# 8.6: Solids

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##### Representation of 3-dimensional structures should usually be made using models, and their representation on a flat surface is difficult. For simplicity, we use a 2-dimensional pattern (plane) to illustrate a 2-dimensional (planar) crystal structure.
 # @ # @ # @ # @ # @ # @ # @ # @
@ # @ # @ # @ # @ # @ # @ # @ #
# @ # @ # @ # @ # @ # @ # @ # @
@ # @ # @ # @ # @ # @ # @ # @ #
# @ # @ # @ # @ # @ # @ # @ # @
@ # @ # @ # @ # @ # @ # @ # @ #
# @ # @ # @ # @ # @ # @ # @ # @
@ # @ # @ # @ # @ # @ # @ # @ #


This pattern or crystal structure is generated by using a unit marked by the centers of any four @ or # signs. The choice is up to you in this case. Such a unit is called a primitive unit. The pattern has a square (or rectangular on some screens) appearance, and to preserve the square, we may use a square unit of

 # @ #             @ # @
@ # @      or     # @ #
# @ #             @ # @


as our unit cell. Such choices result in having two @ and # per unit cell, and these are called centered cells.

Thus, if we know the arrangement of a unit cell, we can use our imagination to build a crystal structure, or use symbols or models to represent a crystal structure. Since each pattern has features shared by many structures, often such a pattern is called a lattice. For example, the diamond, zinc blende, wurtzite, and $$\ce{NaCl}$$ structures have been called lattices; however, the word lattice has a more formal definition by crystal physics and chemists. The above site gives a gallery of lattices.

##### Definition: Crystals

Inorganic Chemistry by Swaddle defined crystals as packed regular arrays of atoms, ions, or molecules in a pattern repeated periodically ad infinitum.