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
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 view a movie of the unit
When these unit cells are stacked into a lattice, they
form a structure such as below.
One simple ionic structure is:
Click on the images below to view the cesium lattice
Notice that you cannot see through the structure; there are
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 images below to view the open structure rotating.
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
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.
We may also consider the neighborhood in which each ion finds itself.
Each Cs+ is surrounded by 8 Cl- (so the Cs+
coordination number is 8) at the corners of each cube.
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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Barbara L. Sauls and Frederick C. Sauls
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