The repulsive core-core energy between neighboring ions separated by r in a binary ionic crystal of 2N ions of alternating positive and negative charges of e is A/r^n ,where n=9 and A=2.23 x 10^-105 Joule.m^9. At the equilibrium nearest-neighbor distance of 0.2624 nm, the repulsive energy is 1.60 x 10^-19 J per unit cell. The crystal is identified as Given k=9 x 10^9 N. m^2.C^-2;] Madelung constants for crystals: Linear crystal= 2ln2, Fluorite=2.519, CsCl=1.7625, ZnS=1.64, NaCl = 1.7475, [MgF2=2.381, CdCl2= 2.244 * (7 ) CsCI O NaCl O Fluorite O MgF2 O Linear crystal O CdCl2 O ZnS O