4. Electron Sharing and      Covalent Bonds  
     Nitrogen and Ammonia

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.

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