[deutdeut]

Do the resonance structures of a molecule are isomers, as they have the same molecular formula but different structures?

[Dolphinsiu]

I don't think so.

Isomers mean they have same molecular formula but different structural formula.

Resonance is a process to stablilize the molecule,resulting in two or more than two resonance hybrids.

You should distinguish them carefully!

[Yggdrasil]

Resonance structures do not represent different structures since the real structure is a hybrid of all of the resonance structures.  It's an important point to remember that resonance structures do not exist in real life.  They come about because of the inability of our conventional systems of drawing chemical structures to represent conjugation and delocalization of electrons.

[deutdeut]

But how I can know whether they two molecules are isomers or not?

[Dolphinsiu]

Give an example C4H8

If you see that CH3CH2CH2CH3 and (CH3)3CH are the two possible structures of C4H8,

then they are structural isomers!

[leakeydong]

resonance structures just one substance, but isomers are different substances.

[deutdeut]

But the defintion says that the molecule which has different structures are isomers. For the resonance structures of benzene, they both have the same molecular formula but certainly different structures (different in bond position), why aren't they are isomers?

[Borek]

Isomers have different atom positions?

[kryptoniitti]

But the defintion says that the molecule which has different structures are isomers. For the resonance structures of benzene, they both have the same molecular formula but certainly different structures (different in bond position), why aren't they are isomers?

The Kékule structures of benzene don't
give authentic representation of the actual
molecule. The bonds are the same type and
the lengths are also equal. The molecule is
often represented as such:

[Dolphinsiu]

Obviously, the two possible structures in Benzene ring are simultaneously appearing in the same benzene, however C4H8(butane) that I give you the two structures cannot be existed simultaneouly in the same butane, but its isomers only exists themselves individually! Is that clear?

[movies]

The term isomer is a vague one.  It also depends on the time scale that you are looking at.  In the butane example, two different conformations of the molecule could be considered isomers on a very short time scale, however, in practical applications they can be considered homomers (the antonym of isomers). 

With resonance structures, you're never looking at two isomerizing species, you are looking at two (or more) extreme possibilities for where the electrons are at.  The reality is somewhere between the extremes (the resonance hybrid).  If we were to try to change the time scale to be faster than an electron can move around, then we run into a quantum mechanical problem, but remember that in quantum mechanics you never see a single electron relative to a nucleus, it's always a probablity function for where the electron might be.  You can't have a time scale fast enough to consider resonace structures to be isomers.

[macman104]

Isomers have a different arrangement of atoms.  Resonance structures represent multiple different bond(read electron) arrangements, which when combined into a hybrid resonance structure represent what the actual electronic arrangement is.