Energetics of the Ionic Bond

When ions are formed as the result of electron transfer between atoms, the charged species produced interact according to Coulomb s law with a force, F, given by 

Sodium chloride is formed from the elements in their standard states with a heat of formation of -411 kJ mol-1  

In the Born-Haber cycle for the formation of NaCl, S is the sublimation enthalpy of Na, 

D is the dissociation enthalpy of Cl2,1 is the ionization potential of Na, E is the electron addition enthalpy of Cl (which is the negative of the electron affinity), and U is the lattice energy. The Born-Haber cycle shows that the lattice energy corresponds to the energy required to separate a mole of crystal into the gaseous ions, and forming the crystal from the ions represents -U. 

In some cases, the energy to be determined is the lattice energy, U, because the enthalpy change for the formation of NaCl(s) is the same for both pathways. Therefore, 

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