Atoms or ions in any solid do not sit rigidly in place. Their thermal
energy causes them to vibrate about fixed positions in the crystal
structure. The higher the temperature, the more energy the atoms
in a solid have, and the more they vibrate.
The melting point of LiF salt crystals, 842°C, is the temperature
at which the ions have enough vibrational energy to shake loose
from the crystal structure and circulate past one another in a liquid.
The boiling point of molten LiF, 1676°C, is the temperature
at which the particles in the melt can break loose and enter the
gas phase. To do so they must pair off into Li-F molecules again.
An isolated ion of one charge is tolerable when it is surrounded
by ions of the opposite charge in a crystal, but isolated gaseous
would require much greater energies. This charge separation is avoided
by the formation of LiF diatomic molecules.
To produce such a LiF
molecule, even though the bond is quite polar, the
must give some of the borrowed electron pair back to .
The high boiling point of the salt demonstrates that this is a hard
thing to do. The
ion does not really want the electron, and accepts it only at high
temperatures (high energies).
In contrast to liquid LiF (a molten, ionic salt), liquid HF already
exists as molecules, so all that is involved in its vaporization
from liquid to gas is overcoming van der Waals forces and hydrogen
bonds. This can be done at temperatures as low as 19.5°C.