In the phenolate ion the oxygen actually becomes part of the delocalized system and enlarges it from six atoms to seven. You can see this by comparing the various resonance structures for phenol and the phenolate ion at the left. Only benzenelike resonance structures are possible for phenol, since the -OH group really is not included in the delocalized system. The phenolate ion has resonance structures in which the oxygen has three lone pairs and a negative charge, and is attached to the ring by a single bond. But other resonance structures can be drawn in which one of the three lone pairs is shifted into a C=0 double bond, and the negative charge shows up at different positions on the ring. These resonance structures tell us that in the ion the oxygen is part of the general seven-atom delocalized system. With greater delocalization, the spacings between p-orbital energy levels are decreased, and the electronic absorption band shifts from 2700 Å to 2870 Å. We cannot see this change because our eyes are insensitive to the ultraviolet, but if all the wavelengths were doubled, benzene would be red, phenol would be purple, and the phenolate ion would be blue-violet. Phenol then would bean acid- base indicator, revealing the acidity of its environment by its color. If added in small amounts to a solution, it would appear purple in acid, where its own dissociation was repressed by a plentiful H+ supply, and blue-violet in base, where most of it had dissociated to the phenolate ion.