LiF behaves entirely differently. The difference in electronegativity
between the two atoms is so great that a diatomic LiF molecule is
in a highly unfavorable state. There is a strong tendency for the
electron pair to be given over completely to the F atom, thereby
turning the molecule into two ions:
(The dots represent outer-shell electrons, and the bond line between
atoms represents an electron pair.)
When this occurs, each
ion attracts all the
ions present, and each
attracts every .
The gas of separate Li-F molecules condenses into a liquid of
ions (see next paget) when the temperature decreases below 1676°C.
Only above this temperature does LiF have enough energy to force
the electron pair back toward the
ion and turn the separate ions into gaseous Li-F molecules.