Your cells are continually extracting chemical energy from molecules of glucose through the process of cellular respiration. This process consumes oxygen and glucose. Biochemists track what happens to each of these two participants by determining which one loses and which one gains electrons -- which one, in other words, is oxidized and which one is reduced.
When two atoms are joined by a chemical bond they are sharing electrons. Oxygen is a far more selfish element than carbon or hydrogen, while carbon is only slightly more selfish than hydrogen. This means that oxygen tends to share electrons unequally with carbon or hydrogen, while carbon and hydrogen tend to share much more equally.
Chemists use these differences to assign an oxidation number to each element in a compound. The oxidation number is the charge that the atoms of that element would have if all of the shared electrons were assigned to the more selfish element. This is basically a kind of mental bookkeeping that will help you figure out who gained electrons in a chemical reaction.
Glucose and Oxygen
In a molecule of oxygen, two oxygen atoms share four electrons. Since they are equally selfish, when you assign the oxidation number each oxygen atom gets two electrons for a total of six that it now owns. Since this is equal to the total number of electrons that an oxygen atom by itself would possess, the oxidation number of oxygen in this compound is zero.
A molecule of glucose, by contrast, has the molecular formula C6H12O6. Since oxygen is more selfish than carbon or hydrogen, when determining oxidation numbers you assign all electrons the oxygen atoms share to oxygen, and thus each oxygen atom has an oxidation number of -2. Hydrogen is the least selfish element present; since hydrogen by itself has only one electron, each hydrogen atom has an oxidation number of +1. Multiplying +1 by 12 and adding it to -2 times 6 gives zero, which means that carbon in this compound has an oxidation number of zero.
Water and Carbon Dioxide
In a molecule of water, oxygen is by far the more selfish element. It shares four electrons with two hydrogen atoms. If we assign all four electrons to oxygen, we find that oxygen has an oxidation number of -2, while the hydrogen atoms have an oxidation number of +1.
In the compound carbon dioxide, one atom of carbon is sharing eight electrons with two atoms of oxygen. If all of the eight shared electrons were assigned to oxygen, the more selfish element, the carbon would have a charge of +4 since it has four fewer electrons than elemental carbon by itself. Its oxidation number is therefore +4. The two oxygen atoms, by contrast, now each have two more electrons than they would if they were by themselves, and so their oxidation number is -2.
Oxidation and Reduction
If you compare the oxidation number of oxygen before and after the reaction, you'll find it has decreased from zero to -2. The oxidation number of carbon, by contrast, has increased from zero to +4. When the oxidation number of an element in a compound increases during a reaction, chemists say that compound has been oxidized or has lost electrons. A decrease in oxidation number, by contrast, indicates the compound has been reduced or gained electrons. In other words, oxygen is reduced during cellular respiration while glucose is oxidized. Oxidizing carbon-containing compounds like glucose releases a lot of energy, which is how your cells get the energy they need to keep you moving.
- Chemical Principles, the Quest for Insight, Fourth Edition, pages F77-F83: Peter Atkins and Loretta Jones
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