After making the skeleton of s- bonds using sp2 hybridization around carbons, four electrons and four p orbitals in butadiene remain unused, and the p orbitals are arranged as in the perspective drawing (a) at the upper left. As in benzene, these p orbitals can be combined in several ways to produce various resonance structures for the molecule. Four resonance structures are shown in drawings (b) - (e) at left. All four carbon atoms are linked in a delocalized electron system. Compounds with alternating single and double bonds, in which p-electron delocalization can occur, are called conjugated molecules. Whether linear or in closed rings, conjugated molecules somewhat larger than butadiene or benzene have the useful property of absorbing visible light, as we shall see in the postscript. Linear conjugated molecules are used in the photoreceptors of the eye, and both linear and aromatic conjugated molecules are put to work in trapping light in photosynthesis.