CBSE Class 12 Chemistry Revision Notes Chapter 4 The d- and f-Block Elements

The d- and f-Block Elements explain transition metals and inner transition metals in the periodic table. In CBSE Class 12 Chemistry, this chapter covers electronic configuration, oxidation states, coloured ions, magnetic behaviour and important compounds.

The d- and f-Block Elements chapter explains metals whose d or f orbitals are progressively filled. The d-block contains groups 3–12, while the f-block contains lanthanoids and actinoids. These elements show special properties because of partly filled d or f orbitals.

Use these CBSE Class 12 Chemistry Revision Notes Chapter 4 to revise the 2026–27 chapter quickly. Start with position and electronic configuration. Then revise transition metal properties, K₂Cr₂O₇, KMnO₄, lanthanoids, actinoids and their applications.

Key Takeaways

• d-Block Elements: They belong to groups 3–12 and involve progressive filling of d orbitals.
• Transition Elements: They have incomplete d subshells in atoms or common ions.
• f-Block Elements: They include lanthanoids and actinoids with progressive filling of 4f and 5f orbitals.
• KMnO₄ and K₂Cr₂O₇: Both are important oxidising agents in Class 12 Chemistry.

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Access 30 Minutes Class 12 Chemistry Chapter 4 The d- and f-Block Elements Notes

This chapter is long, so revise it in blocks. First cover d-block position, electronic configuration and transition element properties.

Then revise K₂Cr₂O₇ and KMnO₄. Keep lanthanoids, actinoids and their comparison for the final round.

The d- and f-Block Elements Class 12 Chemistry Chapter 4 Overview

The d-block elements are placed between s-block and p-block elements in the periodic table. Their d orbitals are progressively filled.

The f-block elements are placed separately at the bottom of the periodic table. Their 4f and 5f orbitals are progressively filled.

The chapter mainly studies transition elements, lanthanoids, actinoids and important compounds of chromium and manganese.

Position of d-Block and f-Block Elements in the Periodic Table

The d-block contains elements of groups 3 to 12.

There are four transition series:

The f-block has two series:

Lanthanum and actinium are usually discussed along with lanthanoids and actinoids because of their close resemblance.

Transition Elements and Inner Transition Elements

Transition elements are metals that have incomplete d subshells either in their neutral atoms or in their common ions.

Zinc, cadmium and mercury are not regarded as true transition elements because they have completely filled d¹⁰ configuration in their atoms and common oxidation states.

Inner transition elements are f-block elements. They include lanthanoids and actinoids.

Electronic Configuration of d-Block Elements

The general electronic configuration of d-block elements is:

(n - 1)d¹–¹⁰ ns¹–²

Here, (n - 1)d represents the inner d orbital, and ns represents the outermost s orbital.

For the first transition series:

Exceptions in Electronic Configuration of Cr and Cu

Chromium and copper show exceptional electronic configurations.

This happens because the energy difference between 3d and 4s orbitals is small.

General Properties of Transition Elements

Transition elements show special properties because they have partly filled d orbitals.

Important properties include variable oxidation states, coloured ions, complex formation, catalytic activity and magnetic behaviour.

Physical Properties

Transition metals show typical metallic properties.

Zn, Cd and Hg are exceptions in some physical properties.

Atomic and Ionic Sizes

In a transition series, atomic and ionic radii generally decrease from left to right.

This happens because nuclear charge increases, but d electrons do not shield the outer electrons very effectively.

Across 3d series, the decrease is small because electrons are added to inner d orbitals.

Lanthanoid Contraction Effect on d-Block Elements

Lanthanoid contraction makes the sizes of 4d and 5d elements very similar.

Example:

Due to similar sizes, Zr and Hf show very similar properties and are difficult to separate.

Ionisation Enthalpy

Ionisation enthalpy generally increases across a transition series.

The increase is not very steep because the added electron enters the inner d orbital.

Important points:

• ns electrons are removed before (n - 1)d electrons.
• Stable d⁵ and d¹⁰ configurations affect ionisation enthalpy.
• Mn²⁺ has stable d⁵ configuration.
• Zn²⁺ has stable d¹⁰ configuration.

Oxidation States of Transition Elements

Transition elements show variable oxidation states because ns and (n - 1)d electrons can take part in bonding.

Manganese shows the maximum number of oxidation states in the 3d series.

The highest oxidation states are usually shown in oxides and fluorides.

Oxygen and fluorine can stabilise high oxidation states because they are small and highly electronegative.

Magnetic Properties of Transition Elements

Transition metal ions may be paramagnetic or diamagnetic.

Paramagnetism arises due to unpaired electrons.

The spin-only magnetic moment is calculated using:

μ = √n(n + 2) BM

Here, n is the number of unpaired electrons.

Example:

Mn²⁺ has d⁵ configuration.

Number of unpaired electrons = 5

μ = √5(5 + 2)

μ = √35

μ = 5.92 BM

Formation of Coloured Ions

Many transition metal ions are coloured because they have partly filled d orbitals.

When light falls on the ion, an electron gets excited from a lower energy d orbital to a higher energy d orbital. The colour seen is the complementary colour of the absorbed light.

Ions with d⁰ or d¹⁰ configuration are usually colourless.

Complex Formation, Catalytic Properties and Alloys

Transition metals form many complex compounds because their ions are small, highly charged and have available d orbitals.

Examples:

Catalytic Properties

Transition metals and their compounds act as catalysts because they can show variable oxidation states and form complexes.

Interstitial Compounds

Interstitial compounds form when small atoms like H, C or N enter the crystal lattice of transition metals.

Examples:

• TiC
• Mn₄N
• Fe₃H
• VH₀.₅₆

Properties:

• High melting points
• Very hard
• Metallic conductivity
• Chemically inert

Alloy Formation

Transition metals form alloys easily because their atomic sizes are similar.

Examples:

Potassium Dichromate K₂Cr₂O₇

Potassium dichromate is an important compound of chromium.

It is used in the leather industry, volumetric analysis and organic chemistry as an oxidising agent.

Preparation of Potassium Dichromate

Potassium dichromate is prepared from chromite ore, FeCr₂O₄.

Step 1: Chromite ore is fused with sodium carbonate in air.

4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

Step 2: Sodium chromate is acidified to form sodium dichromate.

2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O

Step 3: Sodium dichromate reacts with potassium chloride.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Orange crystals of potassium dichromate separate out.

Chromate and Dichromate Equilibrium

Chromates and dichromates interconvert depending on pH.

In acidic medium:

2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O

In basic medium:

Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O

Oxidising Action of K₂Cr₂O₇

In acidic solution:

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Acidified potassium dichromate oxidises:

Example:

Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

Potassium Permanganate KMnO₄

Potassium permanganate is an important compound of manganese.

It is a strong oxidising agent and forms dark purple crystals.

Preparation of Potassium Permanganate

Potassium permanganate is prepared from MnO₂.

Step 1: MnO₂ is fused with KOH and oxidised by air or KNO₃.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

Step 2: Manganate ion is oxidised to permanganate ion.

MnO₄²⁻ → MnO₄⁻

In neutral or acidic solution, manganate disproportionates:

3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O

Properties of KMnO₄

Thermal decomposition:

2KMnO₄ → K₂MnO₄ + MnO₂ + O₂

Oxidising Action of KMnO₄

In acidic medium:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

KMnO₄ oxidises:

Example:

5Fe²⁺ + MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

Permanganate titrations are not done in hydrochloric acid because HCl can get oxidised to chlorine.

Lanthanoids in f-Block Elements

Lanthanoids are the fourteen elements after lanthanum, from Ce to Lu.

They belong to the 4f series.

The general electronic configuration involves 6s² and variable filling of 4f orbitals.

The most common oxidation state of lanthanoids is +3.

Lanthanoid Contraction

Lanthanoid contraction is the gradual decrease in atomic and ionic radii from lanthanum to lutetium.

It happens because 4f electrons shield nuclear charge poorly.

Effects of lanthanoid contraction:

General Properties of Lanthanoids

Ce shows +4 oxidation state because Ce⁴⁺ has noble gas configuration.

Eu²⁺ and Yb²⁺ are known because they get extra stability from f⁷ and f¹⁴ configurations.

Actinoids in f-Block Elements

Actinoids are the fourteen elements after actinium, from Th to Lr.

They belong to the 5f series.

All actinoids are radioactive. The later actinoids are difficult to study because they are available in very small quantities.

Actinoid Contraction

Actinoid contraction is the gradual decrease in atomic and ionic size across the actinoid series.

It is greater than lanthanoid contraction because 5f electrons have poor shielding effect.

General Properties of Actinoids

Actinoids show more oxidation states because 5f, 6d and 7s orbitals have comparable energies.

Lanthanoids and Actinoids Difference

Actinoid chemistry is less smooth than lanthanoid chemistry because actinoids show variable oxidation states and radioactivity.

Applications of d- and f-Block Elements

d- and f-block elements have many industrial uses.

These uses come from their metallic nature, catalytic behaviour, variable oxidation states and special electronic structures.

Quick Revision Table for The d- and f-Block Elements

Important Terms in The d- and f-Block Elements

Useful Links for Class 12 Chemistry