Oxidation and reduction

Sometimes, you want to know if two substances are likely to react with each other or not. And sometimes, you want to know if a substance is likely to pick up electrons or give away electrons. This is useful if you want to build a battery. For elements that usually form ions, it's pretty simple: Metals want to lose electrons, non-metals want to gain electrons. Why is that?

Let's look at the atoms' outer electrons. Here are two elements. Let's call them X and Y. X is a metal. It has few electrons in its outer shell.

Y is a non-metal. It has many electrons in its outer shell, but it needs even more to have a full outer shell. When X and Y react, the metal X gives up its two electrons. Since each Y atom only needs one electron, there needs to be two of those atoms. They form a compound, XY2.

The amount of charge that an element has lost or gained when forming a new compound is called the oxidation state of that element. X lost 2 electrons. This means it has a positive charge of 2. So its oxidation state is plus 2. Y gained 1 electron.

So it has a negative charge of 1. Its oxidation state is minus 1. When element X's oxidation state has increased, we say that it has been oxidized. The word comes from the fact that most elements that react with oxygen gas increase their oxidation state. But the same word is used in reactions with other elements than oxygen.

Y's oxidation state has decreased. It's been reduced. A chemical reaction where oxidation states change is called a redox reaction. In any redox reaction, the total change in oxidation state needs to be unchanged. For each oxidation step up, there needs to be a reduction step down.

Sometimes, when you mix two substances that both want to get rid of electrons, or both want to pick up electrons, nothing happens. But how do you know which substances can be oxidized and which can be reduced? You can look at what oxidation state - or oxidation number - an element has. When a molecule contains only one element, for instance N-two or O-three, and is not in a compound with another element, its atoms always have an oxidation state of zero. If the atoms have formed ions, the ions always have the oxidation number of their charge.

This means that when an element, such as nitrogen, becomes an ion with a charge of minus three... ... each nitrogen atom has gained three electrons... The oxidation state of each atom has gone from zero to minus three... so the nitrogen has been reduced. Here's another example.

A copper ion can have a charge of +1. From there, it can go two ways: it can either get oxidized one step, to form copper "two-plus". Or it can be reduced one step, to a copper atom with oxidation state zero. If the copper "one-plus" ion reacts with something that wants to steal electrons, it will be oxidized. If it reacts with something that wants to give it electrons, it will be reduced.

What was the point of this now? Well, some reactions are more likely to occur when a substance that is easily oxidized and a substance that is easily reduced come together. And it's also useful if you want to know whether a substance wants to gain electrons or if it wants to lose electrons. That's why we need to know an element's oxidation state. It tells us how many electrons it has gained or lost when reacting with other elements.