1. The problem statement, all variables and given/known data Calculate the theoretical lattice energy for MgF2 (Born-Landé equation) Ionic radius Mg+2 (coordination number 6) = 86 pm Ionic radius F- (coordination number 3) = 116 pm Madelung constant = 2.408 n = 7 2. Relevant equations 3. The attempt at a solution I'm having problems with this equation, because its result is very different from the experimental lattice enthalpy of MgF2 (-2962 kJ/mol), and it must be similar. http://img411.imageshack.us/img411/4670/quiqui.jpg [Broken] What's wrong? Maybe the sum of ionic radius? (I added the Mg ionic radius plus 2 times F radius).