aromatic heterocyles
1. They are cyclic compounds with conjugated double bonds.
2. Flat, there can be no sp3 hybridized atoms.
3. Hückel's rule: number of electrons p: 4n +2 with n = 0,1,2......
If it complies with rule 4n it is antiaromatic (especially unstable). In case of not complying
the first two conditions we say that it is non-aromatic.