Oxidation and reduction are two important concepts in chemistry that describe the transfer of electrons between atoms or molecules. Oxidation : Oxidation is the loss of electrons from an atom or molecule. When an atom or molecule loses electrons, it becomes positively charged and is said to be oxidized. Oxidation often involves the addition of oxygen or the removal of hydrogen from a molecule. Reduction : Reduction is the gain of electrons by an atom or molecule. When an atom or molecule gains electrons, it becomes negatively charged and is said to be reduced. Reduction often involves the addition of hydrogen or the removal of oxygen from a molecule. These two processes are often coupled and occur together in chemical reactions. This is known as a redox reaction . In a redox reaction, the species that is oxidized loses electrons to the species that is reduced, which gains electrons. A simple example of a redox reaction is the combustion of methane: ...