Proton tunneling is usually associated with hydrogen bonds. In many molecules that contain hydrogen, the hydrogen atoms are linked to two non-hydrogen atoms via a hydrogen bond at one end and a covalent bond at the other. A hydrogen atom without its electron is reduced to being a proton. Since the electron is no longer bound to the hydrogen atom in a hydrogen bond, this is equivalent to a proton resting in one of the wells of a double well potential as described above. When proton tunneling occurs, the hydrogen bond and covalent bonds are switched. Once proton tunneling occurs, the same proton has the same probability of tunneling back to its original site provided the double well potential is symmetrical.