Q.90 Chemistry ⭐ 360° Option Elimination Active
NEET 2026 Chemistry Question 90 Solution
Topic: Chemical Bonding and Molecular Structure · Official NTA Booklet Code 50
Question
The highest occupied molecular orbital for Ne₂ is
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Verified Against NTA Final Answer Key
Option (4) : σ*_{2p}
📚 Chemical Bonding and Molecular Structure Easy
📋 Given Parameters & Problem Setup
Diatomic Species Ne₂ (Neon dimer)
Total Electron Count 2 × 10 = 20 electrons
Target Highest Occupied Molecular Orbital (HOMO)
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⏱️ Save ~45 seconds The 30-Second Exam Hall Shortcut: ⚡ Full 20-Electron MOT Sequence
Ne₂ fills all available second-row molecular orbitals up to the very highest antibonding level: σ*_{2p}!
1 Total electrons in Ne₂ = 10 + 10 = 20 electrons.
2 Molecular orbital filling order for elements with Z > 7 (O₂, F₂, Ne₂):
3 σ_{1s}² σ*_{1s}² σ_{2s}² σ*_{2s}² σ_{2p_z}² (π_{2p_x}² = π_{2p_y}²) (π*_{2p_x}² = π*_{2p_y}²) σ*_{2p_z}².
4 All 20 electrons completely fill orbitals up to the final σ*_{2p} level.
5 Therefore, the Highest Occupied Molecular Orbital (HOMO) is σ*_{2p}!
6 Option (4) is undeniably correct in 5 seconds flat!
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Why You Can 100% Trust This Shortcut: Universal molecular orbital theory: Second-row diatomic molecules with 20 electrons fill all bonding and antibonding molecular orbitals generated from 1s, 2s, and 2p valence atomic orbitals, ending at σ*_{2p} (bond order = (10 - 10)/2 = 0).
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Step-by-Step Rigorous Workflow & Mathematical Proof
Step 1
Calculate Total Number of Electrons in Ne₂
Neon has atomic number Z = 10. A hypothetical homonuclear diatomic neon molecule (Ne₂) contains 2 × 10 = 20 electrons.
- • Core electrons from 1s: 4 electrons.
- • Valence electrons from 2s and 2p: 16 electrons.
Step 2
Construct the Complete Molecular Orbital Configuration
For homonuclear diatomics with Z > 7, no significant 2s-2p mixing occurs. The molecular orbitals are ordered by increasing energy: σ_{1s} < σ*_{1s} < σ_{2s} < σ*_{2s} < σ_{2p_z} < (π_{2p_x} = π_{2p_y}) < (π*_{2p_x} = π*_{2p_y}) < σ*_{2p_z}
- • Total electrons accommodated = 2 + 2 + 2 + 2 + 2 + 4 + 4 + 2 = 20 electrons.
Step 3
Identify the Highest Occupied Molecular Orbital (HOMO)
The orbital of highest energy that contains electrons in the ground-state electron configuration is defined as the HOMO. In Ne₂, the last two electrons occupy the σ*_{2p_z} (antibonding σ_{2p}) orbital.
- • HOMO is σ*_{2p}.
- • Matches Option (4).
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4-Option Deep Scan
360° Option Elimination Engine Key Feature
Diagnose every choice: Know exactly why the right option works and why distractors fail.
1 A
✗ Incorrect Verdict & Scientific Reason:
Incorrect. π_{2p} holds electrons 11 through 14, well below the highest occupied orbital.
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Student Fallacy (Why candidates pick this trap): Confusing bonding π orbitals with highest occupied level.
2 B
✗ Incorrect Verdict & Scientific Reason:
Incorrect. σ_{2p} holds electrons 9 and 10, far lower in energy than the antibonding orbitals.
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Student Fallacy (Why candidates pick this trap): Thinking of bonding orbitals only.
3 C
✗ Incorrect Verdict & Scientific Reason:
Incorrect. π*_{2p} is the HOMO for O₂ (16 electrons) and F₂ (18 electrons), but Ne₂ has 20 electrons.
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Student Fallacy (Why candidates pick this trap): Confusing the HOMO of F₂ with the HOMO of Ne₂.
4 D
✓ 100% Correct Verdict & Scientific Reason:
Exact match: With 20 electrons, the last orbital filled is the antibonding σ*_{2p} orbital.
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Why this is airtight: None; flawless mastery of Molecular Orbital Theory.
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NTA Trap Alert: Assuming F₂ and Ne₂ Share the Same HOMO
Students frequently memorize that F₂ (18 electrons) has HOMO = π*_{2p}. Ne₂ has 20 electrons, adding 2 more electrons that occupy the final higher-energy σ*_{2p} orbital!
🛡️ How to avoid the -1 mark: Count all 20 electrons: F₂ stops at π*_{2p} (18 e⁻); Ne₂ fills the final σ*_{2p} (20 e⁻).
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Core NEET Syllabus Standard
NEET Chemistry Core Syllabus · Chemical Bonding and Molecular Structure Standard Benchmark
Core Scientific Principle Tested:
"In diatomic second-row molecules with 20 electrons, all 2p bonding and antibonding orbitals are completely filled."
Syllabus Focus: Molecular Orbital Theory - Energy level diagrams and electronic configurations
✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗
20 electrons fill the cup right to the top: σ*_{2p} is the final stop!
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