Resonance structures provide a handy way of deciding how far delocalization extends in a molecule. A set of resonance structures for a molecule must have the atomic nuclei in the same places, and can differ only in the placement of electrons and hence of bonds and charges. When all possible resonance structures have been written for a molecule that cannot be described adequately by simple single and double bonds, then all atoms that are connected by double bonds in at least one of the resonance structures are involved in the delocalized electron system. For benzene these are the six carbon atoms, and the hydrogens play no part in delocalization. Delocalization in carbon compounds almost always involves the combination of a set of p orbitals perpendicular to the plane of a molecular skeleton connected by s- bonds. The double bonds do not have to be alternating around a closed ring for delocalization to occur. In the butadiene molecule shownon the next page, four carbon atoms are connected in a linear chain with two double bonds. Structure (b) is the one usually thought of for butadiene, but it cannot be completely right because all ten atoms in the real molecule lie in a plane, and this would not necessarily be true if the central carbon-carbon bond were a single bond.