Question

Cesium chloride on heating to \[760{\text{K}}\] changes into:
A.\[{\text{CsCl}}\left( {\text{g}} \right)\]
B.\[{\text{NaCl}}\] structure
C.Antifluorite structure
D.\[{\text{ZnS}}\] structure

Answer 1

Hint:Coordination number of metal in cesium chloride is 8 and in sodium chloride type of ionic crystal, coordination number of metal is 6. On heating, crystal with cesium chloride type of crystal lattice changes to sodium chloride type of lattice.

Complete answer:
The chemical formula of cesium chloride is \[{\text{CsCl}}\] , it is an inorganic ionic compound. It is present as impurities in ore such as carnallite, sylvite and kainite. Its molar mass is \[168.36{\text{gmo}}{{\text{l}}^{ - 1}}\] with a melting and boiling point of \[{646^o}{\text{C and }}{1297^o}{\text{C}}\] respectively.
Cesium chloride structure is body centered cubic structure with cesium ions present at body centre or at cubic void and on the other hand, chloride ions are present at every corner in the unit cell. The coordination number is 8 which means each atom is surrounded by 8 chloride ions. This structure is similar to that of cesium bromide, cesium iodide and many metallic alloy structures.
The reason for changing cesium chloride structure to sodium chloride structure on heating the cesium chloride structure at \[760{\text{K}}\] is due to size difference between the ions. This can be understood as, when the size difference between the ions is small or we can say both the ions are similar in size then cesium chloride structure is adopted by the crystal. If the size difference between the ions is large or both the ions are not similar in size then sodium chloride structure is adopted by the lattice. As we can see, in case of cesium chloride, the ionic radius of cesium is 174pm and ionic radius of chloride is 181pm which is of similar size but in sodium chloride, ionic radius of sodium ion is 102pm and ionic radius of chloride is 181pm which is not similar to each other.

Thus, the correct option is B.

Note:
Cesium chloride is being widely used in activities such as techniques involving separation of various DNA, in analytical chemistry, medical diagnostic and treatment, etc.