This is an introduction to oxidation reduction reactions, which are often called redox reactions for short. An oxidation reduction (redox) reaction happens when electrons are transferred between atoms. A loss of electrons is called oxidation, and we say that atom has become oxidized. A gain of electrons is called reduction, and we say that the atoms has become reduced. The two separate parts (oxidation and reduction) of an oxidation reduction (redox) reaction are called half reactions. Two half reactions can be put together to make the whole reaction. Oxidation numbers are numbers that can be written above atoms to show whether they are gaining or losing electrons.