The spreading of valence electrons over two or more bonds in a chemical compound. In certain compounds, the valence electrons cannot be regarded as restricted to definite bonds between the atoms but move over several atoms in the molecule. Such electrons are said to be delocalized. Delocalization occurs particularly when the compound contains alternating (conjugated) double or triple bonds, the delocalized electrons being those in the pi orbitals. The molecule is then more stable than it would be if the electrons were localized, an effect accounting for the properties of benzene and other aromatic compounds. The energy difference between the actual delocalized state and a localized state is the delocalization energy. Another example is in the ions of carboxylic acids, containing the carboxylate group –COO⁻. In terms of a simple model of chemical bonding, this group would have the carbon joined to one oxygen by a double bond (i.e. C = O) and the other joined to O⁻ by a single bond (C−O⁻). In fact, the two C−O bonds are identical because the extra electron on the O⁻ and the electrons in the pi bond of C = O are delocalized over the three atoms. Delocalization of electrons is a feature of metallic bonding. The delocalization energy of molecules can be calculated approximately using the Hückel approximation, as was done originally by Hückel. However, modern computing power enables delocalization energy to be calculated using ab-initio calculations, even for large molecules. See also localization.