Oxidation reactions occur when the reactant loses electrons and may involve the addition of oxygen or the loss of hydrogen, while reduction reactions occur when the reactant gains electrons and may involve the loss of oxygen or the addition of hydrogen. In oxidation, the atom or molecule becomes more positively charged, while in reduction, the molecule becomes more negatively charged.
Because electrons are neither created nor destroyed in a chemical reaction, it is impossible to have oxidation without another reactant undergoing reduction. These type of reactions are called redox reactions. In these reactions, the reactant that gains the electrons is referred to as the oxidizing agent, while the reactant that loses electrons is called the reducing agent. An example of a redox reaction is when copper is heated over a flame, turning its surface black as it oxidizes with oxygen to become copper oxide. The copper acts as a reducing agent, giving up electrons to the oxygen molecule, which acts as the oxidizing agent. Metals are typically reducing agents because during reactions they lose electrons and become positively charged. Before the discovery of electrons, the term "oxidation" was used to describe reactions between an element and oxygen, and reduction, which contains the Latin stem for "to lead back," was used to describe the reverse reaction.