Oxidation Numbers

 

Oxidation number is, simply the charge on each atom, whether it is alone, found as an ion or within a compound. That means that in many (most?) cases, it is relatively easy to determine the oxidation number of an atom. For instance, we know that pure elements are neutral, so the oxidation number on atoms of chlorine in a cloud of chlorine gas (\(Cl_2\)) is 0. Ions with the “ide” ending have the negative charge that makes the atom more stable, so chloride is always -1. That is true whether the chloride is part of a compound, like sodium chloride, or found independently in an aqueous solution. However, chlorine also has some other possible charges like those in the compounds \(HClO, HClO_2, HClO_3\), and \(HClO_4\).

For these more complicated cases, we need a set of rules to help us. The most common set of rules has some serious weaknesses. For instance, I was taught this rule:

Oxygen is always -2 except in peroxides

The problem, of course is that you must then be able to recognize peroxides when you see them. In addition, the rule is still not really correct. It should be that Oxygen is always -2 except pure oxygen, ozone, peroxides and hypoflorous acid.

The rule I was taught for hydrogen was not better:

Hydrogen is always +1, except for metal hydrides

What I will present here is a set of rules of my own invention. There are no exceptions and no other labels or particular types of compounds you need to watch for. It always works and it is always correct.

The McAfoos Method of Assigning Oxidation Numbers 

What follows are a series of rules, NOT steps. In other words, Rule #1 is the most important rule and beats all other rules. Rule #2 beats all others except rule #1, etc.

Rule #1 – The sum of the oxidation numbers = the total charge. This rule assigns oxidation numbers to pure elements, single element ions and the “last” element left without an oxidation number. Examples are shown below.

Rule #2 – Single charge elements get their charge. This rule assigns oxidation numbers to those elements that only have one non-zero charge. The list is short: alkali metals are always +1 (not H), alkaline earth metals are always +2, F is always -1 (not all the halogens), Al (+3), Zn (+2), and Ag (+1). Fair notice: if you forget the last three you’ll still probably be fine.

Rule #3 – Hydrogen is always +1. This doesn’t contradict the rule above if you remember that these rules are in descending importance. In other words, the only times that H is NOT +1 will have already been worked out based on the two rules above.

Rule #4 – Oxygen is always -2. As with the hydrogen rule, any “contradictions” to this rule will already have been found and dealt with.

Rule #5 – The most electronegative element gets its logical negative charges. This rule will only be used on rare occasions, since the earlier rules will handle almost everything.

A word of warning  – Compounds containing multiple instances of the same element should be broken into ions before assigning oxidation numbers. Although this is not generally an issue, when it is an issue, it matters immensely.

The only way to understand how these rules work is to see them in action, so let's take a look.