Solids are divided into two classes on the basis of haphazard and regular arrangement of the building constituents.
|Amorphous solids||Crystalline solids|
|No pattern of arrangement of atoms, ions or molecules and, thus, do not have any definite geometrical shape.||They have fixed and regular geometry because of definite and orderly arrangement of atoms, ions or molecules in three dimensional space.|
|They do not have sharp melting points and do not change abruptly into liquids.||Sharp melting points and on melting change abruptly into liquids.|
|Amorphous solids are isotropic. Their physical properties are the same in all directions.||Crystalline solids are anisotropic. Some of their physical properties are different in different directions.|
|These are considered as pseudo-solids or supercooled liquids.||These are considered as true solids.|
|They are not very rigid. They are distorted by bending or compressing forces.||They are rigid and their shape is not distorted by mild distorting forces|
|They do not have well defined planes. When an amorphous solid is broken, the surfaces of the broken pieces are generally not flat and they intersect at random angles.||Crystals are bound by plane faces. For a given crystalline solid, it is a definite angle and remains always constant no matter how the faces develop. On hammering a crystalline solid, it breaks up into smaller crystals of the same geometrical shape.|
|Amorphous solids do not have any symmetry.||An important property of crystals is their symmetry. There are: (i) plane of symmetry, (ii) axis of symmetry and (iii) centre of symmetry.|
The substances which show the same properties in all directions are said to be isotropic and the substances exhibiting directional differences in properties are termed anisotropic. . Amorphous solids like liquids and gases are said to be isotropic as arrangement of building constituents is random and disordered. Hence, all directions are, equal and therefore, properties are the same in all the directions. Crystalline solids are anisotropic. Magnitude of some of the physical properties of crystalline solids such as refractive index, coefficient of thermal expansion, electrical and thermalconductivities, etc., is different in different directions, within the crystal. For example, in the crystal of silver iodide (AgI), the coefficient of thermal expansion is positive in one direction and negative in the other direction.
All crystals are polyhedra consisting of regularly repeating arrays of atoms, molecules or ions which are the structural units. A crystal is primarily a homogeneous part of a solid substance. It consists of regular patterns of structural units which are bonded by plane surfaces that make definite angles with one another. The geometrical form consisting only of a regular array of points in space is called a lattice or space lattice. A space lattice can be subdivided into a number of small cells known as unit cells. It can be defined as the smallest repeating unit in space lattice which when repeated over and over again results in a crystal of the given substance or it is the smallest block or geometrical figure from which the entire crystal can be built up by its translational repetition in three-dimensions.
On the basis of geometrical considerations, theoretically there can are 32 different combinations of elements of symmetry of a crystal. These are called 34 systems. Some of the system have been grouped together. In all, seven types of basic or primitive unit cells have been recognised among crystals. These are cubic, orthorhombic, tetragonal, monoclinic, triclinic, hexagonal, and rhombohedral.