Aromatic compounds are more stable than theoretically predicted by alkene hydrogenation data; the "extra" stability is due to the delocalized cloud of electrons, called resonance energy. Criteria for simple aromatics - (1) follow Huckel's rule, having 4n+2 electrons in the delocalized cloud, (2) are able to be planar and are cyclic, (3) every atom in the circle is able to participate in delocalizing the electrons by having a p orbital or an unshared pair of electrons

compounds that follow Huckel's rule, having 4n+2 electrons in the delocalized cloud

n=0, 2 electrons: cyclopropenyl cation
 

n=1, 6 electrons:
 


 
n=2, 10 electrons:

compounds that are "anti-aromatic" (less stable than theoretically predicted by alkene data):

 

compounds that are not fully aromatic because every atom can't participate in delocalizing any electrons (the atom that can't participate is indicated by an arrow)

 

compounds that are aromatic but which don't follow Huckel's rule (larger structures can be delocalized in "subcycles," such that every atom participates in some number of resonance structures)