It is defined as the amount of energy released when a NaCl molecule is formed from isolated Na and Cl atoms. The cohesive energy of NaCl molecule is calculated as follows. The ionization energy or the amount of energy needed to remove the outermost valence electron from Na atom is 5.1 eV. That is:
Na + 5.1 eV → Na+ + e− _____ (1.11)
The electron affinity of Cl atom is 3.6 eV; i.e., the amount of energy released when an electron is added to Cl atom is 3.6 eV.
so Cl + e− → Cl− + 3.6 eV _____ (1.12)
The net amount of energy required to form Na+ and Cl‒ ions at infinite separation is [5.1 eV − 3.6 eV] = 1.5 eV.
Thus Na + Cl + 1.5 eV → Na+ + Cl− _____ (1.13)
As the ions possess ...