Q.67 Chemistry ⭐ 360° Option Elimination Active

Vault ↗

NEET 2021 Chemistry Question 67 Solution

Topic: Solid State · Official NTA Booklet Code M1

Question

The correct option for the number of body centred unit cells in all 14 types of Bravais lattice unit cells is:

✓

Verified Against NTA Final Answer Key

Option (4) : 3

📚 Solid State Easy

📋 Given Parameters & Problem Setup

Total Crystal Systems 7 crystal systems

Total Bravais Lattices 14 Bravais lattices

Target Unit Cell Type Body-centred unit cells (BCC)

⚡

The 30-Second Exam Hall Shortcut: ⚡ BCC Systems: Cubic, Tetragonal, Orthorhombic (Count = 3)

⏱️ Save ~20 seconds

Across all 7 crystal systems, body-centred lattices occur in exactly 3 systems: Cubic ( $1$ ), Tetragonal ( $1$ ), and Orthorhombic ( $1$ ). No other system possesses a body-centred lattice! Total body-centred Bravais lattices $= 1 + 1 + 1 = 3$ ! Match Option (4)!

1 Total Bravais lattices = 14 across 7 crystal systems.

2 Recall distribution of Bravais types: - Primitive (P): in all 7 systems (7) - Body-centred (I): in Cubic, Tetragonal, Orthorhombic (3) - Face-centred (F): in Cubic, Orthorhombic (2) - End-centred (C): in Orthorhombic, Monoclinic (2) Total = 7 + 3 + 2 + 2 = 14.

3 Count of body-centred unit cells = 3.

4 Match Option (4).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Solid State crystallography principle on 14 Bravais Lattices.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Reviewing the 14 Bravais Lattices across 7 Crystal Systems

Auguste Bravais demonstrated that only 14 space lattices are mathematically possible in three dimensions across the 7 crystal systems: 1. Cubic (3): Primitive, Body-centred, Face-centred 2. Tetragonal (2): Primitive, Body-centred 3. Orthorhombic (4): Primitive, Body-centred, Face-centred, End-centred 4. Hexagonal (1): Primitive 5. Rhombohedral / Trigonal (1): Primitive 6. Monoclinic (2): Primitive, End-centred 7. Triclinic (1): Primitive

Step 2

Enumerating Body-Centred Unit Cells

Summing the body-centred lattices: - Cubic body-centred: 1 - Tetragonal body-centred: 1 - Orthorhombic body-centred: 1 No other crystal system allows a body-centred Bravais lattice without reducing to a simpler existing lattice.

\text{Total body-centred Bravais lattices} = 1 + 1 + 1 = 3

⭐

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:

7 is the number of PRIMITIVE unit cells (one in each crystal system), not body-centred.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing primitive unit cell count with body-centred unit cell count.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Arbitrary number.

⚠️

Student Fallacy (Why candidates pick this trap): Misremembering the Bravais lattice table.

3 3

✗ Incorrect

Verdict & Scientific Reason:

2 is the number of Face-centred (or End-centred) unit cells, not body-centred.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing body-centred with face-centred Bravais lattices.

4 4

✓ 100% Correct

Verdict & Scientific Reason:

Exact count of body-centred unit cells: Cubic, Tetragonal, and Orthorhombic.

💡

Why this is airtight: None; precise Bravais lattice crystallography knowledge.

⚠️

NTA Trap Alert: Primitive vs. Body-Centred vs. Face-Centred Trap

The Bravais breakdown is: - Primitive = 7 - Body-centred = 3 - Face-centred = 2 - End-centred = 2 Total = 14. Students often confuse the counts of primitive (7) or face-centred (2) with body-centred (3).

🛡️ How to avoid the -1 mark: Mnemonic: "CTO has Body"

\implies

Cubic, Tetragonal, Orthorhombic = 3 systems.

📖

Core NEET Syllabus Standard

NCERT Chemistry · Solid State Standard Benchmark

Core Scientific Principle Tested:

"Only three crystal systems—cubic, tetragonal, and orthorhombic—possess body-centred Bravais lattices."

Syllabus Focus: Crystal Lattices and Unit Cells (14 Bravais Lattices)

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

← Previous (Q.66) All Chemistry Questions Grid Next (Q.68) →

Body Centered Cubic (BCC) Lattice: Relationship Between Edge Length and Atomic Radius

View Solution →

NEET 2021 Q56

Hexagonal Primitive Unit Cell: Number of Atoms, Tetrahedral and Octahedral Voids

View Solution →

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2021 Chemistry Question 67 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ BCC Systems: Cubic, Tetragonal, Orthorhombic (Count = 3)

Step-by-Step Rigorous Workflow & Mathematical Proof

Reviewing the 14 Bravais Lattices across 7 Crystal Systems

Enumerating Body-Centred Unit Cells

360° Option Elimination Engine Key Feature

NTA Trap Alert: Primitive vs. Body-Centred vs. Face-Centred Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — The Solid State

Formula of a Compound and Sum of Stoichiometric Subscripts (x + y)

Octahedral Voids in Face-Centred Cubic (fcc) Unit Cell

Body Centered Cubic (BCC) Lattice: Relationship Between Edge Length and Atomic Radius

Hexagonal Primitive Unit Cell: Number of Atoms, Tetrahedral and Octahedral Voids

Solve 4 NEET Questions a Day — One per Subject