i. The coordination number of constituent particle of the crystal lattice is the number of particles surrounding a single particle in the crystal lattice. 

ii. In case of ionic crystals, coordination number of an ion in the crystal is the number of oppositely charged ions surrounding that ion. 

iii. The stoichiometry and the size of the cation and anion determines the structure of ionic compounds. 

iv. In ionic solids, the bigger ions (usually anions) form close packed structure and smaller ions (usually cations) occupy the voids. If the latter ion is small enough then tetrahedral voids are occupied; but if it is bigger, then octahedral voids are occupied. Generally, for an ionic crystalline structure to be stable, there is definite radiusratio (r⁺ /r^- ) limit for a cation to fit perfectly in the lattice of anions. This also defines the coordination number of the cation. 

v. The limiting values of r⁺/r^- for the coordination number and the hole occupied by the cation are given in the following table: