Oxidation Definition in Chemistry

What Oxidation Means (New and Old Definitions)

In this example of oxidation, zinc atoms in an electrode dissolve in acid, losing electrons to form cations. Dorling Kindersley / Getty Images

Oxidation Definition

Oxidation is the loss of electrons during a reaction by a molecule, atom or ion.

Oxidation occurs when the oxidation state of a molecule, atom or ion is increased. The opposite process is called reduction, which occurs when there is a gain of electrons or the oxidation state of an atom, molecule, or ion decreases.

An example of a reaction is that between hydrogen and fluorine gas to form hydrofluoric acid:

H2 + F2 → 2 HF

In this reaction, hydrogen is being oxidized and fluorine is being reduced. The reaction may be better understood if it is written in terms of two half-reactions.

H2 → 2 H+ + 2 e-

F2 + 2 e- → 2 F-

Note there is no oxygen anywhere in this reaction!

Historical Definition of Oxidation Involving Oxygen

An older meaning of oxidation was when oxygen was added to a compound. This was because oxygen gas (O2) was the first known oxidizing agent. While the addition of oxygen to a compound typically meets the criteria of electron loss and an increase in the oxidation state, the definition of oxidation was expanded to include other types of chemical reactions.

A classic example of the old definition of oxidation is when iron combines with oxygen to form iron oxide or rust. The iron is said to have oxidized into rust. The chemical reaction is:

2 Fe + O2 → Fe2O3

The iron metal is oxidized to form the iron oxide known as rust.

Another example of oxidation where an element combines with oxygen is the reaction between magnesium metal and oxygen to form magnesium oxide. Many metals oxidize, so it's useful to recognize the form of the equation:

2 Mg (s) + O2 (g) → 2 MgO (s)

Oxidation and Reduction Occur Together (Redox Reactions)

Once the electron was discovered and chemical reactions could be explained, scientists realized oxidation and reduction occur together, with one species losing electrons (oxidized) and another gaining electrons (reduced).

A type of chemical reaction in which oxidation and reduction occurs is called a redox reaction, which stands for reduction-oxidation.

The oxidation of a metal by oxygen gas could then be explained as the metal atom losing electrons to form the cation (being oxidized) with the oxygen molecule gaining electrons to form oxygen anions. In the case of magnesium, for example, the reaction could be rewritten as:

2 Mg + O2 → 2 [Mg2+][O2-]

comprised of the following half-reactions:

Mg → Mg2+ + 2 e-

O2 + 4 e- → 2 O2-

Historical Definition of Oxidation Involving Hydrogen

Oxidation in which oxygen is involved is still oxidation according to the modern definition of the term. However, there is another old definition involving hydrogen which may be encountered in organic chemistry texts. This definition is the opposite of the oxygen definition, so it may cause confusion. Still, it's good to be aware. According to this definition, oxidation is the loss of hydrogen, while reduction is the gain of hydrogen.

For example, according to this definition, when ethanol is oxidized into ethanal:


Ethanol is considered oxidized because it loses hydrogen. Reversing the equation, ethanal can be reduced by adding hydrogen to it to form ethanol.

Using OIL RIG to Remember Oxidation and Reduction

So, remember the modern definition of oxidation and reduction concern electrons (not oxygen or hydrogen). One way to remember which species is oxidized and which is reduced is to use OIL RIG. OIL RIG stands for Oxidation Is Loss, Reduction Is Gain.