Structure of pure oxides

In pure oxides

• Metallic cations are surrounded by oxygen ions in a three dimensional crystalline network
• Each cation is surrounded by the maximum number of anions in a closed packed structure, and this number is called coordination number
• Cations of basic oxides such as CaO, MgO, FeO ( etc. have radii smaller than that of cations of
• Structure of an oxide depends on the ratio of radii of cations/anions as shown in the following table 
  
      
  

  Structure CN Cation/anion Examples Cubic 8 1 – 0.732   Octahedral 6 0.732 – 0.414 CaO, MgO, MnO, FeO etc. Tetrahedral 4 0.414 – 0.225 SiO2, P2O5 Triangular 3 0.225 – 0.133

    

CN = Coordination number

As can be seen in the table the basic oxides have octahedral and acidic oxides tetrahedral structure.