4. Electron Sharing and      Covalent Bonds  
     How Many Bonds Per Atom ?

In the most common bonding behavior, an atom makes as many covalent bonds as are needed to fill its outer shell with eight electrons. For atoms of C, N, O, and F, this means 4, 3, 2, and 1 electrons, respectively. If these elements are bonded to hydrogen, we find the expected CH4 methane molecule, NH3 for ammonia, H20 for water, and HF for the hydrogen fluoride molecule. Another way of looking at the bonding behavior is to say that each of the unpaired electrons in C, N, O, and F is available for pairing in a covalent bond. The pair of electrons in the N atom, the two electron pairs in O, and the three pairs in the F atom do not need to find outside electrons to interact with because they already are paired. They are called lone pairs to distinguish them from the electron pairs of a chemical bond, which are called bonding pairs. These lone pairs and bonding pairs for the simple molecules mentioned previously are illustrated in the table on the right hand side of this page and on the right hand side of page 6.

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