IONIC COMPOUNDS We can predict which elements are likely to form ionic compounds based on IE and EA, but how do we calculate the stability of these compounds? • IE and EA occur in the gas phase and these compounds are at 1atm and 25°C and solids. • The electron transfer absorbs energy thus, the overall stability of an ionic compound depends on the interactions of all these ions, and a quantitative measure of this stability is the lattice energy . Lattice energy can not be measured directly but we could do it two ways: • Using Coulomb’s law • Born-Haber cycle. Born-Haber cycle enables us to analyze the energy changes of ionic compound formation, with the application of Hess’s law. 1. Convert the metal to vapor (sublimation) 2. Dissociate the nonmetal into separate gaseous atoms. (diatomic gas using Bond energies) 3. Ionize the metal (IE) 4. Add e- to the nonmetal (EA) 5. Combine the gaseous metal ion with the gaseous nonmetal ion. The reverse of step 5 is the lattice energy.
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