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8.6: Solids

  • Page ID
    226696
  • 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.

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