Oxygen is a curious substance. On the one hand, most organisms need it to live. On the other hand, it can be extremely toxic. It was oxygen's toxicity that posed the problem for the bacteria of a couple of billion years ago. If we are to understand how, we must delve into some basic chemistry.
Chemistry, at root, is the science of moving electrons around. A chemical bond between atoms is actually a pair of electrons shared by two atoms. Oxygen gas, for example, is O2, two oxygen atoms bound together by a covalent bond. We usually represent a covalent bond with a connecting line to indicate the coupling between the atoms, O—O. It is truer to the nature of a chemical bond to write it this way:
where the two dots signify the two shared electrons.
Fundamentally, a chemical reaction is a movement of electrons from some bonds to others. In any chemical reaction, bonds between certain atoms, those in the reactants, are broken at the same time new bonds are formed in the products. This is said by chemists to be a coupled oxidation and reduction: the bond that gives up its electrons is oxidized, while the bond that receives the electrons is reduced (Fig. 6.2).
The art of chemistry is in knowing where the electrons will go if you put atoms or molecules together in such a way that electrons have a "choice" of bonds in which to reside. Chemists have a powerful tool to help them predict these preferences in the reduction potential, or redox potential. The redox potential is a voltage, a measure of the potential energy that can drive electrons from one bond to another. Electrons carry a negative charge, so electrons residing in a bond will tend to move toward other bonds with more positive redox potentials, just as electrons in an electrical current move through a wire from the negative pole of a battery to the positive pole.
Let us illustrate this concept with a simple example; the combining of oxygen and hydrogen to form water:
Redox Potential (V) 0
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