Question

Among the following which one has the highest cation to anion size ratio?
a.) CsI
b.) CsF
c.) LiF
d.) NaF

Answer 1

To find the cation to anion charge ratio, cation that is the positive charge species should be the smallest and anion that is the negative charge species should be the largest.

Complete step by step answer:
Let’s identify the cation and anion present in the given compounds:
Cesium iodide
Cation present in cesium iodide = $C{{s}^{+}}$
Anion present in cesium iodide= ${{I}^{-}}$

Cesium fluoride
Cation present in cesium fluoride= $C{{s}^{+}}$
Anion present in cesium fluoride= ${{F}^{-}}$

Lithium fluoride
Cation present in lithium fluoride=$L{{i}^{+}}$
Anion present in lithium fluoride=${{F}^{-}}$

Sodium fluoride
Cation present in sodium fluoride=$N{{a}^{+}}$
Anion present in sodium fluoride= ${{F}^{-}}$
Now we know that $C{{s}^{+}}$ has the maximum size among the given cations and ${{F}^{-}}$ has the minimum size among the given anions.
So the cation to anion ratio will be maximum in CsF.
So, the correct answer is “Option B”.

Additional Information:
The charge to size ratio determines the polarizing power of the cation. According to the Fajans rule, the power of the cation to cause distortion in the electron cloud of the negatively charged species that is the anion is referred to its polarizing power and the ability of the anion to distort is known as polarisability. Down the group the size of the atom increases similarly the size of the cation increases down the group.

Note: Similarly if the question asks for the highest anion to cation charge ratio then cation that is the positive charge species should be the largest and anion that is the negative charge species should be the smallest.