The compounds possessing aromatic character show the following characteristics:

(i) The compounds must be cyclic in nature and have flat planar structure.
(ii) Their molecular formulae suggest these compounds are highly unsaturated due to the presence of one or more double bonds in the ring but they must behave as saturated compounds.
(iii) They must resist addition reaction and take part in the electrophilic substitution reactions.
(iv) The molecules have delocalised  electron cloud above and below the plane of the ring.
(v) An essential criterion for the aromatic character is that the compound must obey Huckel’s rule. According to this rule, a cyclic compound will behave as aromatic compound if it contains (4n + 2) electrons, where n may be 0, 1, 2, 3 etc. Huckel’s rule can be applied successfully to polycyclic compounds, annulenes and also other non-benzenoid compounds. For example;
(a) Monocyclic systems: Some monocyclic systems having π-electrons (obey Huckel’s rule) possess aromatic character.




(b) Fused ring systems: The polynuclear hydrocarbons such as naphthalene, anthracene and phenanthrene are also aromatic according to Huckel’s rule.


Aromatic ions: Some cyclic ions also exhibit aromatic character. For example


The following compounds are not aromatic:



Cycloheptatriene although obeys Huckel’s rule yet it is not aromatic as it is not planar and can not show resonance.

Annulenes are monocyclic conjugated polyenes which contain an even number of carbon atoms in their molecules. These are represented by the general formula (-CH = CH-)_n where n = 2, 3, 4.....

Aromatic nature in annulenes: According to Huckel’s rule, annulenes containing (4n + 2) electrons and having a coplanar cyclic carbon skeleton should be aromatic in nature. For example,

(i) [4] Annulene. It is non-aromatic because it does not contain  electrons.
 

(ii) [6] Annulene. It is aromatic because it contains  electrons.


(iii) [8] Annulene. It is non-aromatic because it does not contain  electrons.

A resonance may be defined as a phenomenon in which a single compound is supposed to be existing as a hybrid of two or more compounds differing in the distribution of electrons and not of atoms. These different structures of the molecule are known contributing structures or resonating structures or canonical forms. The actual structure that is intermediate between all the contributing structures is called resonance hybrid. Different contributing structures are written by putting a double head arrow ( ↔ ) between them.
Resonance in benzene: Benzene ring has three double bonds in it and is expected to be quite reactive. But benzene is extremely stable. The stability of benzene is explained in terms of resonance. Benzene molecule is a resonance hybrid of the following two main contributing structures:

Due to resonance in benzene, the carbon-carbon bonds in benzene acquire an intermediate character of carbon-carbon single and double bonds. As a result, each carbon-carbon bond length in benzene is 139 pm which lies between standard C – C bond length 154 pm and C=C bond length 134 pm.
Effect of resonance: Due to resonance, the -electron charge in benzene is distributed over a greater area. The density of the charge decreases. As a result, the energy of resonance hybrid also decreases or its stability increases.