Redox - Definitions

If you want to understand chemistry, you have to follow the electrons. If electrons are transferred during a chemical reaction (as opposed to just being rearranged...), then a reduction-oxidation process is taking place. To help keeping track of the electron transfer in redox processes, we can use a couple acronyms/mnemonics related to the definitions of reduction and oxidation.

OIL / RIG - “Oxidation Is Losing electrons” / “Reduction Is Gaining electrons”

LEO / GER - “Losing Electrons is Oxidation” / “Gaining Electrons is Reduction”

Hmm, why is “reduction” associated with gaining electrons? Remember, electrons are negatively charged, so gaining electrons increases the number of negatively charged particles associated with an atom which reduces its net charge. But are we really looking at “charge” to determine redox chemistry? Sometimes it seems like it, but other times the charge doesn't seem to line up with the processes. We really have to look at oxidation number, which is related to charge, but a with some subtle differences. One way to distinguish charge and oxidation number is that “charge” can be used to describe the net overall balance between electrons and protons in a system that might contain multiple atoms, but “oxidation number” describes the balance between electrons and protons for each individual atom in a structure regardless of its size. Oxidation numbers sound important enough for their own post, look for it soon.

Another thing that can cause some mix-ups is the term “oxidation” or {in verb form} “oxidize”. {Or for the British English spellers in the crowd, “oxidise”.} The element oxygen is very often involved in redox reactions. Is oxygen usually undergoing oxidation or reduction? You know you want to say oxidation, the words look so similar... But if we start with molecular oxygen, O₂(g), it's almost always going to gain electrons. Gaining Electrons is Reduction. GER, indeed! The important thing to remember here is that reduction and oxidation are ALWAYS coupled processes. You can't have one without the other. This leads to some other terminology...

The process of one substance undergoing oxidation causes something else in the system to be reduced. The substance that is being oxidized is the reducing agent or reductant because it is causing reduction to take place.
The process of one substance undergoing reduction causes something else in the system to be oxidized. The substance that is being reduced is the oxidizing agent or oxidant because it is causing oxidation to take place.

So if oxygen is usually being reduced, it is a good oxidizing agent.