₇²⁻ (orange) + H₂O
Yellow chromate in alkaline medium; orange dichromate in acidic medium.
- As an oxidising agent in acidic medium (Cr⁶⁺ → Cr³⁺, 6 electrons):
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Important Reactions
The reactions most often asked in the board exam:
Reaction Equation
KMnO₄ preparation (step 1) 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
KMnO₄ preparation (step 2) 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
KMnO₄ in acidic medium MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
KMnO₄ oxidises iron(II) MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
KMnO₄ oxidises oxalate 2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O
Chromate ⇌ dichromate 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O
K₂Cr₂O₇ in acidic medium Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
K₂Cr₂O₇ oxidises iodide Cr₂O₇²⁻ + 6I⁻ + 14H⁺ → 2Cr³⁺ + 3I₂ + 7H₂O
Colour cues to quote: MnO₄⁻ is deep purple, MnO₄²⁻ green, Mn²⁺ almost colourless (pale pink); CrO₄²⁻ yellow, Cr₂O₇²⁻ orange, Cr³⁺ green.
Important Question Patterns
1. Reasoning (2–3 marks): why transition elements show variable oxidation states / colour / catalytic activity; why Zn is not a transition element; why Cr and Cu have anomalous configurations.
2. Magnetic moment (2 marks): count unpaired electrons and apply μ = √(n(n+2)) BM.
3. Lanthanoid contraction (2–3 marks): definition, cause, and consequences (Zr/Hf similarity, difficulty of separation).
4. KMnO₄ / K₂Cr₂O₇ (3 marks): preparation, oxidation state of the metal, and the half-reaction in acidic medium; the chromate–dichromate colour change with pH.
5. Trends (2 marks): atomic radii across a series; melting points; interstitial compounds and alloys.
⚡ Quick Revision
- Transition element = partly filled d subshell in atom or common ion → Zn, Cd, Hg excluded.
- Anomalies: Cr = 3d⁵4s¹, Cu = 3d¹⁰4s¹ (half/fully filled stability).
- Variable oxidation states ← (n−1)d and ns have similar energies; Mn shows the widest range.
- Colour ← d-d transitions; d⁰ (Sc³⁺) and d¹⁰ (Zn²⁺) are colourless.
- Magnetism: μ = √(n(n+2)) BM; unpaired → paramagnetic.
- Catalysts ← variable oxidation states + large surface (Fe Haber, V₂O₅ Contact, Ni hydrogenation).
- Lanthanoid contraction ← poor 4f shielding → Zr ≈ Hf; separation difficult; common state +3.
- KMnO₄: Mn is +7, purple, tetrahedral; acidic medium MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
- K₂Cr₂O₇: Cr is +6; CrO₄²⁻ (yellow, alkaline) ⇌ Cr₂O₇²⁻ (orange, acidic); Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
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