AN IONIC COMPOUND
Ionic compounds generally have more complicated structures than metals. This is probably because:
(1) There are now at least two kinds of particles in the lattice, generally of different sizes.
(2) The cations attract the anions, but like ions repel one another. The structure must balance both types of forces.
(3) Many ions (e.g. nitrate, carbonate, azide) are very non-spherical in shape. They will thus pack differently in different directions.
One simple ionic structure is:
Cesium chloride crystallizes in a cubic lattice. The unit cell may be depicted as shown.
(Cs+ is teal, Cl- is gold).
Click on the unit cell above to explore the structure in VRML.
When these unit cells are stacked into a lattice, they form a structure such as below.
Click on the image below to explore the cesium chloride lattice in horizontal and vertical rotation.
Notice that you cannot see through the structure; there are no channels.
However, the tightly-packed structures make it difficult to view the interior relationships.
The same structure, but with the ions moved further apart allows the interior to be viewed.
Click on the image below to explore the expanded structure in VRML.
One way to describe the crystal is to consider the cations and anions separately. We begin with the larger (gold colored) Cl- ions. Ignoring the Cs+, we note that the Cl- themselves form a simple cubic anion sublattice.
Considering only the Cs+, they form a simple cubic cation sublattice.
This animation shows the CsCl lattice, only the teal Cs+
(the Cs sublattice), and only the gold Cl- (the Cl sublattice).
We can describe the CsCl structure as two interpenetrating simple
cubic cation and anion sublattices.
(The two sublattices are not necessarily the same, although in this example they are.)
Recall that the simple cubic lattice has large interstitial sites
between each 8 atoms. We can therefore think of making the CsCl by
taking a simple cubic Cs lattice and placing Cl into the interstitial sites.
This is obvious if we compare the CsCl unit cell with the simple cubic unit cell showing the interstitial site.
Each Cl- is also surrounded by 8 Cs+ at the
corners of a cube, so the Cl- has CN = 8.
here to go to the next page.
Structure of Crystals
From Unit Cell to Lattice
From Lattice to Unit Cell
Packing & Geometry
Simple Cubic Metals
Close Packed Structures
Body Centered Cubic
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VRMLs are owned and copyright
Barbara L. Sauls and Frederick C. Sauls 2000.
Contact the owners for individual permission to use. blsauls@.kings.edu