9.1 Types of molecular bonds  (Page 2/14)

The positive sodium ion and negative chloride ion experience an attractive Coulomb force. The potential energy associated with this force is given by

where $k{e}^2=1.440\text{eV-nm}$ and $r_0$ is the distance between the ions.

As the sodium and chloride ions move together (“descend the potential energy hill”), the force of attraction between the ions becomes stronger. However, if the ions become too close, core-electron wave functions in the two ions begin to overlap. Due to the exclusion principle, this action promotes the core electrons—and therefore the entire molecule—into a higher energy state. The equilibrium separation distance    (or bond length ) between the ions occurs when the molecule is in its lowest energy state. For diatomic NaCl, this distance is 0.236 nm. [link] shows the total energy of NaCl as a function of the distance of separation between ions.

The total energy required to form a single salt unit is

where $U_{\text{ex}}$ is the energy associated with the repulsion between core electrons due to Pauli’s exclusion principle. The value of $U_{\text{form}}$ must be negative for the bond to form spontaneously. The dissociation energy    is defined as the energy required to separate the unit into its constituent ions, written

Every diatomic formula unit has its own characteristic dissociation energy and equilibrium separation length. Sample values are given in [link] .