When an element (can be an atom or ion or molecule) gain electrons the reaction is called reduction, the element is reduced. The oxidation number of the element decrease.

When an element (can be an atom or ion or molecule) loose electrons the reaction is called oxidation, the element is oxidized. The oxidation number of the element increase.

Redox reactions or oxidation-reduction has to do with the transfer of electrons and the two reactions happen one after the other (in tandem). Examples where oxidation-reduction reactions are found:

• Bacteria has the ability to convert N₂ in the atmosphere for example to NH₄⁺.
  In our human bodies our lungs convert oxygen to water (H₂O) and carbon dioxide (CO₂) by breathing. A large molecule hemoglobin that contain Fe²⁺ is part of this process.
  Green plants convert water to O₂ by photosynthesis.
• Corrosion is the oxidation of a metal surface to its oxide. The elements that are part of this process are water, oxygen and the metal (iron, aluminum). Water on the surface of the metal form an electrochemical cell and reduced oxygen that is present in the air provide the oxide.
• Redox reactions are also found in a battery where an electrochemical process take place that cause a current (electrons) to flow at a constant voltage.
• By applying electricity metals can be prepared or purified or plated in industrial processes.

At the anode of an electrochemical cell, oxidation take place and at the cathode of an electrochemical cell, reduction take place. Electrons are produced at the anode of the battery and this terminal has a negative charge. Electrons are used at the cathode of the battery and this terminal has a positive charge. The terminal with a negative charge and the terminal with a positive charge are the electrodes of an electrochemical cell. For an electrochemical cell to function, it need an electrolyte to supply ions internally for conductance to take place between the positive and negative electrodes.

The negative electrode produce electrons to an external circuit and is oxidized in the discharge reaction. The positive electrode use the electrons from an external circuit and is reduced in the reaction.

Read more: Theodore Brown, Eugene LeMay, Bruce Bursten, Catherine Murphy. 2009. Chemistry The Central Science. Pearson Education Ltd.: London ISBN 978 0 13 235 848 4