Stability of Ionic crystals on the basis of radius ratio Chemistry Notes
→ Cations and anions are closely packed in an ionic crystal. Each ion in a crystal, whether it is anion or cation, is surrounded by oppositely charged ions. Since electrostatic forces of attraction is present between anion and cation, for stability of the crystal, it is necessary that cation should be in contact with more anions and anion should be in contact with more cations.
→ it depends on ratio of cations and anions respectively. Radius ratio increases with increase in radius of cation due to which coordination number of cation also increases. Hence, cation comes in contact of more arijons. For example, let the coordination number of a cation is 3 then there can be following three types of possibilities:
→ In figure (A), cation is so small that it does not contact with an anion but anions are in contact of each other. So, the attraction between cation and anions is less but repulsion is high between anions. Due to this reason, figure (A) will have unatable arrangement.
→ In figure (B), cation and anion are in contact of each other and three anions are also in contact of each other. Due to this reason in this arrangement, attraction and repulsion forces balance each other. So, this arrangement (B) is stable. In figure (C),cation is in contact of anions but anions are not in contact with each other.
→ Here, attraction force is more then repulsion force but vacancy is present between anions. So, the arrangement of figure (C) is not suitable In this position, trigonal planar arrangement will be converted in to tetrahedral structure according to following figure 1.43.
→ In this type of arrangement, a cation contacts to four anions and four anions are touching with each other. So, this arrangement is stable. It can be proved from above example that the geometry of crystal also changes by changing radius ratio.