An oxidation-reduction or redox reaction is a chemical reaction that involves the transfer of electrons between two species/Electrochemistry/Redox_Chemistry/Oxidation-Reduction_Reactions). Oxidation is the loss of electrons, while reduction is the gain of electrons/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction). These reactions are accompanied by energy changes in the form of heat, light, and electricity, and they are vital to some of the basic functions of life, including photosynthesis, respiration, combustion, and corrosion or rusting/Electrochemistry/Redox_Chemistry/Oxidation-Reduction_Reactions). In organic chemistry, oxidations and reductions are different from ordinary redox reactions because many reactions carry the name but do not actually involve electron transfer. Instead, the relevant criterion for organic oxidation is gain of oxygen and/or loss of hydrogen, respectively. The oxidation state (OS) of an element corresponds to the number of electrons that an atom loses, gains, or appears to use when joining with other atoms in compounds. In determining the oxidation state of an atom, there are guidelines to follow/Electrochemistry/Redox_Chemistry/Oxidation-Reduction_Reactions).