Electrons in a hydrogen molecule are more firmly bound when they have two hydrogen nuclei to move around than when each electron has only one nucleus.

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