Ans. The necessary conditions for a molecule to be aromatic are- It should have a single cyclic ring of delocalised n-electrons above and below the plane of the molecule,- It should be planar as complete delocalisation of n-electrons is possible only if the ring is planar to allow a cyclic overlap of p-orbitals,- It should contain Huckel number of electrons, i.e., (4n + 2) n-electrons where n = 0, 1, 2, 3 etc. - A molecule that does not meet any one or more of the above conditions is said to be non-aromatic.