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📘 Class 12 Chemistry – Chapter: The d- and f-Block Elements

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🔷 1. Introduction

• The d- and f-block elements are also called transition and inner transition elements.

• These elements show variable oxidation states, form colored ions, and exhibit paramagnetism.

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🔹 2. d-Block Elements (Transition Elements)

📍 Position in the Periodic Table:

• Groups 3 to 12, Periods 4, 5, 6, 7.

• Elements with incomplete d-orbitals in ground or ionic states.

🧪 General Electronic Configuration:

• (n-1) d¹–¹⁰ ns¹–²

⚛️ Properties:

1. Variable Oxidation States:

• Due to similar energies of ns and (n-1)d orbitals.

• E.g., Mn: +2 to +7; Fe: +2, +3.

2. Color of Compounds:

• Due to d–d transitions (electron jumps between d-orbitals).

3. Magnetic Properties:

• Due to unpaired d-electrons.

• Magnetic moment, μ = √[n(n+2)] BM (n = unpaired electrons)

4. Formation of Complexes:

• Due to small size, high charge, and availability of d-orbitals.

5. Catalytic Properties:

• Transition metals like Fe, Ni, Pt, V₂O₅ act as good catalysts due to variable oxidation states.

6. Alloy Formation:

• Metals mix readily due to similar atomic sizes.

⚠️ Exception:

• Zn, Cd, Hg are not considered transition metals (completely filled d-orbitals).

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🔸 3. f-Block Elements (Inner Transition Elements)

🔽 4f & 5f series:

Series	Name	Atomic Numbers
Lanthanides	4f series	58–71
Actinides	5f series	90–103

📘 General Configuration:

• (n-2)f¹–¹⁴ (n-1)d⁰–¹ ns²

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🌟 Lanthanides

⚛️ Properties:

• Show +3 oxidation state (dominant).

• Show lanthanide contraction: gradual decrease in atomic and ionic size across series.

Effects of Lanthanide Contraction:

• Similarities in size and properties of 4d and 5d elements.

• Poor separation of lanthanides (difficult to isolate).

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☢️ Actinides

⚛️ Properties:

• Show variable oxidation states (+3 to +6 common).

• Most are radioactive.

• Greater tendency for complex formation.

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🔄 Comparison: d-Block vs f-Block

Property	d-Block	f-Block
Orbitals involved	(n-1)d	(n-2)f
Oxidation states	Variable, common +2, +3	Variable, +3 common
Complex formation	High tendency	High tendency
Magnetic property	Paramagnetic	Paramagnetic
Examples	Fe, Cu, Ni	Ce, U, Th

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🧪 Important Compounds:

• Potassium dichromate (K₂Cr₂O₇):

  • Oxidizing agent

  • Orange color

  • Cr⁶⁺ state

• Potassium permanganate (KMnO₄):

  • Strong oxidizing agent

  • Purple color

  • Mn⁷⁺ state

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📌 Key Reactions:

1. Oxidation by KMnO₄ in acidic medium:

   $$Mn{O}_4^{-}+8H^{+}+5e^{-}\to M{n}^{2+}+4H_{2}O$$

2. Oxidation by K₂Cr₂O₇ in acidic medium:

   $$C{r}_2{O}_7^{2-}+14H^{+}+6e^{-}\to 2C{r}^{3+}+7H_{2}O$$

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🔚 Summary

• Transition metals show rich chemistry due to variable oxidation states and d-orbitals.

• Inner transition metals include lanthanides and actinides with f-orbitals.

• Important compounds like KMnO₄ and K₂Cr₂O₇ are strong oxidizing agents.

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Absolutely! Here's a detailed and complete explanation of the d-block elements from Class 12 Chemistry, including concepts, properties, trends, and examples—suitable for exams like CBSE, NEET, and JEE.

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📘 Chapter: The d-Block Elements (Transition Elements) – Class 12 Chemistry

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🔷 1. What are d-block Elements?

• Elements in groups 3 to 12 of the periodic table.

• Their last electron enters the (n-1)d orbital, hence called d-block elements.

• Also called transition elements (except Zn, Cd, Hg).

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🔹 2. General Electronic Configuration

• (n-1)d¹–¹⁰ ns¹–²

• For example:

  • Sc (Z=21): [Ar] 3d¹ 4s²

  • Fe (Z=26): [Ar] 3d⁶ 4s²

⚠️ Note: There are some exceptions due to extra stability of half-filled and fully filled orbitals:

• Cr (Z=24): [Ar] 3d⁵ 4s¹

• Cu (Z=29): [Ar] 3d¹⁰ 4s¹

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🔹 3. Position in Periodic Table

Period	Series Name	Example Elements
4th	3d series	Sc → Zn
5th	4d series	Y → Cd
6th	5d series	La, Hf → Hg
7th	6d series	Ac → elements under development

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🔹 4. Important Characteristics of d-block Elements

✅ (1) Variable Oxidation States

• Due to participation of (n-1)d and ns electrons in bonding.

• Common for many d-block elements to show more than one oxidation state.

Element	Oxidation States
Mn	+2 to +7
Fe	+2, +3
Cr	+2, +3, +6
Cu	+1, +2

📌 Stability of higher oxidation states increases from left to right in a series.

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✅ (2) Formation of Colored Compounds

• Due to d–d electronic transitions within partially filled d-orbitals.

• Absorption of visible light causes electrons to jump between split d-orbitals.

Ion	Color
Cu²⁺	Blue
Fe³⁺	Yellow
Cr³⁺	Green
Mn²⁺	Light pink

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✅ (3) Paramagnetism

• Caused by unpaired d-electrons.

• Greater the number of unpaired electrons, higher the magnetic moment.

$$\mu =\sqrt{n(n+2)}\text{BM}$$​BM

Where n = number of unpaired electrons.

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✅ (4) Catalytic Properties

• Many d-block metals and compounds act as excellent catalysts.

• Reasons:

  • Variable oxidation states

  • Formation of intermediate complexes

  • Adsorption of reactants on the metal surface

Catalyst	Reaction
Fe	Haber process (N₂ + H₂ → NH₃)
V₂O₅	Contact process (SO₂ → SO₃)
Ni	Hydrogenation of oils
MnO₂	Decomposition of H₂O₂

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✅ (5) Formation of Complex Compounds

• d-block elements form complexes due to:

  • Small ionic size

  • High charge

  • Availability of vacant d-orbitals

Example Complex	Central Ion
[Fe(CN)₆]⁴⁻	Fe²⁺
[Cu(NH₃)₄]²⁺	Cu²⁺

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✅ (6) Tendency to Form Alloys

• Transition metals have similar atomic sizes and can replace each other in metallic lattices.

• Examples: Brass (Cu + Zn), Bronze (Cu + Sn), Stainless steel (Fe + Cr + Ni)

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🔹 5. Chemical Reactivity

• Reactivity decreases across the period.

• 1st row (3d series) elements are more reactive than 2nd and 3rd row.

• Formation of stable oxides, sulfides, halides.

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🔹 6. Trends Across the Period and Down the Group

Property	Across a Period	Down a Group
Atomic size	Decreases slightly then constant	Increases slightly
Density	Increases	Increases
Melting/Boiling point	High, varies irregularly	Decreases slightly
Ionization enthalpy	Increases	Decreases
Metallic character	Decreases	Increases

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🔹 7. Uses of d-block Elements

• Fe, Cr, Ni – construction, machinery

• TiO₂ – white pigment

• MnO₂ – dry cells

• Cu, Ag – electrical conductivity

• Catalysts – Ni, Pt, V₂O₅

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🔹 8. Limitations of Transition Metal Definition

• Zn, Cd, Hg have completely filled d-orbitals in both ground and ionic states.

• Hence, not considered true transition elements, though they lie in d-block.

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🔚 Summary

• d-block elements show unique properties due to partially filled d-orbitals.

• They are versatile in oxidation states, magnetic behavior, and chemical reactivity.

• Their ability to form complexes, act as catalysts, and show color makes them chemically significant.

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