A crystalline solid is a homogeneous solid in which the constituent particles, atoms, ions or molecules are arranged in a definite repeating pattern.
Most of the solid substances are crystalline in nature. The arrangement of particles in a crystalline solid is such that the total intermolecular force of attraction is at maximum. The forces responsible for the stability of the solid crystal may be ionic bonds, covalent bonds, hydrogen bonds or van der Waals forces. Crystalline solids are aggregates of many small, tiny crystals. These tiny crystals are called unit cells. A unit cell is a basic repeating structural unit of a crystalline solid.
Two or more substances having the same crystal structure are said to be isomorphous ( iso-same, morphous- form ). Isomorphous substances contain constituent atoms of the substance in the same atomic ratio. Some of the pairs of isomorphous substances are given below:
i) NaF and MgO ( atoms in the ratio 1:1 )
ii) NaNO3 and CaCO3 ( atoms in the ratio 1:1:3 )
iii) K2SO4 and K2SeO4 ( atoms in the ratio 2:1:4 )
iv) Cr2O3 and Fe2O 3 ( atoms in the ratio 2:3 )
The same atomic ratio, similar molecular formula or similar chemical properties of two or more solids are not enough to claim that the substances are isomorphous. Sodium chloride NaCl and potassium chloride KCl have almost all the properties identical, but the crystal structures are different. Sodium chloride and potassium chloride are not isomorphous. A single substance that crystallises in two or more forms under different conditions is called polymorphous ( many forms ). Carbon has two polymorphic forms graphite and diamond. Sulphur also exists in two polymorphic forms rhombic and monoclinic. Calcium carbonate and silicon dioxide also exists in nature in two polymorphic forms. Polymorphic forms are also called allotropic forms. All pure crystalline solids exhibit sharp melting points and melt completely at one constant temperature. The sharp melting point of the crystalline solids are due to the presence of uniform orderly arrangement if constituent particles in the crystalline solids. The physical properties like refractive index, electrical conductance, dielectric constant, etc. of crystalline solids change with change of directions. The stability of crystalline solids to change values of physicals properties when measured in different directions is called anisotropy. The anisotropy in crystalline solid arises because the composition of solid changes with direction. It is evident that the composition of the medium changes with the change of directions AB, CD, EF etc.