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 |