Depending on temperature, RbCl can exist in either the rock-salt or caesium chloride structure. a) What is the coordination number of the cation and anion in each of these structures? b) In which of these structures will Rb have the larger apparent radius?

The answers are given below.

Explanation:

a)In the rock salt or NaCl, six sodium ions surrounds each chloride ions so the coordination number for NaCl is 6. It means there are 6 anions around each cations.  

In CsCl, each Cl ion is surrounded by 8 cesium ions, which means there are 8 anions surrounding 8 cations. The coordination number is 8 for CsCl.

b) The ideal radius of both the structures are

We can answer this by considering the ideal radius  ratio's of both structures, that is r+  /r–  = 0.414 for rock salt and 0.732 for CsCl. Since the anion is the same, so the cation radius will "seem" larger in the CsCl structure type.