Ionic Crystals
These summary notes are based largely on Faure (1998), Chapters 6-8.
The force holding ions together in ionic bonds is proportional to the ionic charge of the
two ions (e1 & e2) and inversely proportional to the distance between them (r)
F=e1e2
r2
Ions pack into crystals according to:
A) charge balance!(to keep the crystal electrically neutral)
B) closest packing!(the closer the cation and anion are, the stronger the bond)
To maximize bond strength, crystals form with geometries that pack the ions as close as
possible depending on the relative size of the cations and anions (the radius ratio).
radius ratio = cation radius
anion radius =r
c
r
a
Radius Ratio
Coordination Number
Bonding Geometry
< 0.155
2
linear
0.155 - 0.414
3
trigonal planar
0.414 - 0.732
4
tetrahedral
“
6
octahedral
0.732 - 1
8
body centered cubic
≥ 1
12
edge centered cubic
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