Simple Ionic Model for the Alkali Halides

2-18 SIMPLE IONIC MODEL FOE. THE ALKALI HALIDES The total potential energy for an ionic alkali halide molecule is 

We need only know the values of the constants b, a, and d in order to calculate potential energies from Eq. (2-47). The exact values of these constants for alkali metal ions and halide ions are not known. However, the alkali metal ions and the halide ions have inert-gas electronic configurations. For example, if LiF is formulated as an ionic molecule, Li+ is isoelectronic with the inert gas He, and F is isoelectronic with the inert gas Ne. Thus the van der Waals interaction in Li+F may be considered approximately equal to the van 

The inert-gas-pair interactions can be measured and values for the b, a, and d constants are available. These values are given in Table 2-9. Using Eq. (2-47), we are now able to calculate the bond energy ofLiF. 

To calculate the bond energy of LiF, we first calculate the energy needed for the process 

We shall calculate this energy in atomic units (au). The atomic unit of distance is the Bohr radius, ao, or 0.529 A. The atomic unit of charge is the electronic charge. The b, a, and d constants in 

---