Solid materials are classified on the basis of arrangement of atoms within the solid. There are two
main categories: Materials in which atoms are placed randomly(lack regular arrangement) in a long
range are called amorphous. Examples are pitch, plastic. Materials in which atoms are placed in a
high ordered structure(regular and regular pattern) in a long range are called crystalline. Examples
are diamond, quartz. Polycrystalline materials are materials with a high degree of short-range order
and no long-range order. These materials consist of small crystalline regions with random
orientation called grains, separated by grain boundaries. Crystals are categorized by their crystal
structure and the underlying lattice. While some crystals have a single atom placed at each lattice
point, most crystals have a combination of atoms associated with each lattice point.
