8. Continuing a search for resonance structures for the alizarin ion from Problem 7, can you draw a structure in which carbonyl groups are present at Positions 9 and 2, and -0 at 1 and 10? What about carbonyl groups at 9 and 1, and -0 at 2 and 10? How many other resonance structures can you find that differ in assignment of the negative charges to different pairs of oxygen?

9. How many different resonance structures can you draw for the alizarin ion in which the negative charges are located on carbon atoms rather than on oxygen atoms?

10. As you try out various resonance models for the alizarin ion, you may come across the empirical observation that only those structures are possible for which the carbon atoms bearing negative charges (either directly or through an attached -0) are separated by an even number of other carbon atoms: 0, 2, 4, - - -. Can you explain this in terms of single and double bonds between carbons in the three rings?