of conjugated chains, there will be a node at every alternate carbon atom in the non-bonding orbital. This is important for the unpaired electron of allyl, which will occupy this non-bonding orbital. If an electron is added to the allyl radical to form the anion, the negative charge will appear at the terminal carbon atoms. If the unpaired electron is removed forming the cation, the resulting positive charge is also spread over the termial carbon atoms. There are three p -molecular orbitals for allyl, the p 1 is bonding, the p 2 orbital is non-bonding and the p 3 is anti-bonding. In the neutral allyl species - there are a total of seventeen valence electrons - of which three fill the p -orbital manifold. A pictorial representation of the energy diagram for the neutral, cationic and anionic allyl species are shown below - ( orbitals are shown only for the cationic species ): In the pentadienyl anion, the negative charge is centred on the carbon atoms in the 1,3 and 5 position - similarly with the positive charge for the cation. These ions are represented in resonance theory as two or three canonical forms: The delocalisation of p -electrons is associated with a lowering of the orbital energy. Therefore the total energy of the occupied p -orbitals of butadiene is lower in energy then two isolated ethene-type double bonds. Further delocalisation of p -electrons occurs in aromatic hydrocarbons. A figure showing the comparative energy levels of the p -orbitals of