Answer: Solids are classified as crystalline and amorphous on the basis of the presence or absence of orderly arrangement of their constituent particles (atoms, ions or molecules).
i. Crystalline solids: A crystalline solid is a homogeneous solid in which the constituent particles (atoms, ions or molecules) are arranged in a definite repeating pattern.
Crystalline solids are further classified as:
a. Isomorphous form:
1. Two or more substances having the same crystal structure are said to be isomorphous (isosame, morphous-form) and the phenomenon is called isomorphism.
2. The constituent atoms of isomorphous substances are in the same atomic ratio. The molecular formula and chemical properties of the isomorphous substances are similar.
3. Following are some examples of pairs of isomorphous substances and their atomic ratios:
4. Some substances are not isomorphous eventhough they have same atomic ratio, similar molecular formula and similar chemical properties. This is because they have different crystal structures.
eg. Sodium chloride (NaCl) and potassium chloride (KCl)
b. Polymorphous / Allotropic form:
1. A single substance that crystallises in two or more forms under different conditions is called polymorphous (poly-many, morphous-form) and the phenomenon is called polymorphism.
2. The polymorphic forms are also called allotropic forms
eg. i. Diamond, graphite and fullerene are three polymorphic forms of carbon.
ii. Rhombic sulphur and monoclinic sulphur are two allotropes of sulphur.
iii. Silicon dioxide and calcium carbonate also have allotropes.
ii. Amorphous solids / Psuedo solids / Super cooled liquids:
The substances that appear like solids but do not have well developed perfectly ordered crystalline structure are called amorphous (no form) solids.
eg. Tar, glass, plastic, rubber, butter, etc.