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There are a huge number of possible crystal types, but a few are by far the most common. Three examples are shown below:

A simple cubic (SC) crystal has a unit cell with atoms on each corner of the cube and nowhere else. While simple, this leaves a lot of empty space in the crystal and thus the simple cubic structure is not seen as often as more closely packed structures

A body centered cubic (BCC) crystal has a unit cell with atoms on each corner of the cube and one in the center of the cube. While there is less empty space in a BCC crystal than in a simple cubic crystal, the FCC crystal below has still less empty space.

A face centered cubic (FCC) crystal has a unit cell with atoms on each corner of the cube and six other faces located on each face of the cube. The FCC structure is "closest packed": there is no way to pack spheres more densely. The FCC structure is one of the most common crystal types.


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