First principles calculations based on DFT have been performed on crystals of halides (X = F, Cl, Br and I) of alkali metals (M = Li, Na, K, Rb and Cs). The calculated lattice energies (U0) are in good agreement with the experimental lattice enthalpies. A new exact formalism is proposed to determine the Born exponent (n) for ionic solids. The values of the Born exponent calculated through this ab-initio technique is in good agreement with previous empirically derived results. Band Structure calculations reveal that these compounds are wide-gap insulators that explains their optical transparency. Projected density of states (PDOS) calculations reveal that alkali halides with small cations and large anions, have small band gaps due to charge transfer from X → M. This explains the onset of covalency in ionic solids, which is popularly known as the Fajans Rule