Q. In chemical terms how is the neuronal repolarization achieved?

Repolarization is the return of the membrane probable from the action potential (+35 mV) to the resting potential (-70 mV).

When the membrane reaches its action potential voltage-gated potassium channels open and voltage-gated sodium channels close. So potassium starts to exit and sodium stops entering into the cell therefore the repolarization is due to exiting of potassium cations from the cell.

The repolarization causes the potential difference temporarily to increase under -70 mV below the resting potential a phenomenon called as hyperpolarization.