Bonding and Tunneling

[Image: see text] Quantum mechanical electron tunneling is proposed as the mediator of chemical bonding. Covalent, ionic, and polar covalent bonds all rely on quantum mechanical tunneling, but the nature of tunneling differs for each bond type. Covalent bonding involves bidirectional tunneling across a symmetric energy barrier. Ionic bonding occurs by unidirectional tunneling from the cation to the anion across an asymmetric energy barrier. Polar covalent bonding is a more complicated type of bidirectional tunneling, consisting of both cation-to-anion and anion-to-cation tunneling across asymmetric energy barriers. Tunneling considerations suggest the possibility of another type of bond—denoted polar ionic—in which tunneling involves two electrons across asymmetric barriers.