This structure shows the halite, or rock-salt structure. The white spheres represent the larger chloride ions and the smaller red spheres represent the sodium ions. Im this strcture the chloride ions form a face-centered cubic array, with the sodium ions residing inside the octahedral holes. While the halite structure is almost always drawn to illustrate its cubic nature, recall from the previous discussion that the face-centered cubic array can be alternately viewed as a series of ABC-stacked hexagonal layers.

View a VRML model of this structure

In this illustration, eight of the octahedral holes are outlined in green. For the sake of clarity, not all of the octahedral holes are outlined. Each of the sodium ions, represented by the red spheres, resides within an octahedral hole. Consider the stoichiometry of this cell for a moment. Each of the white spheres forming the face-centered cubic array resides 1/8 inside a single cell and each of the six atoms on the six faces of the cell lie 1/2 inside the cell. Therefore a total of four sodium ions lie inside a single cell. Now consider the chloride ions. One lies at the center and is entirely contained within the cel.l. The remaining twelve each lie 1/4 within the cell. Therefore a total of four chloride ions lie inside the cell. With four sodium ions and four chlroide ions inside the cell, the stoichiometry is indeed 1:1.

The face-centered array formed by the chloride ions results in a series of octahedral and tetrahedral holes. The octahedral holes, as mentioned above, are filled by the sodium ions. The smaller tetrahedral holes, shown here, remain unfilled. You may ask, why do the sodion ions reside exclusively inside the octahedral holes? This is most likely a size issue; the octahedral holes are larger than the tetrahedral holes.