4. Electron Sharing and      Covalent Bonds  
     Nitrogen and Ammonia

The unit of measure for dipole moments is the debye (abbreviated D) . A proton and an electron held 1 A apart would constitute a dipole moment of 4.8 debye. (The 4.8 factor has no deep significance, but merely comes from the size of the charge on an electron. ) Methane has no dipole moment, and ammonia has a dipole moment of 1.47 D. Water is slightly more polar, with a dipole moment of 1.85 D. If lithium fluoride salt is vaporized at temperatures above 1676C, the gaseous LiF molecules have the quite large dipole moment of 6.33 D. In the preceding chapter we said that most real bonds were intermediates between the extremes of completely covalent and totally ionic. Measured dipole moments allow us to calculate the percent of ionic and covalent character of a bond. Since two charges of +1 and -1, located 1.0 A apart, yield a dipole moment of
u = 4.8 D, we can write

u = 4.8 qr

with q in units of electron charge and r in angstroms. The atoms in an HF molecule are 0.92 A apart, and the measured dipole moment is 1.82 D. Hence,

1.82 = 4.8 x 0.92 q
q = 0.41 of the charge on an electron

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