The energy necessary to break the ionic bond between a sodium ion and a fluorine ion is $\mathbf{4}\mathbf{.99}\mathbf{\text{eV}}$. The energy necessary to separate the sodium and fluorine ions that form the ionic $\mathbf{\text{NaF}}$crystal is $\mathbf{9.30}\mathbf{\text{eV}}$ per ion pair. Explain the difference qualitatively.

Sodium is metal and fluorine is non-metal. So the bond between them can be an ionic bond. Sodium has one valence electron and fluorine is highly electronegative. Sodium needs to lose electrons and fluorine needs to lose electrons to form a chemical bond.

Here not only break all sodium-fluorine connections but also separate all sodium ions from all fluorine ions in order to separate the sodium and fluorine ions that make up the $\text{NaF}$ crystal. Distinguish ions macroscopically since they were just $10$ Bohr radii apart. To separate the oppositely charged ions that attract each other requires more energy. As a result, separating the sodium and fluorine ions requires more energy than breaking the connection per pair.

Hence, it takes more energy to separate the sodium and fluorine ions than it does to break the link per pair.