Final answer:
In an oxidation-reduction (redox) reaction, the primary chemical event is the transfer of electrons, with oxidation being the loss and reduction being the gain of electrons. These processes occur simultaneously and define what redox reactions are.
Step-by-step explanation:
The primary chemical event that occurs in an oxidation-reduction (redox) reaction is the transfer of electrons. In this type of reaction, oxidation, which is the loss of electrons, always occurs simultaneously with reduction, which is the gain of electrons. An example of this would be the reaction between a metal and nonmetal where the metal donates electrons to the nonmetal, therefore the metal is oxidized and the nonmetal is reduced, forming an ionic bond. It is this loss and gain of electrons that define redox reactions.
Oxidation can also be described as the gain of oxygen or the loss of hydrogen, while reduction is the loss of oxygen or the gain of hydrogen. Photosynthesis and the digestion of food are examples of biological redox processes. Photosynthesis involves the reduction of CO2 to store solar energy as chemical energy, and digestion is the oxidation of food to release energy for life processes.