4. Electron Sharing and      Covalent Bonds  
     Nitrogen and Ammonia

Nitrogen has five electrons in its outer shell: one lone electron pair and three unpaired electrons that are available for bonding. Its Lewis diagram is shown on the right. In the simplest nitrogen compound, ammonia (NH3), these three bonding electrons each are paired with one electron from a hydrogen atom. As in methane, the central atom in the ammonia molecule is surrounded by four electron pairs in approximately tetrahedral orientation. There is one critical difference, however, which will lead us to an improvement in the VSEPR theory. The three bonding pairs each are shared between two atoms and hence are attracted by H as well as N. In contrast, the nitrogen lone pair is held only by the N atom. The lone pair is closer to N than the bonding pairs are, and therefore repels the bonding pairs more strongly than would a fourth bonding pair. This extra repulsion by the lone pair pushes the three N-H bonds closer together. In a perfect tetrahedron the
H-N-H angles all would be 109.5 degrees, as in methane. In the ammonia molecule the three H-N-H angles are only 107 degrees. The ammonia molecule has the shape of a pyramid with the lone pair at the apex; and the pyramid is slightly steeper than it would have been if the lone pair had not been closer to the N atom.

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