In trigonal planar arrangement the ratio is -

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HPSC Assistant Professor 2018 Chemistry (Official Paper)
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  1. 0.732 - 1.00
  2. 0.414 - 0.732
  3. 0.225 - 0.414
  4. 0.155 - 0.225

Answer (Detailed Solution Below)

Option 4 : 0.155 - 0.225
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Detailed Solution

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CONCEPT:

Trigonal Planar Arrangement and Ionic Radii Ratios

  • Ionic compounds often form geometric arrangements based on the ratio of the radius of the cation to the radius of the anion ( \(r_c / r_a\) ).
  • Different geometries are stabilized by specific ranges of this ratio.
  • In a trigonal planar arrangement, a cation is surrounded by three anions in a plane forming a triangular shape.

EXPLANATION:

​​

Coordination number

Structure

Example

0.155 - 0.225

3

Trigonal planar

B2O3

0.225 - 0.414

4

Tetrahedral

ZnS

0.414 - 0.732

6

Octahedral

NaCl

0.732 - 1.0

8

Cubic

CsCl

  • The radius ratio (\(r_c / r_a\) ) dictates the preferred geometry:
    • \(r_c / r_a\) = 0.155 - 0.225 : This range is for trigonal planar arrangement where the cation fits snugly between three anions arranged in a triangle.
    • \(r_c / r_a\) = 0.225 - 0.414 : This range stabilizes a tetrahedral arrangement.
    • \(r_c / r_a\) = 0.414 - 0.732 : This range stabilizes an octahedral arrangement.
    • \(r_c / r_a\) = 0.732 - 1.00 : This range stabilizes a cubic arrangement.

The correct option for a trigonal planar arrangement is  0.155 - 0.225

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