Q.16 In NaCl crystal, the arrangement and coordination number of the ions are
(A) fcc and 6
(B) fcc and 4
(C) hcp and 6
(D) hcp and 4
NaCl crystal has a face-centered cubic (FCC) arrangement with each ion surrounded by 6 nearest neighbors of the opposite charge, making option (A) correct.
Crystal Structure Details
NaCl forms a rock salt structure where Cl⁻ ions occupy FCC lattice positions (corners and face centers), and Na⁺ ions fill all octahedral voids. This creates two interpenetrating FCC sublattices shifted by half the body diagonal.
Each Na⁺ bonds octahedrally to 6 Cl⁻ ions, and vice versa, yielding a 6:6 coordination number that maximizes ionic attraction while maintaining stability.
Option Analysis
| Option | Arrangement | Coordination | Valid? | Reason |
|---|---|---|---|---|
| (A) | FCC | 6 | Yes | Matches rock salt structure: octahedral voids give CN=6 |
| (B) | FCC | 4 | No | CN=4 is tetrahedral (ZnS structure), not NaCl |
| (C) | HCP | 6 | No | NaCl uses FCC, not hexagonal close-packed |
| (D) | HCP | 4 | No | Wrong lattice and coordination |
NaCl Crystal Structure Basics
NaCl crystal arrangement coordination number questions test solid-state chemistry fundamentals for GATE Life Sciences. The rock salt structure features FCC packing of anions with cations in octahedral sites.
Unit Cell Breakdown
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Cl⁻ at 8 corners (⅛×8=1) + 6 face centers (½×6=3) = 4 Cl⁻ per unit cell
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Na⁺ in 4 octahedral voids = 4 Na⁺
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Each ion touches 6 opposites along <100> directions
Exam Relevance
GATE aspirants: Radius ratio r⁺/r⁻ ≈0.52-0.73 predicts octahedral (CN=6) coordination, unlike tetrahedral (0.22-0.41) or cubic (0.73-1.00).
