Radius Ratio

Radius Ratio

Ionic compounds occur in crystalline forms. Ionic compounds are made of cations and anions. These ions are arranged in three dimensional array to form an aggregate of the type (A+B)n . Since, the Coulombic forces are non-directional, hence the structures of such crystals are mainly governed by the ratio of the radius of cation  to that of anion  The ratio  to   is called as radius ratio.

 

 
 

 

 

 

 

 

 

 

 

 

 

Table: Limiting Radius ratios and Structure

Limiting radius ratio (r+)/(r)

C.N.

Shape

< 0.155

2

Linear

0.155 – 0.225

3

Planar triangle

0.225 – 0.414

4

Tetrahedral

0.414 – 0.732

6

Octahedral

0.732 – 0.999 or 1

8

Body-centered cubic

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