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The crystal structure of sodium chloride

The crystal structure of sodium chloride - ions • The...

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The crystal structure of sodium chloride The unit cell of sodium chloride is cubic, and this is reflected in the shape of NaCl crystals The unit cell can be drawn with either the Na + ions at the corners, or with the Cl - ions at the corners. If the unit cell is drawn with the Na + ions at the corners, then Na + ions are are also present in the center of each face of the unit cell If the unit cell is drawn with the Cl - ions at the corners, then Cl - ions are are also present in the center of each face of the unit cell
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Within the unit cell there must be an equal number of Na + and Cl - ions. For example, for the unit cell with the Cl - ions at the center of the faces The top layer has (1/8+1/8+1/8+1/8+1/2)=1 Cl - ion, and (1/4+1/4+1/4+1/4)=1 Na + ion The middle layer has (1/2+1/2+1/2+1/2)=2 Cl - ions and
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Unformatted text preview: + ions • The bottom layer will contain the same as the top or 1 each Cl-and Na + ions • The unit cell has a total of 4 Cl-and 4 Na + ions in it. This equals the empirical formula NaCl . Close packing of spheres Many ions are spherical and many small molecules pack in a crystal lattice as essentially spherical entities. Spheres can pack in three-dimensions in two general arrangements: • Hexagonal close packing • Cubic close packing The coordination number is the number of particles surrounding a particle in the crystal structure. • In each packing arrangement above (hexagonal close pack, cubic close pack), a particle in the crystal has a coordination number of 12 • The NaCl (face centered cubic) has a coordination number of 6...
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