Based upon the type of constituent particles occupying the lattice sites and hence the type of binding forces, the various crystalline solids is classified into the following different types:

In these crystals the constituent particles occupying the lattice points are the positive and negative ions (e.g. Na and CI-ions in case of NaCI crystal). These ions are held together by the strong electrostatic forces of attraction. Some of the important characteristics of these crystals are as follows:
(i)    Because of strong electrostatic forces of attraction existing among the ions, they have high melting and boiling points.
(ii)    They are good conductors of electricity in the molten state or in the solution (but not in the crystalline state where the ions are not free to move).
(iii)    They are soluble in polar solvents but insoluble in non-polar solvents.
(iv)    Because of strong electrostatic forces of attraction he ions are closely packed and hence the ionic crystals are hard. However, they are brittle because the stability of the crystals depends upon preservation of there geometric pattern.

The constituent particles occupying the lattice points in this case are the lattice points in this case are the molecules. The intermolecular forces holding the molecules together in the crystal lattice are the weak Vander Waal’s forces. In case of polar molecules. (e.g. HCI, H₂O, NH₃ etc) these Vander Waal’s forces are the dipole-dipole attractions as shown in Fig . In case of non-polar molecules (e.g. H₂, CI₂, CH₄, etc), the Vander Waals forces are the London dispersion forces.
 
                Dipole-dipole attractions

These forces are believed to arise due to momentary dipole produced as a result of distortion of electron cloud of one molecule which produces an induced dipole in the other molecule as shown in Fig.


                London dispersion forces.

(i)    As the Vander Waal’s forces are much weaker than the electrostatic forces existing anionic crystals, the molecular crystals are soft and have low melting and boiling points.
(ii)    They do not conduct electricity in the solid or liquid state or in solution as there are no ions present.
(iii)    They are less soluble in water and more soluble in non-polar solvents. Polar molecular liquids act as solvents for ionic compounds.
(iv)    As dipole-dipole attractions are stronger than London forces, therefore, polar molecular crystals generally have higher melting points and boiling points than crystals of non-polar molecule of comparable molecular size an shape.

In these crystals ,the lattice points are occupied by atoms which are linked together by a network of covalent bounds to form a giant molecule. One of the most common examples of the crystals of this type is that of diamond in which the carbon alums are linked together by covalent bonds to give a three dimensional structure as shown in Fig.

 

             Diamond – A covalent crystal.

Substances of this type have high melting points, high baling points, and low volatility and are extremely hard because of the large number f covalent bonds that have to be broken to destroy the crystal structure. Further, they are non-conductors of electricity.

These crystals consist of positively charged metal ions (kernels) occupying the lattice points which are held together by the metallic bond. The metallic bond arises due to the presence of mobile electros (as the ionization energy of metal sis low). These mobile electrums undergo simultaneous attraction by a number of positive ions and hence the ions are held together.
 
        
          Metallic crystal.

Unlike ionic crystals, the positions of the positive ions be altered without destroying the charge distribution provided by the freely moving electrons. Thus metallic crystals can be easily deformed. That is why the metals are malleable and ductile. The other properties of metals like luster, thermal and electrical conductivity can be explained on the basis of the free moving electrons. Further as the positive ions are closely packed in the crystal lattice, most of the metals posses high milling points and high densities.

It may be noted that a metallic bond differ form a covalent bond in the following two aspects:
(i)    Covalent bond is directional whereas a metallic bond is non-directional.
(ii)    Metallic bond is weaker than covalent bond.

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