The most ordered state of matter in which the constituent particles (atoms, ions, or molecules) are packed as closely as possible due to which solids are rigid and have definite shape and volume.
Classification of Solids:
The various types of solids are classified into following two types depending upon the arrangement of constituent particles-
A solid is said to be crystalline if its constituent particles (i.e. ions, atoms, or molecules) are arranged in a definite geometric pattern in three-dimensional space so that there is a short-range as well as long-range order of the constituent particles.
The constituent particles in a crystal have no transitional motion.
The crystalline substances possess sharp melting points.
In crystalline substances, properties like electrical conductivity, refractive index, thermal expansion, etc. have different values in different directions. This type of behavior is called Anisotropy.
The crystalline solids on being cut with a knife give clean cleavage. Example- NaCl, KCl, CuSO4.
A solid is said to be amorphous if there is no regular arrangement of its constituent particles or at the most, there is only a short-range order of its constituent particles. Example- Rubber, Glass, Pitch, Fused Silica, etc.
The amorphous substances melt gradually over the temperature range.
In amorphous substances, properties like electrical conductivity, refractive index, thermal expansion, etc., are identical in all directions just as in the case of gases, liquids. This property is called Isotropy.
On cutting with a knife, they give irregular surface.