Q.56 Chemistry · Section A ⭐ 360° Option Elimination Active

NEET 2024 Chemistry Question 56 Solution

Topic: The d- and f-Block Elements · Official NTA Booklet Code T1

Question

'Spin only' magnetic moment is same for which of the following ions? A. Ti³⁺ B. Cr²⁺ C. Mn²⁺ D. Fe²⁺ E. Sc³⁺ Choose the most appropriate answer from the options given below:

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Verified Against NTA Final Answer Key

Option (3) : B and D only (Cr2+ and Fe2+)

📚 The d- and f-Block Elements Easy

📋 Given Parameters & Problem Setup

Ion A Ti3+ (Z = 22)

Ion B Cr2+ (Z = 24)

Ion C Mn2+ (Z = 25)

Ion D Fe2+ (Z = 26)

Ion E Sc3+ (Z = 21)

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The 30-Second Exam Hall Shortcut: ⚡ Unpaired Electrons Counting Shortcut

⏱️ Save ~40 seconds

Same spin-only magnetic moment requires IDENTICAL number of unpaired electrons (n)! Cr2+ (3d4 -> 4) and Fe2+ (3d6 -> 4) both have n = 4!

1 Formula: μ_s = √[n(n + 2)] BM. Equal μ requires equal n.

2 Ti3+ (Z=22, [Ar]3d1): n = 1 unpaired electron.

3 Cr2+ (Z=24, [Ar]3d4): n = 4 unpaired electrons.

4 Mn2+ (Z=25, [Ar]3d5): n = 5 unpaired electrons.

5 Fe2+ (Z=26, [Ar]3d6): 6 electrons in five d-orbitals = 1 paired + 4 unpaired -> n = 4 unpaired electrons.

6 Sc3+ (Z=21, [Ar]3d0): n = 0 unpaired electrons.

7 Ions with n = 4 are Cr2+ (B) and Fe2+ (D) -> Option (3) in 10 seconds!

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Why You Can 100% Trust This Shortcut: NCERT Chemistry · The d- and f-Block Elements (Electronic configurations and magnetic properties of first row transition metal ions).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Formula for Spin-Only Magnetic Moment

For transition metal ions of the first series, orbital angular momentum is largely quenched by the crystal/ligand field. The magnetic moment depends exclusively on the number of unpaired electrons n:

\mu_s = \sqrt{n(n + 2)} \; \text{B.M.} \quad (\text{Bohr Magnetons})

Step 2

Determination of d-Electron Count and Unpaired Electrons

Determining the electronic configurations of the 3d transition metal cations: - Ti3+ (Z = 22): [Ar] 3d1 -> n = 1 unpaired electron (μ = √3 = 1.73 BM) - Cr2+ (Z = 24): [Ar] 3d4 -> n = 4 unpaired electrons (μ = √24 = 4.90 BM) - Mn2+ (Z = 25): [Ar] 3d5 -> n = 5 unpaired electrons (μ = √35 = 5.92 BM) - Fe2+ (Z = 26): [Ar] 3d6 -> 5 + 1 paired -> n = 4 unpaired electrons (μ = √24 = 4.90 BM) - Sc3+ (Z = 21): [Ar] 3d0 -> n = 0 unpaired electrons (μ = 0 BM, diamagnetic)

n(\text{Cr}^{2+}) = 4, \quad n(\text{Fe}^{2+}) = 4 \implies \mu_s = \sqrt{4(4 + 2)} = \sqrt{24} \approx 4.90\;\text{B.M.}

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Cr2+ has n = 4 whereas Mn2+ has n = 5.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming adjacent elements in the periodic table have identical magnetic moments.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Ti3+ has n = 1 whereas Fe2+ has n = 4.

⚠️

Student Fallacy (Why candidates pick this trap): Failing to pair electrons in the d6 subshell.

3 (3)

✓ 100% Correct

Verdict & Scientific Reason:

Both ions possess precisely 4 unpaired electrons, yielding identical spin-only moments of 4.90 BM.

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Why this is airtight: None.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Ti3+ is paramagnetic (n=1) while Sc3+ is diamagnetic (n=0).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing 3d0 with 3d1.

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NTA Trap Alert: Counting All 6 d-Electrons in Fe2+ as Unpaired

Students who forget Hund’s rule occasionally write n = 6 for Fe2+, forgetting that the 5 d-orbitals can hold at most 5 parallel spins before pairing begins (n = 10 - 6 = 4).

🛡️ How to avoid the -1 mark: For dn configurations with n > 5: Number of unpaired electrons = 10 - n.

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Core NEET Syllabus Standard

Chemistry Core Inorganic Chemistry · The d- and f-Block Elements Standard Benchmark

Core Scientific Principle Tested:

"The spin-only magnetic moment μ = √[n(n+2)] BM is identical for any pair of ions that possess the same number of unpaired d-electrons."

Syllabus Focus: Magnetic Properties of Transition Metal Compounds

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

d4 and d6 mirror around half-filled d5: both share 4 unpaired spins alive!

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NEET 2024 Chemistry Question 56 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Unpaired Electrons Counting Shortcut

Step-by-Step Rigorous Workflow & Mathematical Proof

Formula for Spin-Only Magnetic Moment

Determination of d-Electron Count and Unpaired Electrons

360° Option Elimination Engine Key Feature

NTA Trap Alert: Counting All 6 d-Electrons in Fe2+ as Unpaired

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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