The structures of simple solids

Structures of Solids

Crystalline solids

     The atoms, molecules or ions pack together in an ordered arrangement
     Such solids typically have flat surfaces, with unique angles between faces and unique
     3-dimensional shape
     Examples of crystalline solids include diamonds, and quartz crystals

Amorphous solids

     No ordered structure to the particles of the solid
     No well defined faces, angles or shapes
     Often are mixtures of molecules which do not stack together well, or large flexible molecules
     Examples would include glass and rubber

The packing of spheres

2.1 Unit cells and the description of crystal structure

Metals and ionic compounds can be treated as 3-d arrays of hard spheres and in many cases satisfactory results for energy calculations can be obtained by completely neglecting the covalent character which is always present but often small.
 In simple cases it is possible to understand and even predict crystal structures based on the notion that the cations and anions pack together as would hard spheres with certain constraints, e.g. cations (often smaller) tend to be surrounded by anions.When atoms, molecules or ions pack in a regular arrangment which can be repeated "infinitely" in three dimensions a crystal is formed. Acrystalline solid, therefore, possesses a long-range order; its atoms, molecules or ions occupy regular positions which repeat in three dimensions. The network of atoms, molecules or ions is known as a crystal latticeor simply as alattice.


Réference: http://www2.latech.edu/~upali/chem481/chem481c3.htm