Bond lengths follow the expected trend that bonds between larger atoms are longer and bonds between smaller atoms are smaller. What is surprising is that bond strength and bond length are inversely related--a short bond is generally stronger than a long one. Another unexpected piece of bond length data shows that there are three common bond lengths for C-C bonds. Our bonding theory must also predict the above trends in bond length.

Bond dipole data provides at least a partial answer for several of the above observations. Because F is a more electronegative atom than C, the electrons in a C-F bond will be polarized toward F. The fluorine atom then acquires a large partial negative charge (δ-) and the carbon atom a large partial negative charge (δ+) as shown in :

Figure %: The dipole of a C-F bond

The crossed arrow underneath the C-F bond indicates that there is a partial positive charge located on C and a partial negative charge located on F. The charge separation leads to a coulombic attraction between the two atoms in the bond and makes the C-F bond stronger than either the C-C or F-F bonds, which have no bond dipoles. This stronger bond will have the nuclei closer than the sum of their atomic radii due to the coulombic attraction between the oppositely charged ends of the molecule.

In the next section we will take a closer look at ionic bonding.