Molten LiF is a rather unusual liquid compared to benzene or water.
The interactions between particles are not the weak van der Waals
forces of benzene, or the moderately strong dipole attractions and
hydrogen bonds of water, but are the extremely strong electrostatic
attractions between ions of opposite charge and the equally strong
repulsions between ions of the same charge. Ions can move past one
another in the fluid, but there is a tendency for each kind of ion
to surround itself with ions of the opposite charge.
If we cool molten LiF to 842°C, the ions move so slowly that
they lock into a regular array, with positive and negative charges
alternating so that ions of the same charge are shielded from one
another. This is the structure of LiF salt crystals, introduced
in Chapter 4 and shown in the margin of the preceding page.
ions remain even in the solid, in marked contrast to the behavior
of HF. The key to this new behavior is the difference in electronegativities
of the atoms.