Looking at resonance structure 1a, it doesn't seem that tropone is aromatic. However, the resonance structure 1b with $\ce{C+-O-}$ satisfies Hückel's rule for aromaticity. Is it correct to say that tropone is then aromatic?
Answer
Tropone, or 2,4,6-cycloheptatrien-1-one, is an aromatic, non-benzenoid hydrocarbon. If you look at the resonance structures in the drawing below you'll see that structure B
depicts a molecule with a continuous, planar loop of 7 p-orbitals that contain 6 pi-electrons. Any planar (or near-planar), cyclic system with a continuous loop of p-orbitals is considered aromatic, or stabilized, if it contains 4n+2 pi-electrons and anti-aromatic if it contains 4n pi-electrons. On this basis, molecules like the cyclopropenyl cation (4n+2, n=0), benzene (n=1), cycloheptatriene (tropylium) cation (n=1), cycooctatetraene dianion (n=2) would be considered aromatic systems. Correspondingly, the cyclopropenyl anion (4n, n=1), cyclobutadiene (n=1), cyclopentadienyl cation (n=1), cyclooctatetraene (if it were a planar molecule, n=2) are considered destabilized or anti-aromatic molecules.
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