Question

what is radius ratio rule and limitations of this rule

Answer 1

Radius ratio: The radius ratio is defined by the ratio of radius of cation and anion of a crystalline solid.the radius ratio of a crystal helps to determine the coordination number cation and anion.It is designated by (r+/r-).The size of a cation of a compound can be determined by the critical radius ratio according to Pauling's rule of crystal structure.

radius ratio(r+/r-)              Co-ordination no.

0.155-0.225                            3

0.225-0.414                            4

0.414-0.732                            6

0.732-1.00                              8

     1                                        12

                                   

Limitation: This rule is applicable when all the cations and anions are perfectly sphere but in maxium cases the atoms or ions are not perfectly sphere.Hence deviation arises.