Answer:
Classification of solids
On the basis of arrangement of constituent particles solids are classified in two classes:
1. Crystalline solids and 2. Amorphous solids
1. Crystalline solids
A solid which has definite ordered arrangement of constituent particles (atoms, molecules and ions) is called crystalline solid.
(a) A crystalline solid consists of a large number of small crystals having a definite geometrical shape. These are known as unit cells.
(b) It has long range ordered arrangement of constituent particles i.e. it has same pattern of arrangement throughout the crystal.
(c) Arrangement of particles repeats itself periodically over the entire crystal.
(d) Crystalline solids have sharp melting points.
(e) Crystalline solids are anisotropic in nature.
Examples of crystalline solids are sodium chloride, quartz, etc.
2. Amorphous solids
Amorphous is a Greek word meaning no form. A solid which has irregular arrangement of constituent particles is called amorphous solid.
(a)Amorphous solid has no ordered arrangement or only very short range order, i.e. there is no periodically repeating pattern.
(b) Constituent particles (atoms, molecules or ions) are scattered and disordered.
(c) The structure of amorphous solids is similar to that of liquids. Hence these are also called pseudo solids or super cooled liquids. Examples of amorphous solids are glass, rubber and plastics.
(d) Amorphous solid melts over a range of temperature.
(e) Amorphous solid can be moulded and blown into various shapes.
(f) Amorphous solids are isotropic in nature. The value of same physical property is same along all the directions.
Isotropy of amorphous solids is due to same irregular arrangement of constituent particles along all the directions.
(g) Amorphous solids on heating and then cooling become crystalline.
Example: Some glass objects from ancient civilisation are found to become milky in appearance because of crystallisation.
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