Ionic compounds are relatively high melting, high boiling, hard, brittle and crystalline. They do not conduct electricity as solids but show high electrical conductivity n molten state or in solution. The properties indicate the presence of essentially immobile ions in ionic solids and relatively mobile ions in their melts or solutions. On te basis of these properties and other spectroscopic investigations, it is reasonable to conclude that he constituent units which make up ionic solids are oppositely charged ions. The electrostatic force of attraction holding the oppositely charged ions is known as the ionic bond.

Since the ionic bond is the consequence of electrostatic attraction between oppositely charged ions, therefore, it is expected to result form the transfer of one or more electrons form highly electropositive atoms to highly electronegative atoms. In other words, the transfer of electrons occurs form an atom with low ionization potential to an atom with high electron affinity. Thus , an ionic bond is likely to be formed most readily between an atom with the lowest ionization potential and an atom with the electron affinity. However, even the combination of the most electropositive cesium atom with the lowest ionization energy of 357. 3KJ mol (energy absorbed’ endothermic) and the electronegative coloring atom with the highest electron affinity of 347 .36KJ mol (energy relapsed , exothermic) is energetically unfavorable (endothermic). This indicates that even for the most ionic bond in C_s⁺CI^-(g) , there is insufficient energy available for its formation. However,this he shortfall is more than compensated by the energy gained through electrostatic attraction in the formation of an ionic crystal. This energy is known as the crystal energy or the lattice energy and sufficient to permit the formation of ions.

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