A Brief History of Oxidation and Reduction

Where do the terms “oxidation” and “reduction” actually come from? Today, we define them by the movement of electrons, but the words themselves were coined long before the electron was even discovered. Their story is a perfect example of how scientific language evolves, starting with simple, tangible observations and gradually becoming more refined and fundamental as our understanding of the universe deepens.

The Age of Oxygen

In the 18th century, the great French chemist Antoine Lavoisier revolutionized the field with his work on combustion. He was one of the first to correctly identify the role of a specific element in the air, which he named oxygen, from the Greek words oxys (acid) and genes (to create), because he initially (and incorrectly) believed it was the basis of all acids.

French chemist Antoine Lavoisier (1743–1794) (Source: Wikipedia)

For Lavoisier and his contemporaries, chemistry was what you could see and weigh. When a substance burned or a metal rusted, it was gaining something from the air. The process of a substance combining with oxygen became known, quite literally, as oxidation.

A classic example is the rusting of iron. \[ \mathrm{4~Fe(s) + 3~O_2(g) \longrightarrow 2~Fe_2O_3(s)} \] If you were to take a piece of pure iron wool, weigh it, and then allow it to rust completely, you would find that the final mass of the rust is significantly greater than the original mass of the iron. That extra mass came from the oxygen atoms that bonded to the iron. The iron was “oxidized”.

The opposite process, reduction, had its roots in metallurgy. To produce pure iron from an iron ore like hematite (Fe₂O₃), a smelter would heat it with charcoal (carbon). \[ \mathrm{2~Fe_2O_3(s) + 3~C(s) \longrightarrow 4~Fe(s) + 3~CO_2(g)} \] In this process, the solid ore was reduced to a smaller mass of pure, shiny metal. The term “reduction” literally meant to reduce a substance back to its simpler, elemental form by removing the oxygen.

For a long time, this was the entire picture. Oxidation was gaining oxygen, and reduction was losing oxygen.

The Hydrogen Connection

As chemistry advanced in the 19th century, scientists realized that many reactions involved the movement of hydrogen atoms. This led to a broader, more fundamental definition of oxidation: the loss of hydrogen. Consequently, reduction came to be understood as the gain of hydrogen. This new perspective didn’t replace the original oxygen-based definition; it expanded it, providing a more complete picture of chemical change.

There is no better place to see this “hydrogen connection” in action than inside our own bodies. The process of metabolizing alcohol in the liver is a perfect textbook example of oxidation through the removal of hydrogen. This detoxification is an elegant two-step pathway, driven by specialized enzymes that use a coenzyme called NAD⁺ (nicotinamide adenine dinucleotide) as the primary oxidizing agent. Its job is to accept the hydrogen atoms that are stripped from the alcohol molecule.

NAD⁺ is a coenzyme found in all living cells and is involved in metabolism. (Source: Wikipedia)

The pathway begins when the enzyme alcohol dehydrogenase (ADH) oxidizes ethanol by removing two hydrogen atoms, converting it into the highly toxic intermediate, acetaldehyde. This toxic molecule is immediately targeted by a second enzyme, aldehyde dehydrogenase (ALDH). This enzyme oxidizes the acetaldehyde further into harmless acetate (the form of acetic acid found in the body), which can then be used for energy.

The entire sequence can be visualized as a continuous flow:

At each stage of the pathway, a molecule of NAD⁺ acts as the oxidizing agent, accepting hydrogen and becoming NADH. This biological process is a prime illustration of the hydrogen connection. It shows a sophisticated oxidation reaction that is fundamentally about the sequential removal of hydrogen, all orchestrated by the elegant machinery of life.

Unification by the Electron

The great unification of these ideas came with the discovery of the electron by J.J. Thomson in 1897. Chemists finally had the fundamental particle they needed to explain what was truly happening beneath the surface of these reactions.

They realized the old definitions were just two sides of the same coin. The real driving force was the transfer of electrons, a concept best understood through electronegativity.

Electronegativity is a measure of an atom’s ability to attract shared electrons in a chemical bond. Oxygen is very electronegative, while most metals and hydrogen are much less so.

With this new insight, the old rules suddenly made perfect sense when viewed through the lens of electronegativity:

• Why is gaining oxygen an oxidation? Because oxygen is so electronegative, when it bonds to an atom like iron, it pulls electron density away from it. The iron atom has effectively lost a share of its electrons to the oxygen. Since the loss of electron density is the very definition of oxidation, this old rule fits perfectly.

• Why is losing hydrogen an oxidation? Because hydrogen has a low electronegativity. When it’s bonded to a more electronegative atom like carbon, as in ethanol, the carbon atom “wins” the electron tug-of-war. If that hydrogen is removed, the carbon atom loses the electron density it once controlled from that bond. Again, this loss of electron density is precisely what we now define as oxidation.

This unifying principle, that the key event is a change in electron control, allowed chemists to state the modern, fundamental definitions that govern all redox chemistry:

• Oxidation is the Loss of electrons (or electron density).
• Reduction is the Gain of electrons (or electron density).

This simple, powerful framework explains every case. In some reactions, like the rusting of iron, the electron loss is a complete transfer. Neutral Fe atoms (oxidation state 0) become Fe³⁺ ions (oxidation state +3) by literally losing three electrons each. The smelting of iron ore is the reverse, a reduction, because the Fe³⁺ ions gain those electrons back to become neutral Fe atoms.

The story of the word “oxidation” is the story of chemistry itself. It began with a simple, visible observation about a single element and evolved over a century into a deep, fundamental principle about the invisible dance of electrons that governs all chemical change.