= Rock salt structure
The <rock salt structure> of ordinary <sodium chloride> has a <coordination number> of six for each kind of <ion>: six nearest neighbours of the opposite sign surround each <ion> along three perpendicular directions. Its conventional cubic <unit cell> contains four <formula units>; sharing corner, face, and edge sites between adjacent cells must be included in that count. Sixfold local coordination is compatible with $1:1$ overall <stoichiometry>.<Chemistry 2e>[§7.1, Figure 7.3; §10.6, “The Structures of Ionic Crystals”, NaCl unit-cell accounting]{ref}
The checkerboard plane in the diagram is a slice through this three-dimensional arrangement. It leaves out the two nearest neighbours above and below the plane. Repeating a complete even-by-even patch gives equal numbers of signs; the displayed odd-by-odd crop has a boundary imbalance and is not a complete neutral <ionic crystal> or a <unit cell>. Other <salts> need not have the <rock salt structure> or the same <coordination number>.
Back to article page
