#valence e- =
2(4) + 4(6) +
(Remember to add the two electrons for the -2 overall
atoms rarely bond to each other, but carbon atoms do, so we place the carbon
atoms in the center of the structure and attach the oxygen atoms to them. If we
put two oxygen atoms on each carbon atom, we are more likely to get a final
structure that satisfies the requirement for four bonds to each carbon atom.
34 - 5(2) = 24
oxygen atoms commonly have one bond and three lone pairs (Table 12.1), we try
the experiment of placing the remaining electrons as three lone pairs on each
oxygen atom. This leaves the carbon atoms with only six electrons each, so we
know that we will need to convert lone pairs into bonds in Step #5.
#5: Because we
are short four electrons (or two pairs) to provide octets for the carbon atoms,
we convert two lone pairs into bonds.
#6: The carbon
atoms and two of the oxygen atoms (the ones with two bonds and two lone pairs)
have their most common bonding pattern. The oxygen atoms with one bond and three
lone pairs lead us to Step #7..
#7: There are no
#8: With its double bonds and adjacent atoms with
lone pairs, our structure meets one of the requirements for resonance. To
compose the resonance structures, we imagine the electron pairs shifting as
shown by the small arrows below. It is as if a lone pair drops down to form
another bond, pushing a bond off to form a lone pair. Remember that we do not
believe this is really happening. We just find it is useful to think of
resonance structures in this way.