What are the necessary conditions for any system to be aromatic ? 

The necessary conditions for a molecule to be aromatic are : 

(i) It should have a single cyclic cloud of delocalised p-electrons above and below the plane of the molecule. 

(ii) It should be planar. This is because complete delocalization of p-electrons is possible only if the ring is planar to allow cyclic overlap of p-orbitals. 

(iii) It should contain Huckel number of electrons, i.e., (4n + 2) p-electrons where n = 0, 1, 2, 3...... etc. A molecule which does not satisfy any one or more of the above conditions is said to be non-aromatic