If order to understand the factors which influence the formation of ionic compound let us examine the energy changes taking place during the formation of ionic bond.
The formation of ionic bond from the gaseous atoms can be considered to proceed in three steps:
For example, let us consider the formation of ionic compound AX(s) from atoms A and X in gaseous state.
(a) Formation of gaseous cation.
This step involves loss of electron by the gaseous atom and is an endothermic process. The energy required for this process is expressed in terms of ionsation energy(E ) which is defined as the energy required to remove the most loosely bound electron from the gaseous atom to make ‘it a gaseous cation.
Now lesser the value of ionisation energy, greater is the tendency of atom to form cation. For example, alkali metals form cations more easily as compared to alkaline earth metals because of low values of their ionisation energies.
(b) Formation of gaseous anion.
This step involves gain of electron by the gaseous atom and is an exothermic process. The energy released in this process is expressed in terms of electron affinity E ea which is defined as the energy released when a gaseous atom takes up an electron to form monovalent gaseous anion.
-ve sign represents energy released.
It may be noted that this process becomes endothermic in case of formation of divalent or trivalent anions because second and successive electron affinities are positive because of requirement of energy. For example, formation of O2- (g) ion from O(g) atom is endothermic
Now larger the value of electron affinity greater is the tendency of atom to form anion. For example, halogens have largest tendency to form anion due to the large values of their
(c) Packing of ions of opposite charges to .form ionic solid. This step involves close packing of gaseous ions of opposite charges to form a crystal lattice. It is an exothermic process and the energy released during this process is expressed in terms of lattice enthalpy (.1.L H) which defined as the energy released when gaseous ions of opposite charge pack up together to form 1 mole of ionic crystal.
A+ (g) + X- (g) AX (s); ∆LH6 = – z kJ
The close packing of the gaseous ions of opposite charge has been shown in Fig. 7.4.
Fig. 7.4. Close packing of gaseous ions of opposite charge to form solid ionic crystal.
Lattice enthalpy plays a key role in the formation of compound. The magnitude of lattice enthalpy gives an about the inter-ionic forces and it also provides a measure the stability of ionic compound. It depends upon following factors:
(i) Size of the ions. Smaller the size of ions, lesser is inter-nuclear distance and greater will be the attraction, hence, more negative will be the value of enthalpy. For example, ionic size of K+ (133 pm) is than that of Na+ (95 pm). Consequently, lattice enthalpy NaCl (- 769.