4. (20 marks) The net potential energy En between two adjacent ions is sometimes represented by: C EN = D + r rn in which r is the interionic separation and C, D and n are constants whose values depend on the specific material. (a) Derive an expression for the bonding energy E, in terms of the equilibrium interionic separation ro and the constants D and n. (b) Derive another expression for E, in terms of ro, C and n. = 6 = and n= 100000 (c) Suppose that three parameters C 1.5, D = plot En, Er, and EA versus r up to 1. 5, superimpose on a single