Q.72 Chemistry ⭐ 360° Option Elimination Active

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NEET 2021 Chemistry Question 72 Solution

Topic: The s-Block Elements · Official NTA Booklet Code M1

Question

The structures of beryllium chloride in solid state and vapour phase, are:

(1)Chain and dimer, respectively
(2)Linear in both
(3)Dimer and Linear, respectively
(4)Chain in both

✓

Verified Against NTA Final Answer Key

Option (1) : Chain and dimer, respectively

📚 The s-Block Elements Easy

📋 Given Parameters & Problem Setup

Compound Beryllium chloride (BeCl2)

States Evaluated Solid state and vapour phase

⚡

The 30-Second Exam Hall Shortcut: ⚡ BeCl2 Structures: Solid = Polymeric Chain; Vapour = Dimer (at < 1200 K)

⏱️ Save ~20 seconds

Verbatim NCERT Group 2 Elements: In the solid state, $BeCl_2$ forms a polymeric chain with chloro-bridges where Be is $sp^3$ hybridized. In the vapour phase at moderate temperatures, it exists as a chloro-bridged dimer ( $Be_2Cl_4$ with $sp^2$ Be), dissociating into linear monomer ( $sp$ ) only above $1200\text{ K}$ . Hence, solid state and vapour phase are Chain and dimer respectively! Match Option (1)!

1 Solid state: Electron-deficient beryllium achieves an octet through coordinate bridging with chlorine lone pairs, forming a 1D polymeric chain.

2 Vapour phase (below 1200 K):

BeCl_2

exists primarily as a bridged dimer:

Cl-Be(\mu-Cl)_2Be-Cl

3 Above 1200 K: Dimer dissociates into linear monomer (

Cl-Be-Cl

4 Respectively: Chain and dimer.

5 Match Option (1).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT s-Block Elements section on anomalous properties of Beryllium.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Structure of BeCl2 in the Solid State

Beryllium has only 4 valence electrons in monomeric $BeCl_2$ , making it severely electron deficient. In the solid state, $BeCl_2$ forms an infinite polymeric chain structure. Each beryllium atom is tetrahedrally surrounded by four chlorine atoms via two normal covalent bonds and two coordinate bonds from chlorine lone pairs ( $sp^3$ hybridized Be):

\cdots -\text{Be}(\mu-\text{Cl})_2\text{Be}(\mu-\text{Cl})_2\text{Be}- \cdots \quad (\mathbf{\text{Polymeric Chain Structure}})

Step 2

Structure of BeCl2 in the Vapour Phase

When heated into the vapour phase, the polymeric chain breaks down into volatile chloro-bridged dimers, $(BeCl_2)_2$ or $Be_2Cl_4$ , where each beryllium is $sp^2$ hybridized with a vacant $p$ orbital. (Only at extreme temperatures exceeding $1200\text{ K}$ does the dimer dissociate into a linear monomer $Cl-Be-Cl$ with $sp$ hybridization):

2\,\text{BeCl}_2 \rightleftharpoons \text{Be}_2\text{Cl}_4 \quad (\mathbf{\text{Chloro-bridged Dimer}})

Step 3

Conclusion

In the solid state and vapour phase, the structures are chain and dimer respectively, corresponding to Option (1).

⭐

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:

Verbatim NCERT factual statement: solid = polymeric chain, vapour = dimer.

💡

Why this is airtight: None; textbook inorganic structure precision.

2 2

✗ Incorrect

Verdict & Scientific Reason:

BeCl2 is linear only in the high-temperature monomeric vapour phase (>1200 K), not in solid or standard vapour.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming simple Lewis structure Cl-Be-Cl applies in all physical states.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Swaps/distorts the phases; solid is a polymeric chain, not a dimer.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming solid is dimer and vapour is linear monomer.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Polymeric chain cannot persist in the gas/vapour phase due to thermal entropy.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming the solid polymeric chain survives vaporization intact.

⚠️

NTA Trap Alert: Linear Monomer Temperature Condition Trap

Students who remember that $BeCl_2$ is linear often choose Option (2) or (3). $BeCl_2$ is linear ONLY in the gas phase at temperatures ABOVE $1200\text{ K}$ . Under standard vapour conditions, it exists as a dimer ( $Be_2Cl_4$ ), and in the solid state as a polymeric chain.

🛡️ How to avoid the -1 mark: Solid = Chain; Vapour = Dimer; Vapour (>1200 K) = Linear monomer.

📖

Core NEET Syllabus Standard

NCERT Chemistry · The s-Block Elements Standard Benchmark

Core Scientific Principle Tested:

"In solid state, BeCl2 has a polymeric chain structure, while in the vapour phase it forms a chloro-bridged dimer which dissociates into linear monomer only at high temperatures (~1200 K)."

Syllabus Focus: Group 2 Elements: Anomalous Properties of Beryllium

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Chemistry Question 72 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ BeCl2 Structures: Solid = Polymeric Chain; Vapour = Dimer (at < 1200 K)

Step-by-Step Rigorous Workflow & Mathematical Proof

Structure of BeCl2 in the Solid State

Structure of BeCl2 in the Vapour Phase

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Linear Monomer Temperature Condition Trap

Core NEET Syllabus Standard

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