Class 12 Chemistry Chapter 4: The d- and f-Block Elements | Complete NCERT Notes

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Chapter 4: The d- and f-Block Elements

Syllabus: NCERT Rationalised 2025–26
Subject: Inorganic Chemistry
Pre-requisite: Chapter 3: Chemical Kinetics

1. Introduction

The elements of the periodic table are divided into four major blocks, namely s-block, p-block, d-block and f-block, depending upon the subshell in which the differentiating electron enters.

The d-block and f-block elements occupy the central and bottom positions of the periodic table and exhibit a wide range of chemical and physical properties. These elements play a significant role in industrial processes, biological systems and modern technology.

The d-block elements are commonly known as transition elements because they show a gradual transition in properties from highly electropositive s-block elements to electronegative p-block elements.

According to NCERT, a transition element is defined as an element which has incompletely filled d-orbitals in its ground state or in any one of its commonly occurring oxidation states.

Important Exception: Zn, Cd and Hg Although Zinc (Zn), Cadmium (Cd) and Mercury (Hg) belong to the d-block of the periodic table, they are not considered transition elements.

This is because they possess a completely filled d-subshell ($d^{10}$ configuration) in both their ground state and in their common oxidation states.

As a result, these elements do not show the characteristic properties of transition elements such as variable oxidation states, formation of coloured ions and paramagnetism.

The f-block elements are also called inner transition elements because the differentiating electron enters the inner f-orbitals. They consist of two important series:

Lanthanoids (4f-series): Elements from atomic number 58 (Cerium) to 71 (Lutetium)
Actinoids (5f-series): Elements from atomic number 90 (Thorium) to 103 (Lawrencium)

In this chapter, we shall study the electronic configuration, general properties, important compounds and applications of the d- and f-block elements strictly in accordance with the NCERT Class 12 Chemistry syllabus.

2. The d-Block Elements

The d-block elements are those elements in which the d-orbitals are progressively filled. They are located in the centre of the periodic table between the s-block and p-block elements.

The d-block consists of elements belonging to Groups 3 to 12 of the periodic table. Based on the filling of d-orbitals, these elements are divided into four transition series.

First Transition Series (3d): Sc (21) to Zn (30)
Second Transition Series (4d): Y (39) to Cd (48)
Third Transition Series (5d): La (57) to Hg (80)
Fourth Transition Series (6d): Incomplete series (Ac onwards)

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2.1 General Electronic Configuration

$(n-1)d^{1-10} \; ns^{1-2}$

The (n−1)d and ns orbitals are very close in energy. Therefore, electrons from both orbitals can participate in bonding. This explains the characteristic properties of transition elements such as variable oxidation states and coloured compounds.

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2.2 First Transition Series (3d Series): Electronic Configuration

The first transition series consists of elements from Scandium (Sc) to Zinc (Zn). Their electronic configurations are given below. This table is highly important for board examinations, especially for questions related to oxidation states and magnetic properties.

Element	Atomic No.	Electronic Configuration	Common Oxidation State
Sc	21	[Ar] 3d¹ 4s²	+3
Ti	22	[Ar] 3d² 4s²	+2, +3, +4
V	23	[Ar] 3d³ 4s²	+2, +3, +4, +5
Cr	24	[Ar] 3d⁵ 4s¹	+2, +3, +6
Mn	25	[Ar] 3d⁵ 4s²	+2, +4, +7
Fe	26	[Ar] 3d⁶ 4s²	+2, +3
Co	27	[Ar] 3d⁷ 4s²	+2, +3
Ni	28	[Ar] 3d⁸ 4s²	+2
Cu	29	[Ar] 3d¹⁰ 4s¹	+1, +2
Zn	30	[Ar] 3d¹⁰ 4s²	+2

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2.3 Anomalous Electronic Configurations

Chromium (Z = 24)

Expected: [Ar] 3d⁴ 4s²
Actual: [Ar] 3d⁵ 4s¹

Copper (Z = 29)

Expected: [Ar] 3d⁹ 4s²
Actual: [Ar] 3d¹⁰ 4s¹

3. General Properties of d-Block Elements

The d-block (transition) elements exhibit several characteristic properties which arise due to the presence of partially filled d-orbitals and high effective nuclear charge. These properties show regular trends along each transition series, with a few important exceptions.

3.1 Atomic and Ionic Radii

In the first transition series (3d series), the atomic radii generally decrease from Sc to Cr due to increasing effective nuclear charge. Beyond chromium, the radii remain almost constant up to copper and show a slight increase for zinc.

Fig 3.1: Trend of atomic radii in 3d series

Ionic radii decrease with increase in oxidation state:

Fe²⁺ > Fe³⁺
Higher oxidation state results in stronger nuclear attraction and hence smaller ionic size.

3.2 Ionisation Enthalpy

Ionisation enthalpy is the energy required to remove an electron from an isolated gaseous atom. Transition elements show a gradual increase in first ionisation enthalpy (IE₁) from left to right.

However, the increase is not regular due to electron-electron repulsions and stability of half-filled and completely filled d-orbitals.

Because of these factors, trends like Mn vs Fe and Cu vs Zn are frequently asked in board examinations.

3.3 Oxidation States

Transition elements show variable oxidation states because both $ns$ and $(n-1)d$ electrons participate in bonding.

Lower oxidation states: loss of $ns$ electrons
Higher oxidation states: involvement of $(n-1)d$ electrons

The highest oxidation state increases from Sc to Mn and decreases from Fe to Zn.

3.4 Melting and Boiling Points

Transition metals generally have high melting and boiling points due to strong metallic bonding involving both $ns$ and $(n-1)d$ electrons.

The melting point increases from Sc to V, but shows a sharp dip at Manganese (Mn), then increases again towards iron and cobalt, and finally decreases towards zinc.

Fig 3.2: Variation of melting points in 3d series (Mn dip)

Zinc, cadmium and mercury have low melting points because of completely filled d-orbitals, resulting in weak metallic bonding.

3.5 Density

Transition metals possess high densities due to their high atomic masses, small atomic sizes and efficient packing of atoms.

Density generally increases across a transition series. Iron, cobalt and nickel show particularly high densities.

3.6 Enthalpy of Atomisation

Transition elements show high enthalpies of atomisation, indicating strong interatomic bonding. This is due to the participation of both $ns$ and $(n-1)d$ electrons in metallic bonding.

Enthalpy of atomisation = Energy required to convert 1 mole of metal atoms from solid state to gaseous state

3.7 Electrical Conductivity

Transition metals are good conductors of heat and electricity due to the presence of free electrons. Silver and copper are among the best electrical conductors and are widely used in electrical applications.

4. Formation of Coloured Ions & Magnetic Properties

4.1 Formation of Coloured Ions

Most transition metal compounds are coloured. This characteristic property arises due to d–d electronic transitions of electrons in partially filled d-orbitals.

In a free metal ion, all five d-orbitals are degenerate (same energy). When ligands approach the metal ion to form a complex, the d-orbitals split into two sets of different energies: t_2g (lower energy) and e_g (higher energy). This phenomenon is known as Crystal Field Splitting.

Fig 4.1: Crystal field splitting of d-orbitals in an octahedral complex.

If the energy gap between t_2g and e_g levels corresponds to the energy of visible light, absorption occurs and colour is observed.

Colourless ions: Sc³⁺ (3d⁰), Zn²⁺ (3d¹⁰)
Coloured ions: Ti³⁺, Fe²⁺, Cu²⁺

4.2 Complementary Colour Concept (NCERT Mandatory)

The colour observed is always the complementary colour of the light absorbed by the compound.

Colour Absorbed	Complementary Colour Observed
Red	Green
Orange	Blue
Yellow	Violet
Green	Red
Blue	Orange
Violet	Yellow

Board Tip:
If a compound absorbs blue light, it appears orange in colour.

4.3 Standard NCERT Example: [Ti(H₂O)₆]³⁺

Ion: [Ti(H₂O)₆]³⁺

Oxidation State of Ti: +3

Electronic Configuration: [Ar] 3d¹

Explanation:
The single d-electron undergoes a d–d transition by absorbing yellow-green light. Hence, the complex appears violet (purple), the complementary colour.

4.4 Magnetic Properties of Transition Elements

Magnetic behaviour of transition metal ions depends on the number of unpaired electrons present in their d-orbitals.

Paramagnetic: Contain unpaired electrons (attracted by magnetic field).
Diamagnetic: All electrons paired (repelled by magnetic field).

The magnetic moment can be calculated using the spin-only formula:

$$ \mu = \sqrt{n(n+2)} \ \text{BM} $$

Worked Numerical (NCERT Format)

Given: Fe³⁺ ion

Electronic configuration: Fe = [Ar] 3d⁶4s²

For Fe³⁺: [Ar] 3d⁵

Number of unpaired electrons (n): 5

Formula: $\mu = \sqrt{n(n+2)}$

Substitution: $\mu = \sqrt{5(5+2)} = \sqrt{35}$

Magnetic Moment: 5.92 BM

5. Important Compounds of d-Block Elements

5.1 Potassium Dichromate (K₂Cr₂O₇)

5.1.1 Preparation of Potassium Dichromate

Potassium dichromate is prepared from chromite ore (FeCr₂O₄) by the following steps:

Step 1: Formation of Sodium Chromate

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

Step 2: Formation of Sodium Dichromate

2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + H₂O

Step 3: Conversion to Potassium Dichromate

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ ↓ + 2NaCl

5.1.2 Chromate–Dichromate Equilibrium (Effect of pH)

Chromate (CrO₄²⁻, yellow) and dichromate (Cr₂O₇²⁻, orange) ions exist in equilibrium. The position of equilibrium depends on pH.

2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O

Acidic medium: Dichromate ion predominates (orange colour)
Basic medium: Chromate ion predominates (yellow colour)

5.1.3 Structure of Dichromate Ion

The dichromate ion (Cr₂O₇²⁻) has a butterfly structure. It consists of two CrO₄ tetrahedra sharing one oxygen atom.

5.1.4 Oxidising Action of K₂Cr₂O₇

Potassium dichromate acts as a strong oxidising agent only in acidic medium.

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Example:

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

5.2 Potassium Permanganate (KMnO₄)

5.2.1 Preparation of KMnO₄

Potassium permanganate is prepared from pyrolusite ore (MnO₂).

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O

5.2.2 Structure of Permanganate Ion

The permanganate ion (MnO₄⁻) has a tetrahedral structure with manganese in +7 oxidation state.

5.2.3 Oxidising Action of KMnO₄

(A) Acidic Medium:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Reaction with Oxalate (Board Favourite):

2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

Reaction with Iodide:

2MnO₄⁻ + 10I⁻ + 16H⁺ → 2Mn²⁺ + 5I₂ + 8H₂O

6. The f-Block Elements (Lanthanoids & Actinoids)

The f-block elements, also called inner transition elements, are placed separately at the bottom of the periodic table. They involve the progressive filling of f-orbitals.

6.1 Lanthanoids (4f-Series)

Lanthanoids consist of 14 elements from Cerium (Z = 58) to Lutetium (Z = 71), following Lanthanum. In these elements, the 4f-orbitals are gradually filled.

6.1.1 Oxidation States

The most common oxidation state of lanthanoids is +3. Some elements also show +2 or +4 oxidation states due to stability of half-filled or fully filled f-orbitals.

Ce⁴⁺: Stable due to empty 4f orbital
Eu²⁺, Yb²⁺: Stable due to half-filled / fully filled 4f orbitals

6.1.2 Chemical Reactivity of Lanthanoids

Lanthanoids are highly electropositive and reactive metals. Their reactivity is comparable to alkaline earth metals.

Chemical Behaviour (NCERT Flow):

With Oxygen: Burn readily to form oxides (Ln₂O₃)
With Water: Slowly react with cold water, faster with hot water, forming hydroxides
With Acids: React readily to liberate hydrogen gas
With Nitrogen: Form nitrides (LnN)
With Sulphur: Form sulphides (Ln₂S₃)

6.1.3 Lanthanoid Contraction

Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoid elements with increase in atomic number.

Cause:
Poor shielding effect of 4f-electrons. As nuclear charge increases, outer electrons are pulled closer to the nucleus.

Consequences of Lanthanoid Contraction

Similarity in size of second and third transition series elements (Zr ≈ Hf)
Difficulty in separation of lanthanoids
Decrease in basic strength of lanthanoid hydroxides

6.2 Actinoids (5f-Series)

Actinoids include 14 elements from Actinium (Z = 89) to Lawrencium (Z = 103). They involve progressive filling of 5f-orbitals.

6.2.1 Oxidation States

Actinoids show a wide range of oxidation states (+3 to +7) because 5f, 6d and 7s orbitals have comparable energies.

6.2.2 Actinoid Contraction

Similar to lanthanoids, actinoids also show a decrease in atomic and ionic radii with increase in atomic number. This is known as actinoid contraction.

Important NCERT Point:
Actinoid contraction is greater than lanthanoid contraction because the shielding effect of 5f-electrons is even poorer than that of 4f-electrons.

6.2.3 Radioactive Nature

All actinoids are radioactive. Many of them have very short half-lives and are synthetic.

6.3 Applications of d- and f-Block Elements

Mischmetal (Ce-rich alloy): Used in lighter flints and ignition devices
Transition Metals: Used as catalysts (Fe, Ni, V₂O₅)
Lanthanoids: Used in glass polishing, permanent magnets and phosphors
Uranium & Plutonium: Used as nuclear fuels

7. Formula & Trends Summary (Chapter 4)

This table provides a complete NCERT-based academic summary of formulas, trends, and exceptions related to d- and f-Block Elements. It is intended for board-exam revision, not as a substitute for theory.

Property / Concept	Formula / Trend	NCERT Explanation / Exam Note
General Electronic Configuration (d-Block)	(n−1)d^1–10 ns^1–2	Exceptions: Cr (d⁵s¹), Cu (d¹⁰s¹)
Transition Element Definition	Incomplete d-orbital in ground or oxidation state	Zn, Cd, Hg are NOT transition elements
Atomic Radii (3d Series)	Decrease → nearly constant → slight increase	Due to increasing nuclear charge and d-electron shielding
Ionisation Enthalpy (IE)	Slow increase across series	Irregularities due to half-filled and fully-filled d-subshells
Enthalpy of Atomisation	High values; maximum in middle of series	Strong metallic bonding due to large number of unpaired d-electrons
Hardness of Transition Metals	High hardness	Directly related to high enthalpy of atomisation
Oxidation States	Maximum in middle of series (Mn: +7)	Both (n−1)d and ns electrons participate in bonding
Standard Electrode Potential (E°)	Generally negative → becomes less negative → positive (Cu)	Cu has positive E° (+0.34 V) due to high hydration enthalpy and atomisation energy
Reducing Power of Metals	Decreases from left to right	More negative E° → stronger reducing agent
Magnetic Moment (Spin-only)	μ = √[n(n+2)] BM	n = number of unpaired electrons
Coloured Ions	d–d transition	d⁰ and d¹⁰ ions are colourless
Lanthanoid Contraction	Gradual decrease in size from Ce → Lu	Cause: Poor shielding effect of 4f electrons
Actinoid Contraction	Greater than lanthanoid contraction	Cause: Even poorer shielding by 5f electrons
Oxidising Power (Acidic Medium)	MnO₄⁻ > Cr₂O₇²⁻	Frequently asked 1-mark MCQ
Chromate–Dichromate Equilibrium	2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O	Yellow ⇌ Orange (pH dependent)
Geometry of MnO₄⁻	Tetrahedral	Mn in +7 oxidation state
Geometry of Cr₂O₇²⁻	Butterfly structure	Two tetrahedra sharing one oxygen atom

8. Worked Reasoning & Numerical-Type Questions

This section includes standard NCERT numericals and reasoning-based questions frequently asked in Class 12 Board Examinations. All explanations strictly follow NCERT-approved thermodynamic logic.

8.1 Numerical: Magnetic Moment (Spin-only Formula)

Example 1:
Calculate the spin-only magnetic moment of Fe³⁺ ion.

Step 1: Atomic number
Fe = 26

Step 2: Electronic configuration
Fe = [Ar] 3d⁶ 4s²

Step 3: Configuration of Fe³⁺
Fe³⁺ = [Ar] 3d⁵

Step 4: Number of unpaired electrons
For d⁵, n = 5 (Hund’s rule)

Step 5: Formula used
μ = √[n(n + 2)] BM

Step 6: Substitution
μ = √[5(5 + 2)] = √35

Final Answer:
Magnetic moment of Fe³⁺ = 5.92 BM

8.2 Reverse Numerical: Finding Unpaired Electrons

Example 2:
A metal ion shows a magnetic moment of 3.87 BM. Find the number of unpaired electrons.

Step 1: Formula
μ = √[n(n + 2)]

Step 2: Square both sides
(3.87)² = n(n + 2)

Step 3: Approximation
15 ≈ n(n + 2)

Step 4: Solve
n = 3

Answer:
Number of unpaired electrons = 3

8.3 Reasoning: Standard Electrode Potential (E° of Copper)

Question:
Why does copper have a positive standard electrode potential (E° = +0.34 V)?

Correct NCERT Explanation:

For a metal to oxidise, the following energies must be compensated:

Ionisation enthalpy
Enthalpy of atomisation

Although Cu²⁺ has hydration enthalpy, it is not sufficiently high to compensate for:

High ionisation enthalpy of Cu
High enthalpy of atomisation of Cu

Conclusion:
Therefore, oxidation of Cu(s) to Cu²⁺(aq) is not energetically favourable, and copper shows a positive standard electrode potential.

8.4 Reasoning: Disproportionation Reaction

Question:
Why is Cu⁺ unstable in aqueous solution?

Explanation:

Cu⁺ undergoes disproportionation in aqueous solution:

2Cu⁺(aq) → Cu²⁺(aq) + Cu(s)

This happens because Cu⁺ has lower hydration enthalpy than Cu²⁺.
The Cu²⁺ ion is more stabilised in aqueous medium.

Hence:
Cu⁺ is thermodynamically unstable in aqueous solution.

8.5 Trend-Based Reasoning Questions

Question:
Why do transition metals generally have high melting points?

Because of strong metallic bonding due to participation of both (n−1)d and ns electrons, resulting in high enthalpy of atomisation.

STEP–9: Practice Questions

(A) Very Short Answer Questions (1 Mark)

Why are Zn, Cd and Hg not considered transition elements?
Answer: Because they have completely filled d-orbitals (d¹⁰) in their ground state as well as in their common oxidation states.
Name the element of 3d-series showing maximum number of oxidation states.
Answer: Manganese (Mn).
Which ion is colourless: Sc³⁺ or Ti³⁺?
Answer: Sc³⁺, because it has no d-electrons (3d⁰).

(B) Short Answer Questions (2–3 Marks)

Why do transition metals exhibit variable oxidation states?
Answer: Because the energies of (n−1)d and ns orbitals are comparable. Hence, both types of electrons can participate in bonding, leading to different oxidation states.
Why do transition metals and their compounds show catalytic activity?
Answer: Due to their ability to adopt multiple oxidation states and to form intermediate complexes with reactants, which lower the activation energy of reactions.
Explain why Mn²⁺ compounds are more stable than Fe²⁺ compounds.
Answer: Mn²⁺ has a half-filled d⁵ configuration, which is extra stable, whereas Fe²⁺ has d⁶ configuration.

(C) Reasoning / Conceptual Questions (NCERT Favourite)

Why does copper have a positive standard electrode potential (+0.34 V)?
Answer: Copper has high ionisation enthalpy and high enthalpy of atomisation. Its hydration enthalpy is not sufficient to compensate for these factors. Hence oxidation of Cu to Cu²⁺ is not favourable, resulting in a positive E° value.
Why is Cu⁺ unstable in aqueous solution?
Answer: Cu⁺ undergoes disproportionation in aqueous solution:
2Cu⁺ → Cu²⁺ + Cu
This occurs because Cu⁺ has lower hydration enthalpy than Cu²⁺.
Why do transition metals form coloured compounds?
Answer: Due to d–d transitions. When visible light is absorbed, electrons are promoted from lower d-orbitals to higher d-orbitals. The observed colour is the complementary colour of the absorbed light.

(D) Numerical Problems (Step-by-Step)

Numerical 1:
Calculate the magnetic moment of Fe³⁺ ion (Z = 26).

Step 1: Electronic configuration of Fe = [Ar] 3d⁶4s²

Step 2: Fe³⁺ = [Ar] 3d⁵

Step 3: Number of unpaired electrons (n) = 5

Step 4: μ = √[n(n + 2)] = √[5(7)] = √35

Answer: μ ≈ 5.92 BM

Numerical 2:
A transition metal ion shows a magnetic moment of 2.83 BM. Find the number of unpaired electrons.

Step 1 (Formula):
μ = √[n(n + 2)]

Step 2 (Observation):
The given magnetic moment (2.83 BM) is very close to √8.

Step 3 (Verification):
For n = 2 → √[2(2 + 2)] = √8 ≈ 2.83 BM

Answer: Number of unpaired electrons = 2

STEP–10: Academic FAQ

Frequently Asked Questions (NCERT-Based)

Q1. Why do transition metals show variable oxidation states?
Answer: Transition metals show variable oxidation states because the energy difference between (n−1)d and ns orbitals is very small. Hence, both orbitals can participate in bond formation, leading to different oxidation states.
Q2. Why are Zn, Cd and Hg not considered transition elements?
Answer: Although Zn, Cd and Hg belong to the d-block, they have completely filled d-orbitals (d¹⁰) in both their ground state and common oxidation states. Therefore, they do not satisfy the definition of transition elements.
Q3. Why do most transition metal compounds exhibit colour?
Answer: Due to d–d transitions. When visible light is absorbed, electrons are excited from lower energy d-orbitals to higher energy d-orbitals. The colour observed is the complementary colour of the light absorbed.
Q4. What is lanthanoid contraction?
Answer: Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoids with increasing atomic number. It occurs due to the poor shielding effect of 4f electrons, which increases effective nuclear charge.
Q5. Why is actinoid contraction greater than lanthanoid contraction?
Answer: Actinoid contraction is greater because 5f electrons have an even poorer shielding effect than 4f electrons. As a result, the increase in effective nuclear charge is more pronounced in actinoids.
Q6. Why is Cu⁺ unstable in aqueous solution?
Answer: Cu⁺ undergoes disproportionation in aqueous solution:
2Cu⁺ → Cu²⁺ + Cu
This happens because Cu²⁺ has much higher hydration enthalpy than Cu⁺, making the reaction thermodynamically favourable.

STEP–11: Internal Linking & Pre-requisite Check

Academic Continuity (NCERT-Aligned)

This chapter is academically connected with earlier NCERT chapters of Class 12 Chemistry. Only published and verified chapters are linked below to maintain syllabus continuity and academic integrity.

Pre-requisite Chapter (Verified):
Class 12 Chemistry Chapter 3: Chemical Kinetics
Relevance: Concepts of reaction rate, redox processes, and catalytic behaviour are essential for understanding oxidation states and reactions of transition metals.
Related Physical Chemistry Chapter (Verified):
Class 12 Chemistry Chapter 1: Solutions
Relevance: Helps in understanding hydration enthalpy, stability of ions in solution, and aqueous behaviour of transition metal ions.

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