[a confused chiral girl]

for the compound structure, pyrene, the calculation after huckel's rule is n=3.5, which is not an integer. but it is still a aromatic compound, how can we tell whether it is or not when we are just given a structure when there are weird exceptions like this? 

[Yggdrasil]

See wikipedia: http://en.wikipedia.org/wiki/H%C3%BCckel%27s_rule

[lemonoman]

Also http://members.aol.com/logan20/outlin14.html  -- under headings "Huckel Rule" and "Other Aromatic Compounds" will be helpful.

The answer to your exact question lies under one of those two headings...but the rest of the info is great too...and if you're studying Huckel's Rule will come in handy

[a confused chiral girl]

thank you for the replies! 

in the wikipedia, it says that the huckel's rule is usually not followed when more than 3 rings are linked together.
in the second link, it says that the rule is meant for mono-cyclic ring systems.
So generally in most cases, is it when 2 rings are joined together, huckel's rule may be dismissed or has to be at least greater than 3 rings?

[Ψ×Ψ]

Have you tried it for naphthalene? 

[FeLiXe]

the mathematical derivation only works with monocyclic systems. For everything else you need the whole hückel theory and not just this rule