Solids: Thermodynamics and Bonding

· There is a net "Cost" of creating an Ion Pair at Infinite Separation:
(E_F - E_I) = - 1.4 eV, and at first this seems an unfavorable situation.
· The diagram opposite shows these initial reactions, and suggests the next step in the process.
· Once the ions are formed they can attract each other and form a molecule of sodium chloride. This process gives rise to a cohesive energy for the molecule of 7.9 eV, and more than compensates for the energy spent in creating the ions.

From: Kittel, "Introduction to Solid State Physics,"
Wiley (1996)