Q.53 Chemistry · Section A ⭐ 360° Option Elimination Active

NEET 2024 Chemistry Question 53 Solution

Topic: Chemical Bonding and Molecular Structure · Official NTA Booklet Code T1

Question

Match List I (Molecule) with List II (Number and types of bond/s between two carbon atoms). List I: A. ethane; B. ethene; C. carbon molecule, C₂; D. ethyne. List II: I. one σ-bond and two π-bonds; II. two π-bonds; III. one σ-bond; IV. one σ-bond and one π-bond. Choose the correct answer from the options given below:

✓

Verified Against NTA Final Answer Key

Option (1) : A-III, B-IV, C-II, D-I

📚 Chemical Bonding and Molecular Structure Easy

📋 Given Parameters & Problem Setup

Molecule A Ethane (C2H6)

Molecule B Ethene (C2H4)

Molecule C Carbon molecule (C2)

Molecule D Ethyne (C2H2)

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The 30-Second Exam Hall Shortcut: ⚡ The Unique C2 Molecular Orbital Hack

⏱️ Save ~40 seconds

Ethane = 1σ (III), Ethene = 1σ + 1π (IV), Ethyne = 1σ + 2π (I), and C2 has ONLY 2π bonds (II)!

1 Ethane: H3C-CH3 single bond = one σ-bond (A-III).

2 Ethene: H2C=CH2 double bond = one σ-bond and one π-bond (B-IV).

3 Ethyne: HC≡CH triple bond = one σ-bond and two π-bonds (D-I).

4 C2 molecule has 12 electrons: (σ1s)² (σ*1s)² (σ2s)² (σ*2s)² (π2px)² (π2py)². Both bonding pairs reside in degenerate π-orbitals -> Two π-bonds and ZERO σ-bonds (C-II)!

5 Match: A-III, B-IV, C-II, D-I -> Option (1) in 5 seconds!

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Why You Can 100% Trust This Shortcut: NCERT Chemistry · Chemical Bonding and Molecular Structure (Molecular orbital configuration of C2 and hybridization).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Bonding in Ethane, Ethene, and Ethyne

In valence bond theory: Ethane involves sp³-sp³ axial overlap forming one σ-bond. Ethene involves sp²-sp² axial overlap (one σ-bond) and sideways 2p-2p overlap (one π-bond). Ethyne involves sp-sp axial overlap (one σ-bond) and two mutually perpendicular sideways 2p-2p overlaps (two π-bonds).

\text{Ethane: } 1\sigma, \quad \text{Ethene: } 1\sigma + 1\pi, \quad \text{Ethyne: } 1\sigma + 2\pi

Step 2

Molecular Orbital Treatment of C2

For homonuclear diatomic molecules with Z ≤ 7, the π2px and π2py molecular orbitals are lower in energy than the σ2pz orbital due to 2s-2p mixing. The 12-electron configuration of C2 is:

\sigma_{1s}^2 \; \sigma_{1s}^{*2} \; \sigma_{2s}^2 \; \sigma_{2s}^{*2} \; (\pi_{2p_x}^2 = \pi_{2p_y}^2)

Step 3

Evaluation of Bond Order and Nature in C2

The bond order is (8 - 4)/2 = 2. The four valence electrons responsible for the double bond reside entirely in the degenerate π2px and π2py bonding orbitals. Consequently, the double bond in C2 consists entirely of two π-bonds with no σ-bond.

N_b = 8, \; N_a = 4 \implies \text{Bond Order} = \frac{8 - 4}{2} = 2 \; (\text{both are } \pi\text{-bonds})

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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)

✓ 100% Correct

Verdict & Scientific Reason:

Perfect alignment across all four species.

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Why this is airtight: None.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Inverts C2 and ethyne bond descriptions.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming C2 has a triple bond.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Assigns triple bond to ethane.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing ethane with ethyne.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Inverts ethane and ethene.

⚠️

Student Fallacy (Why candidates pick this trap): Switching single and double bond counts.

⚠️

NTA Trap Alert: Assuming All Double Bonds Consist of 1σ and 1π

Students routinely assume every double bond must contain one σ and one π bond. In C2, MOT proves both bonds are exclusively π bonds!

🛡️ How to avoid the -1 mark: Remember this classic NCERT line: "In C2, the double bond consists of both pi bonds because of the presence of four electrons in two pi molecular orbitals."

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Core NEET Syllabus Standard

Chemistry Core Inorganic/Physical · Chemical Bonding and Molecular Structure Standard Benchmark

Core Scientific Principle Tested:

"In diatomic carbon C2, the 4 outermost valence electrons occupy degenerate π2p bonding orbitals, making the bond order 2 consisting of two π bonds."

Syllabus Focus: Molecular Orbital Theory and Electronic Configuration of Diatomics

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

C2 is the Pi-Pi star: Two pi bonds and no sigma at all!

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NEET 2024 Chemistry Question 53 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The Unique C2 Molecular Orbital Hack

Step-by-Step Rigorous Workflow & Mathematical Proof

Bonding in Ethane, Ethene, and Ethyne

Molecular Orbital Treatment of C2

Evaluation of Bond Order and Nature in C2

360° Option Elimination Engine Key Feature

NTA Trap Alert: Assuming All Double Bonds Consist of 1σ and 1π

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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