# 6.11B: Structure - Caesium Chloride ($$CsCl$$)

This page is going to discuss the structure of the molecule caesium chloride, CsCl. CsCl is a white solid with a mass of 168.36g/mol. It is a hydroscopic molecule because it attracts water molecules from the surrounding environment.

### Introduction

Caesium Chloride is a type of unit cell that is commonly mistakened as Body-Centered Cubic. This misconception is easy to make, since there is a center atom in the unit cell, but CsCl is really a Non-closed packed structure type.

CsCl has a boiling point of 1303 degrees Celsius, a melting point of 646 degrees Celsius, and is very soluble in water. For the most part this molecule is stable, but is not compatible with strong oxidizing agents and strong acids. Although it is not hazardous, one should not prolong their exposure to CsCl.

CsCl is an ionic compound that can be prepared by the reaction:

Cs2CO3 + 2HCl -> 2 CsCl + H2O + CO2

CsCl crystallize in a primitive cubic lattice which means the cubic unit cell has nodes only at its corners. The structure of CsCl can be seen as two interpenetrating cubes, one of Cs+ and one of Cl-. The ions are not touching one another. Touching would cause repulsion between the anion and cation. Some may mistake the structure type of CsCl with NaCl, but really the two are different. CsCl is more stable than NaCl, for it produces a more stable crystal and more energy is released.

Figure 1.  CsCl Coordination Cubes

Anions and cations have similar sizes. Each Cs+ is surrounded by 8 Cl at the corners of its cube and each Cl- is also surrounded by 8 Cs+ at the corners of its cube. The cations are located at the center of the anions cube and the anions are located at the center of the cations cube. There is one atom in CsCl. To determine this, the following equation is given:

8 Corners of a given atom x 1/8 of the given atom's unit cell = 1 atom

Figure 2.      Unit Cell of CsCl  (http://en.wikipedia.org/wiki/Caesium_chloride

### Applications

Caesium chloride is used in centrifugation, a process that uses the centrifugal force to separate mixtures based on their molecular density. It is also used in the preparation of electrically conducting glasses. Radioactive CsCl is used in some types of radiation therapy for cancer patients, although it is blamed for some deaths.

References

1. Carter, C. Barry., and M. Grant. Norton. "Binary Compounds." Ceramic Materials Science and Engineering. New York, NY: Springer, 2007. pg87-88
2. Quéré, Yves. "Stable Structure of Halides." Physics of Materials. Amsterdam: Gordon and Breach Science, 1998.

### Problems

What kind of bond does CsCl have?

If you add 349.6g CsCl to a beaker that contains 5 ml of water, what is the molarity of the solution?

Why does CsCl crystallize in a primitive cubic lattice and not into another form of cubic lattice?