MP Board Class 12 Chemistry Ch4: d and f Block Elements

Chapter 4: The d- and f-Block Elements (d एवं f-ब्लॉक तत्व) is one of the most scoring chapters of MP Board Class 12 Chemistry. You study transition (d-block) elements, their electronic configuration, general properties, important compounds like K₂Cr₂O₇ and KMnO₄, and f-block elements — lanthanoids and actinoids. These complete notes with tables, formulas and board questions are prepared for the MP Board 2027 exam. Practice free chapter tests and previous year papers at mpboard.ai.

🧪 1. Introduction to d-Block (Transition) Elements

The d-block elements are those in which the last electron enters the d-orbital of the penultimate (n–1) shell. They occupy groups 3 to 12, between the s-block and p-block elements, and form a bridge between the electropositive s-block metals and the less electropositive p-block elements — hence they are called transition elements.

By IUPAC definition, a transition element has a partially filled d-orbital in the ground state or in any common oxidation state. Therefore Zn, Cd and Hg are not transition elements — they have a completely filled (n–1)d¹⁰ configuration in the ground state and in their common +2 oxidation state.

⚛️ 2. Electronic Configuration of d-Block Elements

The general electronic configuration is (n–1)d^1–10 ns^1–2. In the 3d series the 4s orbital fills before 3d because it has lower energy. Two elements show anomalous configurations due to the extra stability of half-filled and fully-filled d orbitals:

• Chromium (Cr): [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²)
• Copper (Cu): [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²)

📊 3. General Properties of Transition Elements

Transition elements differ from s-block and p-block elements in many properties. The table below summarises the general properties with reasons — exactly the pattern MP Board asks in long-answer questions:

🎨 4. Oxidation States, Colour, Magnetic and Catalytic Properties

Variable Oxidation States

Because the (n–1)d and ns orbitals have very close energies, transition elements show variable oxidation states. For example, Mn shows +2 to +7 and Cr shows +2 to +6. The highest oxidation state equals the group number up to group 7 (Mn +7); after group 8 it becomes less common.

Colour of Transition Metal Ions

Most transition metal ions are coloured due to d–d transitions: visible light excites an electron from a lower d-orbital to a higher d-orbital, absorbing a particular wavelength, and the transmitted light gives the colour. Ions with no unpaired d-electrons, such as Zn²⁺ (d¹⁰) and Sc³⁺ (d⁰), are colourless.

Magnetic and Catalytic Properties

Substances with unpaired electrons are paramagnetic (attracted by a magnetic field); transition metals and ions show paramagnetism due to unpaired d-electrons. Fe, Co and Ni are ferromagnetic in the solid state and are used for permanent magnets. As catalysts, transition metals work through variable oxidation states and intermediate complex formation: V₂O₅ (contact process), Fe (Haber process), Ni (hydrogenation of oils) and Pt (Ostwald process).

🧪 5. Important Compounds: K₂Cr₂O₇ and KMnO₄

Potassium Dichromate (K₂Cr₂O₇)

Potassium dichromate forms orange-red crystals and is a strong oxidising agent. It is prepared from chromite ore (FeCr₂O₄): the ore is fused with sodium carbonate and lime in air to give Na₂CrO₄, which is leached with water, acidified with dilute H₂SO₄ to Na₂Cr₂O₇, and then treated with KCl to crystallise out K₂Cr₂O₇.

In acidic medium: Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O. Uses: volumetric analysis (dichromate titration), tanning of leather and chrome pigments.

Potassium Permanganate (KMnO₄)

Potassium permanganate forms dark violet crystals. It is prepared from pyrolusite ore (MnO₂): MnO₂ is fused with KOH and an oxidising agent to give green K₂MnO₄, which is then oxidised electrolytically to KMnO₄.

KMnO₄ is a powerful oxidising agent whose product depends on the medium: acidic medium → Mn²⁺, neutral medium → MnO₂ (brown), strongly alkaline medium → MnO₄^2– (green). Uses: disinfectant, water purification, permanganate titrations and laboratory oxidations.

🌍 6. f-Block Elements: Lanthanoids and Actinoids

In f-block elements the last electron enters the f-orbital of the antepenultimate (n–2) shell. They are placed below the main periodic table in two series:

The common oxidation state of lanthanoids is +3; Ce (+4), Tb (+4), Eu (+2) and Yb (+2) are exceptions stabilised by empty, half-filled or fully-filled 4f orbitals. Actinoids show a wider range (+3 to +7) because 5f, 6d and 7s orbitals have close energies. All actinoids are radioactive; thorium and uranium occur in nature, the rest are synthetic.

📉 7. Lanthanoid Contraction and Comparison

Lanthanoid contraction is the steady decrease in the atomic and ionic radii of lanthanoids from La³⁺ (103 pm) to Lu³⁺ (86 pm). It occurs because the 4f electrons have a very poor shielding effect, so the increasing nuclear charge pulls the outer electrons closer.

Important consequences of lanthanoid contraction:

• Similar chemical properties — separation of lanthanoids is very difficult (done by ion-exchange and solvent extraction).
• Decrease in basicity of hydroxides from La(OH)₃ to Lu(OH)₃.
• Similar sizes of Zr/Hf, Nb/Ta and Mo/W in the 2nd and 3rd transition series.

📋 8. Key Points Quick Revision Table

✅ 9. Important Points for MP Board Exam

• Elements with a partially filled d-orbital in ground state or common oxidation state are transition elements.
• Zn, Cd, Hg are d-block but not transition elements (d¹⁰).
• General configuration: (n–1)d^1–10 ns^1–2; Cr and Cu are exceptions.
• Transition metals show variable oxidation states, coloured ions, paramagnetism, catalytic activity, and form complexes and interstitial compounds.
• K₂Cr₂O₇ (orange) is prepared from chromite; KMnO₄ (purple) from pyrolusite.
• Lanthanoid contraction = decrease in radii from La to Lu due to poor shielding of 4f electrons.
• Actinoids are all radioactive and show more variable oxidation states (+3 to +7).
• In acidic medium KMnO₄ gains 5e⁻ and K₂Cr₂O₇ gains 6e⁻ per molecule.

📝 10. Important Questions for MP Board 2027

✏️ Very Short Answer (1 Mark)

1. What are transition elements?
2. Why is Zn not regarded as a transition element?
3. What is lanthanoid contraction?
4. What is the oxidation state of Mn in KMnO₄?
5. Name the ore from which potassium dichromate is prepared.
6. Write the electronic configuration of Cr and Cu.

📝 Short Answer (2–3 Marks)

1. Why do transition elements show variable oxidation states?
2. Why are transition metal ions coloured? Explain with examples.
3. Why are transition metals and their compounds good catalysts?
4. What are interstitial compounds? State any two properties.
5. How is potassium permanganate prepared from pyrolusite ore?
6. Why do transition elements form complexes?
7. What are the consequences of lanthanoid contraction?

📚 Long Answer (5–6 Marks)

1. Describe the general characteristics of d-block (transition) elements.
2. Compare lanthanoids and actinoids on the basis of electronic configuration, oxidation states and chemical behaviour.
3. Explain the preparation, properties and uses of potassium permanganate (KMnO₄).
4. Explain the preparation, properties and uses of potassium dichromate (K₂Cr₂O₇).
5. Discuss the magnetic and catalytic properties of transition elements.

❓ 11. Frequently Asked Questions (FAQs)

📖 More Resources: Download MP Board Class 12 Chemistry previous year papers and practice sets from mpboard.ai. For video explanations and chapter-wise PYQ discussions, visit our Class 12 Chemistry Course.