Chemistry Reference
In-Depth Information
symmetry or crystals of the
Cu-type
. The elementary cube has one
sphere in every face center - therefore this structure is also called
face-centered cubic
.
3. The name “face-centered cubic” is possible to point out the difference to
the
body-centered
cubic packing of spheres. It is not a close-packing
anymore, the coordination number is 8 (see picture). Metal crystals of
tungsten and alkaline metals have this structure, the
W-type
.
M 6.7: The nine-spheres packing shows the elementary cube of body-centered
cubic metal structures. Draw the crystal structure next to the picture:
draw a perspective cube, instead of the balls only give the central points
of the balls and connect these points. Compare it with M6.4.
Structures of salt crystals: The structure of many metal crystals can be
illustrated by packings of spheres of
one
kind - the structure of salt crystals
needs
two
kinds of spheres. Models for the sodium chloride structure and
three other salt crystals will be built in the following (drawing models of
the crystal structures are shown at the end of this section).
M 6.8: Na
+
ions in sodium chloride are to be represented with red balls (
d
¼
12 mm), Cl
ions with white balls (
d
30 mm). With the help of the
triangle frame build a close-packing of spheres with both kinds of balls.
Draw the layers of balls.
M 6.9: Determine the coordination number for both kinds of balls. Draw the layers
of balls to visualize the coordination number of both kinds of balls.
¼
M 6.10:
In the close-packing of spheres there are two different-sized kinds of holes
or gaps. Determine the number of balls that form those two different gaps.
Draw the gap-producing balls for both kinds of holes in the form of the
layers of balls: (a) for big gaps, (b) for small gaps.
Information: Two different types of holes can be found in the close-
packing of spheres. Convince yourself with the help of the model M 6.8:
1. Big holes are formed by six balls with octahedral geometry: octahedral
gaps (OG)
2. Small holes are formed by four balls with tetrahedral geometry: tetra-
hedral gaps (TG)
