Oxidation happens when an atom loses one or more of its electrons to an atom of a different element. Originally, this term only referred to oxygen combining with another element or elements to form a compound called an oxide.
When an element’s atoms lose electrons, it’s said to be oxidized. When an element’s atoms gain electrons, it’s called the oxidizing agent and is said to be reduced. When the atoms of an element gain electrons, it’s called reduction. Oxidation and reduction always occur simultaneously, so chemists describe this reaction with the terms oxidation-reduction or redox.
Oxygen is the oxidizing agent in most oxidation-reduction reactions. For example, rusted iron is caused by oxygen combining with iron to form iron oxide on its surface. The iron atoms give up electrons to the oxygen atoms. The iron is oxidized, and the oxygen is reduced. The same thing happens during the oxidation of copper, but the result is a greenish coating called copper oxide. In either case, the metal itself isn't weakened by oxidation, but years of exposure to air and water give the surface a patina.
Burning coal is rapid oxidation. Rusting iron is slow oxidation. The respiration of plants and animals is also oxidation.
The halogen elements of fluorine, chlorine, bromine, iodine and astatine are also oxidizing agents. When chlorine combines with sodium, it forms sodium chloride, or common salt, and the sodium atoms give up electrons to the chlorine atoms. Thus, the sodium is oxidized, and the chlorine is reduced.