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
< 0.1552Linear
0.155 – 0.2253Planar triangle
0.225 – 0.4144Tetrahedral
0.414 – 0.7326Octahedral
0.732 – 0.999 or 18Body-centered cubic

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