Oxidations and reductions involve the escape of electrons from molecules or ions of one substance and their capture by other chemical substances. As with the movement of entire molecules discussed in previous sections, free energy is the key to understanding this escaping tendency.

Oxidation-reduction (redox) reactions are important because they are the principal sources of energy on this planet, both natural or biological and artificial. Oxidation of molecules by removal of hydrogen or combination with oxygen normally liberates large quantities of energy.

The synthesis of reduced organic molecules (sugars) by photosynthetic green plants is the main device for trapping and storing solar energy on this planet.

Oxidation either can involve the outright loss of electrons:

or the shifting away of bonding electrons toward a more electronegative atom: