Cesium chloride is being studied as an alternative treatment for cancer. Its medicinal effect is thought to be related to the large size of its cation. Estimate the density of cesium chloride from its crystal structure.

ANTICIPATE The densities of ionic compounds are typically a few grams per cubic centimeter, so expect a density of a similar magnitude.

PLAN The density of the bulk solid is the same as that of the unit cell. The volume of the cesium-chloride unit cell is the cube of the length of one of its sides, and that length can be worked out from the Pythagorean theorem. The mass of the unit cell is obtained by counting the net number of ions of each kind and using the mass of each ion (its molar mass divided by Avogadro’s constant; the molar mass of an ion is negligibly different from that of its parent element). The density is obtained by dividing the mass by the volume. Assume that cations and anions touch either along diagonals (as in cesium chloride) or along edges, as in rock salt.