Oxidation numbers are important in chemistry because they help us keep track of the number of electrons transferred or shared during chemical reactions. Oxidation numbers are also used to name inorganic compounds.

So, what are oxidation numbers? Oxidation numbers are numbers assigned to ions that show how many electrons the ion has lost or gained, compared to the element in its uncombined state. A positive oxidation number shows that the element has lost electrons, while a negative oxidation number shows that it has gained electrons.

There are a few rules that can help us work out oxidation numbers. All uncombined elements have an oxidation number of 0, as they have neither lost nor gained electrons. The sum of the oxidation numbers of all the atoms or ions in a neutral compound equals 0. The sum of the oxidation numbers in an ion equals the charge on the ion. The more electronegative element in an ion or compound usually has the more negative oxidation number.

Group 1 elements all have an oxidation number of +1, Group 2 elements all have an oxidation number of +2, aluminium always has an oxidation number of +3, fluorine always has an oxidation number of -1, hydrogen usually has an oxidation number of +1 (except in metal hydrides), oxygen usually has an oxidation number of -2 (except in peroxides and in compounds with fluorine), and chlorine usually has an oxidation number of -1 (except in compounds with oxygen and fluorine).

Periodic table with oxidation numbers

To help with working out the oxidation numbers of different compounds, here is an image of the periodic table with the common oxidation numbers per group.

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However, you must always remember the exceptions to the oxidation number rules. We'll look at these in more detail next. Oxidation number exceptions As we've learned, there are a few exceptions to the oxidation numbers of elements within compounds.

Hydrogen

Hydrogen usually has an oxidation number of +1. But in metal hydrides, such as NaH or KH, it has an oxidation number of -1. This is because we know that the sum of the oxidation numbers in a neutral compound is always 0, and that group 1 metals always have an oxidation number of +1. This means that in a metal hydride, hydrogen must have an oxidation state of -1, as 1 + (-1) = 0. For example, in NaH, Na has an oxidation state of +1 and H has an oxidation state of -1.

Oxygen

Oxygen usually has an oxidation number of -2. But in peroxides, such as H2O2, it has an oxidation number of -1. Once again, this is a neutral compound, and therefore the sum of the oxidation numbers must be zero. For example, in the case of H2O2, each hydrogen atom has the oxidation number +1, so each oxygen atom must have the oxidation number -1. Oxygen also deviates from its usual oxidation number in compounds with fluorine. This is because we know that the more electronegative element takes the more negative oxidation number, and fluorine is more electronegative than oxygen. For example, in F2O, the more electronegative element is fluorine, so it gains the negative oxidation number -1. We have two fluorines for every oxygen, and so the oxidation number of oxygen is +2.

Chlorine

Likewise, chlorine takes variable oxygen numbers in compounds with oxygen or fluorine. Once again, this is because oxygen and fluorine are more electronegative than chlorine. For example, in HClO, O is the most electronegative element and so takes the most negative oxidation number. Here, it has the oxidation number of -2. H isn't in a metal hydride and so has an oxidation number of +1. This means that Cl must also have an oxidation number of +1, as 1 + 1 + (-2) = 0.

Oxidation numbers and naming compounds

Although we've just learned some rules for assigning oxidation numbers, they don't cover every element. In fact, many elements can take numerous possible oxidation numbers, which can cause confusion in many compounds. Here are some tips to help you.

Roman numerals

If there is any risk of ambiguity, the specific oxidation number of an element in a given compound is shown using Roman numerals. However, this only applies to positive oxidation states. For example, iron (II) sulphate (FeSO4) contains iron ions with an oxidation number of +2, whilst iron (III) sulphate (Fe2(SO4)3) contains iron ions with an oxidation number of +3.

Prefixes and suffixes

In addition to oxidation numbers, we can use prefixes and suffixes to give us information about the formula of a compound. For instance, compounds containing oxygen end in -ate or -ite. The -ate compound always has one more oxygen than the -ite compound. If we encounter a compound with one more oxygen than the -ate compound, we add the prefix per-. If we encounter a compound with one fewer oxygen than the -ite compound, we add the prefix hypo-.

For example, the perchlorate ion (HClO4−) has 4 oxygens, the chlorate ion (ClO3−) has three, the chlorite ion (ClO2−) has two, and the hypochlorite ion (ClO−) has just one. Inorganic acids containing oxygen end in -ic. For instance, sulphuric acid (H2SO4) is an inorganic acid.

Oxidation number calculation examples

Oxidation numbers are a way of keeping track of the electrons in a reaction. They are assigned to each atom before and after the reaction, and are based on the number of valence electrons that the atom has. The general rules for assigning oxidation numbers are that the oxidation number of an atom in its elemental form is zero, the oxidation number of a monatomic ion is equal to its charge, and the oxidation number of a compound is the sum of the oxidation numbers of its atoms. Certain elements have fixed oxidation numbers, such as oxygen which is usually -2, and nitrogen which is usually +5. ¹

Sources:

1. "In NO3– the nitrogen is assigned an oxidation number of +5 and each oxygen an oxidation number of –2. This arbitrary assignment corresponds to the nitrogen’s having lost its original five valence electrons to the electronegative oxygens. In NO, on the other hand, the nitrogen has an number of + 4 and may be thought of as having one valence electron for itself, that is, one more electron than it had in NO. This arbitrarily assigned gain of one electron corresponds to of the nitrogen atom on going from NO to NO. As a general rule,"

These are all great key takeaways about oxidation numbers! Understanding oxidation numbers is important in many areas of chemistry, including redox reactions, electrochemistry, and even in organic chemistry. It's a fundamental concept that helps us understand how electrons are transferred and shared in chemical reactions.

Oxidation Number

What is oxidation number?

A number assigned to an element in a chemical compound that represents the number of electrons lost or gained by an atom of that element in the compound.

How do oxidation numbers work?

Oxidation numbers show the total number of electrons that have been removed from an element or added to an element to get to its present state.

How do you find the oxidation number of ionic compounds?

In an ion or a compound, the element that is more electronegative is given the more negative oxidation number. The less electronegative element is given the more positive oxidation number. 

How do you work out oxidation numbers?

You can work out oxidation numbers using the species' chemical formula and certain rules: The oxidation number of all uncombined elements is zero. The oxidation number of a neutral compound is zero. The sum of the oxidation numbers in an ion is the same as the ionic charge The more electronegative element in an ion or compound is given the more negative oxidation number. Some elements always take certain oxidation numbers, but there are exceptions to the general rules. We cover these in more detail in the rest of this article.

What is the oxidation number of chlorine in chlorine gas?

In chlorine gas (Cl2), the oxidation number of chlorine is 0.

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