Explain radius ratio rule for ionic compounds.

Explanation

(1) In crystals, cations tend to get surrounded by the largest
possible number of anions around it.
(2) If `r^(+) and r^(-)` are the radii of cation and anion respectively , then
`r^(+)//r^(-)` represents the radius ratio.
(3) Greater tlhe radius ratio, greater is the coordination number of cation.
(4) The geometry of the unit cell depends upon the coordination
numbers of cation and anion.
(5) If the cation is extremely small and anion is extremely large,
then radius ratio is very small and packing of anions may be very close
to each other, and system will be unstable due to repilsion and hence
the structure changes into another suitable and stable arrangement.