Structure World

NaCl has a cubic unit cell. It is best thought of as a face-centered cubic array of anions with an interpenetrating fcc cation lattice (or vice-versa). The cell looks the same whether you start with anions or cations on the corners. Each ion is 6-coordinate and has a local octahedral geometry.

Fig. 1 A single unit cell of NaCl	Fig. 2 A 3x3x3 lattice of NaCl

Shown below are two crystallographic planes in NaCl. Notice that the (111) plane is hexagonally closest packed.

Fig. 3 The (111) plane of NaCl.	Fig. 4 The (100 plane of NaCl.

Something that surprises novices is that the 111 plane does not contain the body centered atom. It is important for the beginning student to recognize that the 111 plane does not cut through 1/2, 1/2, 1/2! Take a look at the models above and below to convinvce yourself of this very important fact!

Here is a polyhedral representation of the NaCl unit cell. We could have drawn octahedra around either of the two atoms. In the case below, we have drawn them around the corner atoms and changed the color to improve the image contrast:

Shown here is a Chime model you can rotate and manipulate. (I need to update these to jmol; just haven't found the time yet) Notice that you can see alternating hcp layers that contain only Na and only Cl atoms if you manipulate it correctly. See the drawing of the 100 plane shown above if this orientation is not clear.