6. The internal energy of a crystal is mainly defined by the lattice energy Consider the lattice energy as the total potential energy of one mole of NaCl ionic (a) Estimate the Madelung constant by the initial three groups of neighboring ions. crystal. (b) Apply the estimated Madelung constant for calculating the Coulomb energy. (c) Estimate the Born exponent from the attached table. (e) Apply the derived Born constant for calculating the Repulsion energy. (d) Derive the Born constant (f) Calculate the total Cohesive energy. [15 points]