The
measured dipole moment of the HF molecule is what would be found if
41% of a full electron charge were shifted toward the F atom and away
from the H by a distance corresponding to the actual H-F distance
of 0.92 A (see right). A purely covalent bond has equal sharing and
no dipole moment; and in a purely ionic bond the electron is entirely
shifted from one atom to the other. Hence we can say that the HF bond
in this example has 41% ionic character and 59% covalent, or electron-sharing,
character.
The same calculation tells a different story for gaseous LiF molecules,
which are obtained from LiF salt at high temperatures. The distance
between Li and F atoms is 1.52 A, and the measured dipole moment is
u = 6.33 D. The magnitude of the charge can be calculated:
6.33 = 4.8 x 1.52 q
q = 0.87 of the charge on an electron
The bond in LiF is 87 % ionic and only 13 % covalent.