4. Electron Sharing and      Covalent Bonds  
     Nitrogen and Ammonia

Nitrogen cannot make long chains in the way that carbon can, for two reasons. The lone electron pairs on adjacent nitrogen atoms in a -N-N-N-N- chain repel one another more strongly than bonding pairs in the C-H bonds of hydrocarbons do. This weakens the N-N bond and makes it only half as strong as a C-C bond. The second factor is the great stability of the pieces of the ruptured chain, N2 molecules. Nitrogen has three unpaired electrons for bonding, and it can share all three with a neighboring atom to produce the NN molecule shown at the right. Pure nitrogen therefore is a diatomic gas, N2 , similar to H2. Because the triple bond is quite strong, N2 is a stable and relatively unreactive molecule. If "quadruple bonds" were possible, with four electron pairs shared between two atoms, then perhaps carbon also would be a diatomic gas, C2 . But this is not possible, so carbon bonds to more than one neighbor and forms the three-dimensional structures of graphite and diamond. The sudden change in properties between solid diamond and gaseous nitrogen is one of the most dramatic among the chemical elements.

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