[Bublik]

Can there be aromatic rings that do not have 5 or 6 carbons? For instance, can there be an aromatic ring that has 7,8,9 or 10 members?

I'm asking this because I just learned about Huckel's rule. 4n+2 pi electrons makes it seem like there can exist an aromatic ring that has for instance, 10 pi electrons.
That would visually look like 1,3,5,7,9-Cyclodecapentaene.
If this were a planar molecule, it would be considered aromatic, because it follows all four requirements. However, that would make it a decagon with internal angles of 144, which deviate too much from the ideal sp2 hybridization angle of 120. Thus, the molecule is not stable, does not prefer to be planar and is not therefore aromatic.

So then why is there this rule of 4n+2 pi electrons? Isn't the only possible case when there is 6 pi electrons? (Note: I'm aware that 5-membered heterocyclic rings can have atoms with lone pairs that contribute to aromaticity. I count those lone pairs like 2 pi electrons.)

As I was writing this question, I thought of naphthalene. It has 10 pi electrons, but is the contour of the molecule itself considered an aromatic ring?
I think I might have answered my own question now, but, is that the only possible case where you can have an aromatic ring with more than 6 pi electrons? Would a molecule like that always appear like two separate fused aromatic rings, and its contour would follow Huckel's rule?

I hope this was clear. All help would be greatly appreciated. 

[Corribus]

Porphyrins satisfy Huckel's rule, with 18 pi electrons.

Huckel's rule is an approximation and does not take into account structural factors which may reduce stability.  Some porphyrins with bulky substituents have significant deviations from planarity.  This does diminish their resonance stabilization but does not (necessarily) negate their aromaticity.

Also, Huckel's rule really only applies to monocyclic systems.  There are a number of polycyclic aromatic system that fail Huckel's test but still have aromatic resonance stabilization.

[Bublik]

Hmm... thanks!

[Altered State]

In this link you have a bunch of aromatic compounds examples, with explanations about why are they aromatic:

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/react3.htm

Hope it helps