The ionisation energy of potassium is 4.34 eV and the electron affinity of chlorine is 3.61 eV. The Madelung constant for the KCl structure is 1.748 and the closest distance etween ions of opposite sign is 0.314 nm. On the basis of these data,what is the cohesive energy of KCl. Compare this with the observed cohesive energy of 6.42 eV and what may be the reasons for any discrepancy.