Question

Which has the lowest anion to cation ratio?
A.$LiF$
B.$NaF$
C.$CsI$
D.$CsF$

Answer 1

The anion to cation ratio is defined as the ratio of the ionic radius of the anion to that of the cation in a cation-anion compound or an ionic compound. It is dependent on the position of elements in the periodic table.

Complete step by step answer:
Lithium is present in the first group and in the second period of the periodic table. Whereas, fluorine is present in the group 17 and in the same period of the periodic table. As one moves along the period from left to right, the size of the ion reduces and there is not much difference between the sizes of lithium and fluorine.
Just after fluorine comes sodium and both have the same number of electrons in the valence shell in their ionic forms, but the size of sodium is bit large since it belongs to group 1. Coming to $CsI$, both caesium and iodine are from the same period so they must have some similarities in their sizes.
Lastly, for $CsF$, caesium is at the bottom of the group and the first member of period 6 while fluorine is the last member of period 2 and first member of group 17, hence they would have a lot of difference in their sizes.

So, the lowest anion to cation ratio is that for $CsF$, option D.

Note:
The radius of the atom of an element and its subsequent cation or anion is dependent primarily upon the position of the element in the periodic table and the charge on the cation or anion.