Q.52 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 52 Solution

Topic: Chemical Bonding and Molecular Structure · Official NTA Booklet Code G5

Question

Amongst the following, the total number of species NOT having eight electrons around central atom in its outer most shell, is NH₃, AlCl₃, BeCl₂, CCl₄, PCl₅:

✓

Verified Against NTA Final Answer Key

Option (2) : 3

📚 Chemical Bonding and Molecular Structure Easy

📋 Given Parameters & Problem Setup

Set of molecules NH₃, AlCl₃, BeCl₂, CCl₄, PCl₅

Target condition Central atom NOT having 8 electrons in outermost shell

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The 30-Second Exam Hall Shortcut: ⚡ Valence Electron Quick Scan: Octet = 8; Non-Octet ≠ 8

⏱️ Save ~30 seconds

NH₃ (8 e⁻, octet) and CCl₄ (8 e⁻, octet) are normal octets. AlCl₃ (6 e⁻, hypo), BeCl₂ (4 e⁻, hypo), and PCl₅ (10 e⁻, hyper) violate octet rule! Total = 3 non-octet species in 10 seconds!

1 Inspect each central atom valence shell:

2 - NH₃: N has 3 single bonds + 1 lone pair = 6 + 2 = 8 electrons (Octet complete).

3 - AlCl₃: Al has 3 valence electrons, forming 3 single bonds = 6 electrons (Incomplete octet / Hypovalent) ⟹ COUNT #1.

4 - BeCl₂: Be has 2 valence electrons, forming 2 single bonds = 4 electrons (Incomplete octet / Hypovalent) ⟹ COUNT #2.

5 - CCl₄: C has 4 valence electrons, forming 4 single bonds = 8 electrons (Octet complete).

6 - PCl₅: P has 5 valence electrons, forming 5 single bonds = 10 electrons (Expanded octet / Hypervalent) ⟹ COUNT #3.

7 Count the non-octet species: AlCl₃, BeCl₂, and PCl₅ = 3 species.

8 Lock Option (2) 3 immediately!

🛡️

Why You Can 100% Trust This Shortcut: Chemical Bonding · Octet Rule Exceptions (Incomplete octets occur in electron-deficient Group 2 and 13 monomer halides; expanded octets occur in Period 3 elements utilizing d-orbitals).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Definition of Lewis Octet Completeness

According to the Lewis-Kossel octet rule, main-group atoms tend to gain, lose, or share electrons to attain a stable valence configuration of eight electrons (s² p⁶). Species deviating from 8 valence electrons around the central atom are classified as either electron-deficient (incomplete octet, < 8 e⁻) or hypervalent (expanded octet, > 8 e⁻).

\text{Octet Criterion: Valence electrons around central atom } = 8

Step 2

Individual Molecule Electron Count

Evaluate the total valence electrons surrounding the central atom in each monomeric Lewis structure:

\begin{aligned} \text{NH}_3 &: 3\text{ bond pairs} + 1\text{ lone pair} = 6 + 2 = 8\ e^- \quad (\text{Complete octet}) \\[4pt] \text{AlCl}_3 &: 3\text{ bond pairs} + 0\text{ lone pairs} = 6\ e^- \quad (\text{Incomplete octet, } < 8) \\[4pt] \text{BeCl}_2 &: 2\text{ bond pairs} + 0\text{ lone pairs} = 4\ e^- \quad (\text{Incomplete octet, } < 8) \\[4pt] \text{CCl}_4 &: 4\text{ bond pairs} + 0\text{ lone pairs} = 8\ e^- \quad (\text{Complete octet}) \\[4pt] \text{PCl}_5 &: 5\text{ bond pairs} + 0\text{ lone pairs} = 10\ e^- \quad (\text{Expanded octet, } > 8) \end{aligned}

Step 3

Summation of Non-Octet Species

The species not having eight electrons around their central atom are AlCl₃ (6 e⁻), BeCl₂ (4 e⁻), and PCl₅ (10 e⁻). Thus, exactly 3 species violate the octet rule.

\text{Total non-octet species} = 3 \quad (\text{AlCl}_3, \text{BeCl}_2, \text{PCl}_5)

⭐

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:

Only counts PCl₅ (assuming only expanded octets violate the rule).

⚠️

Student Fallacy (Why candidates pick this trap): Candidate forgot that incomplete octets (< 8 e⁻) are also non-octet species.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate count: AlCl₃ (6 e⁻), BeCl₂ (4 e⁻), and PCl₅ (10 e⁻) all lack exactly 8 electrons.

💡

Why this is airtight: None. Candidate correctly included both hypovalent and hypervalent molecules.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Only counts hypovalent species (AlCl₃ and BeCl₂) or misses PCl₅.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate forgot that having 10 electrons in PCl₅ means it does NOT have 8 electrons.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Overcounting NH₃ or CCl₄.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate miscounted the lone pair on nitrogen in NH₃.

⚠️

NTA Trap Alert: Forgetting That "NOT 8" Includes BOTH Less Than 8 AND More Than 8

Some students only look for electron-deficient species (< 8, giving AlCl₃ and BeCl₂ = 2 species). Others only look for expanded octets (> 8, giving PCl₅ = 1 species). The prompt asks for species NOT having 8 electrons, which includes ALL deviations from 8: AlCl₃ (6), BeCl₂ (4), and PCl₅ (10)!

🛡️ How to avoid the -1 mark: Read "NOT having 8" literally: Any count ≠ 8 qualifies.

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Chemical Bonding and Molecular Structure Standard Benchmark

Core Scientific Principle Tested:

"Exceptions to the octet rule include the incomplete octet of central atom (BeCl₂, AlCl₃) and the expanded octet (PCl₅, SF₆)."

Syllabus Focus: Kossel-Lewis Approach and Limitations of the Octet Rule

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 52 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Valence Electron Quick Scan: Octet = 8; Non-Octet ≠ 8

Step-by-Step Rigorous Workflow & Mathematical Proof

Definition of Lewis Octet Completeness

Individual Molecule Electron Count

Summation of Non-Octet Species

360° Option Elimination Engine Key Feature

NTA Trap Alert: Forgetting That "NOT 8" Includes BOTH Less Than 8 AND More Than 8

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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