Example #2
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## EXAMPLE #2 Drawing Resonance Structures:

Draw a reasonable Lewis structure for the oxalate ion, C2O42-. The structure is best described in terms of resonance, so draw all of its reasonable resonance structures and the resonance hybrid that summarizes these structures. Ionic compounds containing the oxalate ion have many uses, including the bleaching and cleaning of textiles.

Solution:

Step #1:           C2O42-     #valence e-  =  2(4)  +  4(6)  +  2  =  34

(Remember to add the two electrons for the -2 overall charge.)

Step #2: Oxygen 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.

Step #3:  #e- remaining  =  34 - 5(2)  =  24

Step #4:  Because 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.

Step #5: Because we are short four electrons (or two pairs) to provide octets for the carbon atoms, we convert two lone pairs into bonds.

Step #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..

Step #7:  There are no reasonable alternatives.

Step #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.

We follow the guidelines to draw the resonance hybrid that summarizes these structures and provides the best description of the bonds in the oxalate ion:

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