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 – 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|