What are Unit Cells?

  Unit Cells and Crystal Systems

Crystal structures may be described in various ways:

The most common, and one which gives all the necessary information,is to refer thes tructure to the unit cell.

The structure is given by the size and shapeof the cell and the positions of the atoms, i.e. atomic coordinates, inside the cell.

However, a knowledge of the unit cell and atomic coordinates alone is often insufficient to give a revealing picture of what the structure looks like in 3D.

Unit Cells

The latter is obtained only by considering a larger part of the structure, comprising perhaps several unit cells, and by considering the arrangement of atoms relative to each other, their coordination numbers, interatomic distances, types of bonding,etc.

It then becomes possible to find alternative ways of visualizing structures and also to compare and contrast different types of structure.

Two of the most useful ways of describing structures are based on close packing and space-filling polyhedra.


 Cubicand Hexagonal Close Packing

Many metallic, ionic, covalent and molecular crystal structures can be described using the concept of close packing. The guiding factor is that structures are usually arranged to have the maximum density.

The principles involved can be understood by considering the most efficient way of packing equal-sized spheres in three dimensions.

 Close Packed Structures

The coordination number of six is the maximum possible for a planar arrangement of contacting, equal-sized spheres.Lower coordination numbers are, of course, possible,  where each sphere has four nearest neighbors, but the layers are no longer closepacked,cp.

The most efficient way to pack spheres in three dimensions is to stack cp layers on top of each other. There are two simple ways to do this, resulting in hexagonal close packed and cubic close packed structure.

The spaces between the atoms in crystalline structures are very less due to high intermolecular forces. This results in crystals having high melting and boiling points. The intermolecular force is also uniform throughout the structure. Crystals have a long-range order, which means the arrangement ofatoms is repeated over a great distance.

Since the inorganic materials are mostly solid state materials this is why we discussed all crystalline systems and You have learned about

1. Seven Crystal systems

 2. Bravais Lattices system 

3. Close packed structures

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