Themes > Science > Chemistry > Inorganic Chemistry > Chemical Bonding > Ionic Compounds In this section we will discuss Lattice Energy, the change in energy when ions are packed together to form ionic solids. One way to determine Lattice energy is with Coulomb's law, which states that:                         Lattice energy= k( Q1Q2/r) where Q is the numerical charge, r is the shortest distance between nuclei and k is Coulomb's constant. When dealing with Lattice energy, you are usually asked to find the overall energy in a reaction which takes a lot more work. In this form, you calculate the energy for every step of the reaction and find the overall energy change, as in this example: Li (s)+ 1/2 F2(g) ----> LiF(g) 1. Sublimination of Li (s) to Li(g) = 161kJ 2. Form Li+ ions, Li (g) to Li+(g) + e- = 520 kJ 3. Breaking F-F bond, 1/2 F2 (g) to F (g) =77 kJ 4. Form F- ions, F(g) = e- to F-(g) = -328 kJ 5. Formation of LiF, Li+(g) + F-(g) to LiF (s) = -1047 kJ Net overall: -617 kJ/mole, a highly favored reaction Information provided by : http://library.thinkquest.org