4. Electron Sharing and      Covalent Bonds  
     Nitrogen and Ammonia

Similar calculations can be carried out for H20 and NH3 , but they are complicated by the fact that two or three individually polar bonds are pointing in different directions. What is measured for the whole molecule is their vector sum. In calculating the dipole moment of an entire molecule, one can add the individual dipole moments of the polar bonds as if they were small vectors, of length proportional to the dipole moments, and pointing from the negative ends of the bonds to the positive ends, as shown at the right. When the geometry of the molecule is taken into account correctly, the conclusion is that the H-F bond in HF is 41% ionic, the O-H bond in water is 33% ionic, and the N-H bond in ammonia is 27 % ionic. This agrees with what one would expect from the decrease in electronegativities found in going from F to O to N. We can obtain no information from dipole moments about the polarity of the C-H bond in a methane molecule, because the four tetrahedrally arranged C-H bonds would add up to a net molecular dipole moment of zero, even if the individual bonds had a large dipole moment.

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