Solids may be loosely categorized into two groups. Amorphous solid has no long-range order in their molecular or atomic structure. By their nature Amorphous solids are not easily studied, since the powerful analytical methods which are described in Topic A6, for example, are not applicable to such disordered structures. In contrast crystalline solid which consist of ordered three-dimensional array of a structural motif and such as an molecule, atom or ion. This internal order is reflected in the familiar macroscopic structure of crystalline materials, which typically have highly regular forms with flat crystal faces. It is this order and regularity which enables much simpler structural studies of crystalline materials.
The Seven Crystal System