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The d- and f-Block Elements – Class 12 Chemistry Chapter 4 Notes & NCERT PDF

Class 12ChemistryChapter 4NCERT book: Chemistry Part I - Class 12

Notes and a simple summary of Chapter 4 of the NCERT Class 12 Chemistry book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 4: The d- and f-Block Elements

Introduction

Unit 4 studies the d-block (Groups 3-12) and the f-block (the lanthanoids and actinoids placed at the foot of the periodic table). The d-block elements are called transition elements because their properties are transitional between the reactive s-block and the p-block. Four transition series exist: 3d (Sc-Zn), 4d (Y-Cd), 5d (La, Hf-Hg) and 6d (Ac, Rf-Cn). The f-block has the 4f series (lanthanoids, Ce-Lu) and the 5f series (actinoids, Th-Lr). Iron, copper, silver and gold carried humanity through the Bronze and Iron Ages; today these elements are catalysts, pigments, alloys and the heart of biological molecules. For the board exam this unit is heavy on reasons ("account for", "explain why"), so learn every trend together with its cause.

Key Concepts

4.1 Position in the Periodic Table

The d-block sits in the middle of the table; the d orbitals of the penultimate energy level are being filled. By the IUPAC definition, a transition metal has an incomplete d subshell either in the neutral atom or in its ions. Zinc, cadmium and mercury (Group 12) have full d¹⁰ configurations in the ground state and in their common oxidation states, so they are not regarded as transition metals, although they are studied with them.

4.2 Electronic Configurations

General configuration: (n-1)d¹⁻¹⁰ ns¹⁻². Because (n-1)d and ns energies are very close, exceptions occur: Cr = 3d⁵4s¹ and Cu = 3d¹⁰4s¹ (extra stability of half-filled and completely filled d sets). Pd is 4d¹⁰5s⁰. The unfilled d orbitals explain colour, paramagnetism, variable oxidation states and catalytic activity.

4.3 General Properties of the Transition Elements

  • 4.3.1 Physical properties - hard, high melting and boiling points and high enthalpies of atomisation, because both (n-1)d and ns electrons take part in metallic bonding. Melting points peak around d⁵ and fall afterwards; Zn has the lowest enthalpy of atomisation of the 3d series (126 kJ mol⁻¹).
  • 4.3.2 Atomic and ionic sizes - radii decrease slowly across a series. The 4d and 5d radii are almost equal (Zr 160 pm, Hf 159 pm) because of the lanthanoid contraction (poor shielding by 4f electrons).
  • 4.3.3 Ionisation enthalpies - increase across a series, but irregularly; the loss of exchange energy explains the high third ionisation enthalpy of Mn (d⁵ to d⁴) and of Fe²⁺.
  • 4.3.4 Oxidation states - variable, usually differing by one unit. Mn shows the most (+2 to +7). Sc shows only +3 and Zn only +2. Higher states are stabilised by the small, highly electronegative oxide and fluoride ions (e.g. MnO₄⁻ with Mn in +7).
  • 4.3.5-4.3.7 Electrode potentials - Cu has a positive E°(Cu²⁺/Cu) = +0.34 V: its high enthalpy of atomisation plus ionisation is not balanced by hydration enthalpy, so Cu does not liberate H₂ from acids. The stability of Mn²⁺ (d⁵) and Zn²⁺ (d¹⁰) explains their less negative-than-expected values.
  • 4.3.9 Magnetic properties - paramagnetism from unpaired electrons; spin-only formula μ = √(n(n+2)) BM.
  • 4.3.10 Coloured ions - d-d transitions absorb visible light; the complementary colour is seen. Sc³⁺, Ti⁴⁺, Cu⁺ and Zn²⁺ (d⁰ or d¹⁰) are colourless.
  • 4.3.11 Complex formation - small, highly charged ions with vacant d orbitals.
  • 4.3.12 Catalytic properties - variable oxidation states and surface adsorption: V₂O₅ (Contact process), finely divided Fe (Haber process), Ni (hydrogenation).
  • 4.3.13 Interstitial compounds - small atoms (H, C, N) trapped in metal lattices: TiC, Mn₄N, Fe₃H, VH₀.₅₆, TiH₁.₇. They are non-stoichiometric, hard and high melting.
  • 4.3.14 Alloys - formed readily because metallic radii are within about 15% of each other.

    • 4.4 Some Important Compounds

    Potassium dichromate (K₂Cr₂O₇) is made from chromite ore. Fusion with sodium carbonate in air gives sodium chromate; acidification gives sodium dichromate; KCl converts it into the less soluble K₂Cr₂O₇. Chromate and dichromate interconvert with pH. In dichromate the Cr-O-Cr bond angle is 126°. It is a strong oxidant in acid (E° = 1.33 V).

    Potassium permanganate (KMnO₄) is made by fusing MnO₂ with KOH and an oxidant to give green K₂MnO₄, which disproportionates in neutral or acid solution to purple permanganate. It dissolves at 6.4 g per 100 g water at 293 K and decomposes at 513 K. Manganate (d¹) is paramagnetic; permanganate (d⁰) is diamagnetic.

    4.5 The Lanthanoids

    Configuration mainly 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². +3 is the characteristic oxidation state. Ce⁴⁺ is a strong oxidant (E° Ce⁴⁺/Ce³⁺ = +1.74 V); Eu²⁺ and Yb²⁺ are reductants; Tb⁴⁺ is an oxidant. Lanthanoid contraction is the steady decrease in atomic and ionic size across the series. They are reactive metals (E° about -2.2 to -2.4 V; Eu -2.0 V). Mischmetall (about 95% lanthanoid, 5% iron, traces of S, C, Ca, Al) is used in bullets, shells and lighter flints.

    4.6 The Actinoids

    Fourteen elements Th to Lr, all radioactive. Am is [Rn]5f⁷7s². They show more oxidation states than lanthanoids (up to +7 at Np) because 5f, 6d and 7s energies are close. The actinoid contraction is greater than the lanthanoid contraction because 5f electrons shield even more poorly.

    4.7 Some Applications

    Iron and steels for construction; TiO₂ in pigments; MnO₂ in dry cells; Zn and Ni in batteries; coinage metals Cu, Ag, Au; catalysts such as V₂O₅, TiCl₄ with Al(CH₃)₃ (Ziegler catalysts), Fe, Ni and PdCl₂; AgBr in photography.

    Worked Examples

  • Spin-only moment of a divalent ion, Z = 25. Mn²⁺ = [Ar]3d⁵, n = 5. μ = √(5 × 7) = √35 = 5.92 BM.
  • Cr²⁺ is reducing but Mn³⁺ is oxidising, though both are d⁴. Cr²⁺ → Cr³⁺ gives d³ (stable half-filled t₂g). Mn³⁺ → Mn²⁺ gives the extra-stable d⁵.
  • Balance MnO₄⁻ with Fe²⁺ in acid. MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O; Fe²⁺ → Fe³⁺ + e⁻ (×5). Net: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O.
  • Balance dichromate with Fe²⁺. Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O. Six electrons per dichromate, because each Cr falls from +6 to +3.
  • Chromite fusion (balanced). 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂.
  • Manganate disproportionation. 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O (Mn +6 goes to +7 and +4).

    • Key-Facts Sheet

  • Transition metal: incomplete d subshell in atom or ion; Zn, Cd, Hg excluded.
  • Cr 3d⁵4s¹; Cu 3d¹⁰4s¹; Pd 4d¹⁰5s⁰.
  • Mn: +2 to +7; Sc only +3.
  • E°(Cu²⁺/Cu) = +0.34 V, the only positive value in the 3d series.
  • μ = √(n(n+2)); n = 1 gives 1.73 BM, n = 5 gives 5.92 BM.
  • Zr 160 pm vs Hf 159 pm, a consequence of the lanthanoid contraction.
  • Cr₂O₇²⁻: Cr +6; Cr-O-Cr angle 126°; acid E° = 1.33 V.
  • MnO₄⁻: Mn +7, purple, diamagnetic; MnO₄²⁻: Mn +6, green, paramagnetic.
  • Ce⁴⁺/Ce³⁺ E° = +1.74 V.
  • Mischmetall is about 95% lanthanoid and 5% Fe.

    • Board Exam Focus

    The in-text and end-of-unit questions ask you to account for trends. Model outlines:

    Q. Why do transition elements show variable oxidation states? (n-1)d and ns energies are close, so a varying number of d electrons can be used in bonding. Give Mn (+2 to +7) as the example. Contrast with p-block elements, whose states differ by two units.

    Q. What is lanthanoid contraction? State its consequences. Definition: the steady decrease in size from La to Lu, caused by imperfect shielding by 4f electrons. Consequences: Zr/Hf almost equal in size and hard to separate; basicity of hydroxides decreases from La(OH)₃ to Lu(OH)₃; the 4d and 5d series show similar properties.

    Q. Why is E°(Mn³⁺/Mn²⁺) much more positive than for Cr or Fe? Mn²⁺ is d⁵ (half-filled, extra stable), so Mn³⁺ readily accepts an electron. Fe³⁺ is itself d⁵, so Fe³⁺/Fe²⁺ is lower.

    Other favourites include the interconversion of chromate and dichromate with pH, the preparation equations for KMnO₄ and K₂Cr₂O₇, and comparing lanthanoids with actinoids.

    Key Terms

  • Transition element (संक्रमण तत्व)
  • Inner transition element (आंतरिक संक्रमण तत्व)
  • Lanthanoid contraction (लैंथेनॉयड आकुंचन)
  • Oxidation state (ऑक्सीकरण अवस्था)
  • Paramagnetism (अनुचुंबकत्व)
  • Diamagnetism (प्रतिचुंबकत्व)
  • Spin-only magnetic moment (केवल-प्रचक्रण चुंबकीय आघूर्ण)
  • Enthalpy of atomisation (परमाणुकरण एन्थैल्पी)
  • Interstitial compound (अंतराकाशी यौगिक)
  • Alloy (मिश्रधातु)
  • Catalyst (उत्प्रेरक)
  • Disproportionation (असमानुपातन)
  • Mischmetall (मिश्रधातु धातु / मिशमेटल)

    • Common Mistakes

  • Calling Zn a transition metal; it has d¹⁰ in both the atom and Zn²⁺.
  • Writing Cr as 3d⁴4s² or Cu as 3d⁹4s².
  • Removing 3d electrons before 4s when forming ions; 4s electrons leave first (Fe²⁺ = 3d⁶).
  • Using n = total d electrons instead of unpaired electrons in μ = √(n(n+2)).
  • Forgetting that actinoid contraction is greater than lanthanoid contraction.
  • Balancing permanganate equations with the wrong electron count; Mn goes from +7 to +2 in acid, which is 5 electrons.

    • 💡 Key Learning Points

    • ✓Define transition elements by the IUPAC rule (incomplete d subshell in atom or ion) and explain why Zn, Cd and Hg are excluded.
    • ✓Write 3d configurations including the exceptions Cr (3d⁵4s¹) and Cu (3d¹⁰4s¹), and derive configurations of ions by removing 4s electrons first.
    • ✓Explain trends in atomisation enthalpy, ionisation enthalpy, oxidation states, E° values, colour, magnetism, catalysis, interstitial compounds and alloys.
    • ✓Describe the preparation, structure and oxidising reactions of K₂Cr₂O₇ and KMnO₄ with balanced ionic equations.
    • ✓Compare lanthanoids and actinoids: configurations, oxidation states, lanthanoid/actinoid contraction and its consequences.

    👨‍🏫 Teaching Tips

    • →Build a 'why' wall: every property (colour, magnetism, catalysis, variable valency) is written with its structural cause beside it.
    • →Plot the 3d enthalpy-of-atomisation and E°(M²⁺/M) data from the unit and ask students to explain every bump (Mn, Cu, Zn).
    • →Practise spin-only moments for Sc³⁺ to Zn²⁺ as a table drill; students count unpaired electrons, not total d electrons.
    • →Use half-reaction bookkeeping for MnO₄⁻ and Cr₂O₇²⁻ titrations so students can build any ionic equation rather than memorise it.
    • →Show the colour change of chromate (yellow) to dichromate (orange) with pH to anchor the equilibrium.
    • →Use a Zr/Hf radius comparison to make lanthanoid contraction concrete before listing its consequences.
    • →Close with a lanthanoid-vs-actinoid comparison chart built by the class from the text.

    📋 Assessment Questions

    1. Can the student give correct configurations for Cr, Cu, Fe²⁺, Mn²⁺ and Cu⁺ and state which ions are coloured?
    2. Can the student calculate spin-only magnetic moments and link them to the number of unpaired electrons?
    3. Can the student write balanced preparation and oxidation equations for K₂Cr₂O₇ and KMnO₄?
    4. Can the student explain the lanthanoid contraction and at least three of its consequences?
    5. Can the student justify anomalies such as the positive E° of copper and the stability of Mn²⁺?

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    All chapters in Chemistry Part I - Class 12

    1. 1. Solutions
    2. 2. Electrochemistry
    3. 3. Chemical Kinetics
    4. 4. The d- and f-Block Elements
    5. 5. Coordination Compounds