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Bizarro By Dan Piraro
Practice PS #6 is posted online. You should be able
to do Q 1–5, 7. 1 2
Last time Packing efficiency
Unit cell We assume that the particles themselves are spheres
and not cubes.
Particles Coordination #
Simple cubit 1 6
For particles of the same size, the higher the
coordination of the crystal the greater the number of
particles in a given volume.
Corner contributiosn 8 particle per corner Both hexagonal closest packing, hcp, (based on
1 hexagonal unit cell) and cubic closest packing, ccp,
Face contributionis 2 particle per face (based on fcc unit cell) provide 74% packing
Edge contributionis particle per edge
See figure 12.28 A video clip for simple cubic, body centered cubic,
face centered cubic, packing and the NaCl and ZnS
crystal structures was shown in class.
The clip was uploaded to eClass.
I recommend that you view it or even download it.
For NaCl and ZnS, check for yourself that the
empirical formula based on the number of particles in
the unit cell matches the known formula. 5 6
At room temperature, iron crystallizes in the bcc Denote edge length as
structure. The edge of the cubic cell is found to be
287pm. What is the radius of an iron atom?
bcc = the corner atoms are in contact with the center
atom but not with other corners (see fig. 12.27).
The corner–to–center distance = 2 r (2 atoms in
contact, each having a radius r)
Pythagoras equation for right angle triangle
2 2 2 We’re interested in the left side triangle as it has 2r
c = a + b
and radius is what we’re after in this example.
But what is the value of k?