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Closest Packing and Crystal Structures

Skills to develop

Closest Packing and Crystal Structures

We use models for sciences. Some models are mathematical formulas, some are theories, and some are physical representations.

The arrangements of molecules, atoms or ions are called crystal structures . The packing of spheres is often used to model metal crystal structures. Atoms, molecules, and ions are too small, so we use balls and sticks to represent them. These models enable us to see the invisible world of atoms and molecules.

There are two types of closest packing, the hexagonal and the face-centered closest packing, and they are called hcp and fcc (or ccp) respectively. The hcp packing has an ABABAB... two-layer sequence, where A and B represent the location of the layers. In other word, the third layer is exactly above the first layer in hcp. The fcc packing has a three-layer sequence, ABCABC... rather than the two-layer sequence of the hcp. The two types of packing are shown below:

Since these structures have been discussed in Solids, work on the questions below.

Confidence Building Questions

) cchieh@uwaterloo.ca