39. In what ways can different resonance structures differ from one another, and in what ways must they be alike? How is the number of different resonance structures that can be drawn for a given molecule correlated approximately with the energy of the molecule?

40. Sketch the s-bond skeleton of the butadiene molecule. How many electrons and AO's are used in this framework? What kind of hybridization is used around the carbon atoms?

41. How many bonding electrons, and how many and what kind of AO's, remain unused in butadiene after the s-bond skeleton is built? How are these electrons and AO's used for further bonding in butadiene? Over which atoms in the molecule does delocalization extend?

42. Draw several resonance structures for butadiene. Show how each structure assigns the four electrons not involved in s- bonding among the four carbon atoms.

43. How do the energy levels in aromatic molecules change with an increase in the extent of delocalization, and how is this reflected in the energy wavelengths of absorption or radiation?

44. How is the color that we see in a chemical compound related to the energy that it absorbs? If a molecule absorbs wavelengths in the blue region, why does it appear orange to our eyes instead of blue?

45. Why does absorption shift to longer wavelengths when the phenol molecule dissociates to the phenolate ion? What prevents phenol from being a useful acid-base indicator?