 The first challenge comes from the incompatibility between the ontology of quantum mechanics and the apparent ontology of the chemical bonds. Electrons cannot be distinguished in principle  (Identity and Individuality in Quantum Theory)  and hence quantum mechanical descriptions of bonds cannot depend on the identity of particular electrons. If we interpret the structural conception of bonding in a Lewis-like fashion, where bonds are composed of specific pairs of electrons donated by particular atoms, we can see that this picture is incompatible with quantum mechanics. A related objection notes that both experimental and theoretical evidence suggest that electrons are delocalized, “smeared out” over whole molecules. Quantum mechanics tells us not to expect pairs of electrons to be localized between bonded atoms. Furthermore, Mulliken argued that pairing was unnecessary for covalent bond formation. Electrons in a hydrogen molecule “are more firmly bound when they have two hydrogen nuclei to run around than when each has only one. The fact that two electrons become paired … seems to be largely incidental” (1931, p. 360). Later authors point to the stability of the H2+ ion in support of this contention.