RESONANCE

as u posted the name of the question, its resonance.

pi-pi conjugaton exists na.

y resonance?

resonance not possible for non-conjugated system ....

skygirl please explain a bit more abt ur ans.
according to me the reason is resonance.

as u can see... bonds 1,2,3 has some similarity in bond length... due to little resonance...(little similarity as charge separation so 2nd structure is not stable...)

But bonds 4,5,6 has nothing to do with that.. so bond lengths are accordingly...different for single and double bonds...