Cycloheptatrienone is stable, but cyclopentadienone is so reactive that it can't be isolated. Taking into account the polarity of the carbonyl group, draw a resonance structure of cyclopentadienone that illustrates a reason for its instability.
The resonance contributor in cyclopentadienone (as shown in the attachment below) results into the compound having a positive charge on the carbonyl group, C=O which accounts for a highly reactive anti-aromatic 4π system. And this illustrates the reason for its instability.
