 Defenders of the structural conception of bonding respond to these challenges by noting that G.N. Lewis’ particular structural account isn’t the only possible one. While bonds on the structural conception must be sub-molecular and directional, they need not be electron pairs. Responding specifically to the challenge from quantum ontology, they argue that bonds should be individuated by the atomic centers they link, not by the electrons. Insofar as electrons participate physically in the bond, they do so not as individuals. All of the electrons are associated with the whole molecule, but portions of the electron density can be localized. To the objection from delocalization, they argue that all the structural account requires is that some part of the total electron density of the molecule is responsible for the features associated with the bond and there need be no assumption that it is localized directly between the atoms as in Lewis’ model (Hendry 2008, 2010b).