Oxidation is the loss of one or more electrons by an atom during a chemical reaction. In living organisms, chemical energy is stored in high-energy electrons that are transferred from one atom to another in reactions involving oxidation and reduction.
Oxidation in which oxygen is involved
Oxygen in the air has a great tendency to combine with other compounds, especially at high temperatures. When this combination occurs oxidation is said to take place.
Coke consists almost entirely of the element carbon. It does not combine with oxygen at ordinary temperatures, but it will do so when the temperature is high enough. Combination with oxygen of the air then occurs and if plenty of air is available, the carbon is converted to carbon dioxide gas. At the same time energy is released, which make itself apparent in the form of heat and light. It is for this reason that coke is used as a fuel. Many elements will burn in this way when their temperatures become high enough. When the burning begins excess heat energy is given out, some of which is used to keep the temperature of the element raised so that it remains burning. "Burning" involves the combination of elements with oxygen. Hydrogen gas burns to give steam, and sulfur produces sulfur dioxide on burning.
Several metals will burn when heated in air, forming the metallic oxide, and giving out heat and light. Magnesium, for example, emits a bright white light, and for this reason was used in photographers' flash bulbs and as a constituent of aerial flares. Other metals do not release so much energy when they combine with oxygen on heating. They merely glow.
Combination with oxygen will take places in some cases without the application of heat. Metals tarnish and become covered with a layer of oxide. Rusting of iron, in which an impure iron oxide is produced, is brought about by air in the presence of water. Rusting might be called a slow, flameless combustion.
Yellow phosphorus is a dangerous substance because it has a low ignition temperature. It must be stored under water, for when it is brought into contact with air it spontaneously bursts into flame without being heated.
Many compounds will burn, especially those which contain carbon and hydrogen. Wood has been burnt ever since people had a knowledge of fire; gasoline is burnt in the internal combustion engine. Gasoline is hydrocarbon, i.e., its molecule consists of carbon atoms combined with hydrogen atoms. Its ignition temperature is low and this means great inflammability. The molecules of gasoline split and react with the oxygen to form steam and carbon dioxide.
Oxidation processes are very important in living organisms. Both animals and plants need oxygen in order to live. The cells of which living things are made require energy to perform their functions and this is released from food materials by "burning" them. Actually this involves a series of complex chemical reactions which are oxidations.
Oxidation as the removal of electrons
Chemists don't only use the term oxidation for reactions in which oxygen is added to a substance. Oxidation is applied to certain reactions in which oxygen plays no part whatsoever. Oxidation has been extended to all reactions in which electrons are removed from a substance. For instance, when magnesium burns, each atom of magnesium loses two electrons from its outer shell, so exposing the shell of eight inside and so becoming more stable. Since the negative charges of the electrons in a neutral atom just balance the positive charges of the nucleus, the loss of two electrons produces a net gain by each magnesium atom of two positive charges. The two electrons from each magnesium atom are taken up by one oxygen atom. thus the octet of electrons in the outer shells of both the magnesium and oxygen atoms are complete and so stability is achieved.
Chlorine is a good oxidizing agent. When magnesium is heated in chlorine, magnesium chloride is formed. Each magnesium atom loses two electrons, and so becomes an ion. Two chlorine atoms (each of which has seven outer electrons) take the two electrons lost from the magnesium atom, and each chlorine achieves a completed shell of eight electrons. The chlorine atoms become chloride ions with a single negative charge. The magnesium ions and chloride ions are held together in magnesium chloride by an electrovalent bond, (i.e., by the attraction which exists between two objects charged with unlike charges. Both in burning oxygen and reacting with chlorine, the magnesium atoms have behaved in a very similar way, in that they have become magnesium ions by the loss of electrons. The chemist regards both reactions as oxidations, since electrons have been lost.
Oxidation in organic compounds
Oxidation is easy to recognize when an element changes changes oxidation state to become an ion. However when dealing with organic molecules, atoms are joined by covalent bonds rather than ionic ones so oxidation is not as easily recognized. In organic molecules, an atom is oxidized if during a reaction it becomes bonded to a more electronegative element which pulls electron density away from it. In general, oxidation occurs if the oxygen content of a covalently bonded molecule increases or if the hydrogen content decreases. For example, the carbon atom in methane (CH4) is more reduced than the carbon atom in carbon dioxide (CO2).