At very high temperatures, around 1700°C, both hydrogen fluoride
and lithium fluoride are two-atom molecular compounds in the gas
phase. The electronegativities of Li and F are more different (1.0
and 4.0) than those of H and F (2.1 and 4.0), so the LiF molecule
is more polar than HF. Measured dipole moments reflect this difference:
6.33 D for LiF versus only 1.82 D for HF.
From these values and the observed bond lengths, one can calculate
that the Li-F bond in the gaseous diatomic molecule is 87% ionic,
whereas the bond in H-F is only 41% ionic.
Li has a far greater tendency to lose its outer electron to F and
to become a +1 ion than H does.