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=====Periodic Systems in Different Dimensions (5 minutes)=====
  * Periodic systems can take form in all three spatial dimensions:

    * // 3-D periodic system example: // The naturally occuring photonic crystal Opal.

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Below we can also see a cartoon interpretation of the unit cells in Opal.  A 2-D periodic system can be acquired by taking only a single cross-section of unit cells in the pyramid.

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\textbf{Image Sources: The Wikipedia article about opal crystals.}

    * // 1-D periodic system example: // a string of atoms in single-molecule conduction.

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  * In real physical cases, the systems always physically exist 3-Dimensionally; however, the dimensionality of the periodicity is all that concerns us now.
  * Visit [[http://departments.kings.edu/chemlab/animation/nbo.html|here]] for some unit cell animations. Cesium Chloride is a good example.


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