Characteristics Of Ionic Compounds

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Characteristics of Ionic Compounds

(i)    Melting and boiling points.

Melting and boiling points are mostly dependent on the nature of binding forces operating in the crystalline solid. Melting involves breakdown of the crystal lattice .Since electrostatic interactions between+ ve and –ve ions in the crystal are generally strong, therefore, their break-down requires higher energies and hence the ionic compounds will have higher melting and boiling points.

(ii)    Hardness.

The electrostatic interactions between oppositely charged ions determine hardness of an ionic compound. The electrostatic interaction between  oppositely charged ions depends on,
            (i) ionic radii and                                   (ii) charges on the ions.

(iii)    Brittleness.

The constituent units which make up ionic solids are oppositely charged ions. It may be observed that if requisite energy is supplied to the crystal layers, the movement of layers of
                                                                    (a)                                                    (b)

ions brings ions of like charges near other and this causes strong repulsions    which lead to the breakdown of the crystal. For these reason ionic crystals, inspire of their hardness, are brittle and can be powdered easily.

(iv)    Cleavage.

The phenomenon of cleavage is observed in ionic crystals. That is , in crystal the splitting of layers is easier in some directions than others. For example, in a crystal of sodium chloride cleavage can be achieved only along planes parallel to cube faces. Any attempt to cleave such a crystal along an arbitrarily plane will shatter the crystal as shown in.


(v)    Solubility.

 Ionic compounds are soluble in highly polar solvents . Water is a good solvent for dissolving ionic compounds because of two reasons:

(a) Iit has a large dipole moment which provides high hydration energy due to strong ion-solvent dipole interactions. If hydration energy (solvation energy) is greater than the crystal energy, the lattic breaks.
      (b) It has high dielectric constant ( 78.5 at 250C ) which is responsible for reducing attractive forces between cations and anions in solution. This keeps the unlike charges apart in solution.

Thus , suitable solvents for dissolving ionic compounds should have high dipole moment and preferably high dielectric constant.

(vi) Conductance.

 In an ionic crystal, the ions are held in fixed positioning he lattice ,that is, they are immobile. Therefore ionic solids do not conduct electricity in the solid state. However, in molten state or in solution the crystal lattice breaks and ions are relatively free to move in an electric field. Thus hey conduct electricity due to increase in relative mobility of ions in fused state or in solution.

(vii) Stereochemistry.

The constituent ions of an ionic solid possess an orderly arrangement in a definite geometric pattern which reports itself throughout the crystal. Crystals are always bond by plane faces and the faces always meet at some fixed angles. For any particular substance, the angle between corresponding pair of aces is always the same in all crystal.

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