Oxidation Number Rules and Examples

What Is an Oxidation Number?

It's important to know if an atom loses or gains electrons when combining with other atoms to form compounds. How do we keep track of the electrons lost or gained by an atom? We do this by looking at an atom's oxidation number.

An oxidation number is a number that is assigned to an atom in a substance. The oxidation number could be positive, negative, or zero, and it indicates if electrons are lost or gained. In other words, the oxidation number is a number that helps us keep track of electrons in an atom.

Here, the oxidation number of calcium is +2 and the oxidation number of oxygen is -2.

If the oxidation number is positive, then this means that the atom loses electrons, and if it is negative, it means the atom gains electrons. If it is zero, then the atom neither gains nor loses electrons.

Calcium has a +2 charge, which means it lost two electrons. Oxygen has a -2 charge, which means it gained two electrons.

If an atom loses electrons, its oxidation number is positive, so we can say that this atom undergoes oxidation. If an atom gains electrons, its oxidation number is negative, so we can say that the atom undergoes reduction.

You can remember this by thinking of the phrase OIL RIG: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).

Rules For Assigning Oxidation Numbers

In chemistry, it is important to follow a set of rules to assign oxidation numbers. Here are the most important rules and exceptions to remember when assigning oxidation numbers:

Rule 1: In its pure elemental form, an atom has an oxidation number of zero

What exactly is a pure element? A pure element is an atom that is not combined with any other elements. This table shows examples of atoms in their pure elemental form:

Take note that even though the element has subscripts like the 2 in O2 and the 8 in S8, these are still considered pure elements because they are not combined with any other elements.

Rule 2: The oxidation number of an ion is the same as its charge

An ion is an atom with a charge due to the gain or loss of electrons. The charge is indicated as the superscript after the atom. If an atom has this, then this means that it is an ion. For example, the potassium ion has a +1 charge, so the oxidation number is +1. For the bromine ion, the charge is -1, so the oxidation number is -1. It is important to note that if only a positive (+) or negative (-) is shown, the charge is assumed to be +1 or -1.

Here is another example. The barium ion has a +2 charge, so the oxidation number is +2. For the sulfur ion, the charge is -2, and the oxidation number is -2.

Rule 3: The oxidation number of metals is +1 in Group 1 and +2 in Group 2

Unless it is in pure elemental form, the oxidation number of a metal is +1 in Group 1 and +2 in Group 2. It is important to note that although hydrogen is in Group 1, hydrogen is not a metal, so hydrogen is not included in this rule. In the following illustration, the metals for Group 1 and 2 are indicated.

So, the oxidation number for lithium is +1 because it is a metal that belongs in Group 1, and the oxidation number of magnesium is +2 because it is a metal that belongs in Group 2.

Rule 4: Hydrogen has two possible oxidation numbers: +1 and -1

Hydrogen has an oxidation number of +1 when it is bonded to nonmetals, which are highlighted on the right side of the following periodic table.

For the compound hydrochloric acid, hydrogen is bonded to chlorine, a nonmetal, so the oxidation number of hydrogen is +1.

Hydrogen has an oxidation number of -1 when it is bonded to a metal.

For the compound sodium hydride, hydrogen is bonded to sodium, which is a metal, so the oxidation number of hydrogen is -1.

Rule 5: Oxygen has three possible oxidation numbers: +2, -2 and -1

In general, oxygen has an oxidation number of -2. The only exceptions are peroxides, where oxygen has an oxidation number of -1, and in the compound of oxygen difluoride, where it has an oxidation number of +2.

Rule 6: The oxidation number of fluorine in any compound is -1

A few examples of fluorine-containing compounds are hydrogen fluoride or hydrofluoric acid, sulfur hexafluoride, and sodium fluoride. The oxidation number of fluorine in each of these compounds is -1.

Rule 7: For the other halogens (Cl, Br, I), the usual oxidation number is -1, except when they are combined with oxygen or fluorine.

In the compound sodium chloride, the oxidation number of chlorine is -1. In the compound hypochlorous acid, the oxidation number of chlorine is +1; and in the compound perchloric acid, the oxidation number of chlorine is +7.

Rule 8: The sum of the oxidation numbers in a neutral compound is equal to zero

Here are a few examples of neutral compounds:

Let's take a look at two examples to see how to calculate the oxidation numbers of the elements in a neutral compound using the rules we have learned so far.

Example 1:

Here we have the molecule hydrogen fluoride, which is a neutral compound, so its oxidation number is 0. To determine the oxidation numbers of the elements hydrogen and fluorine in this compound, we multiply the number of hydrogen atoms by the oxidation number of hydrogen. Then, multiply the number of fluorine atoms by the oxidation number of fluorine.

Hydrogen has an oxidation number of +1, and we have 1 hydrogen atom:

1 * (1) = 1

Fluorine has an oxidation number of -1, and we have 1 fluorine atom:

1 * (-1) = -1

(-1) + 1 = 0

Example 2:

Here we have the molecule magnesium chloride as a neutral compound. Using the same process as Example 1:

Magnesium has an oxidation number +2 because it is a Group 2 metal, and we have 1 magnesium atom:

1 * 2 = 2

We have 2 chlorine atoms, and we know that we have a neutral molecule. Since the magnesium atom has a +2 oxidation number, this means that each chlorine atom must have a -1 oxidation number. 2 chlorine atoms give us a total of -2.

(-2) + 2 = 0

Rule 9: The sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion

A polyatomic ion is a substance made of more than one atom. The following are examples of polyatomic ions:

Let's look at a couple of examples to understand the oxidation number for polyatomic ions:

Example 1:

The sulfate polyatomic ion has 1 sulfur atom and 4 oxygen atoms, with an overall charge of -2. According to Rule 5, oxygen has an oxidation number of -2 in this compound. We have 4 oxygen atoms, each with an oxidation number of -2, so the overall charge from oxygen is -8.

We know that the overall charge of the sulfate molecule is -2; this means that the sulfur atom must have an oxidation number of +6.

6 + (-8) = -2

Example 2:

The ammonium ion has 1 nitrogen atom and 4 hydrogen atoms, with an overall charge of +1. According to Rule 4, when hydrogen is bonded to a nonmetal it has an oxidation number of +1. We have 4 hydrogen atoms, each with an oxidation number of +1, so the overall charge from hydrogen is +4.

We know that the overall charge of the ammonium molecule is +1; this means that the nitrogen atom must have an oxidation number of -3.

4 + (-3) = 1

Lesson Summary

Oxidation numbers can be positive, negative, or zero, and they are assigned to atoms. The oxidation number indicates whether or not an atom undergoes oxidation (positive) or reduction (negative). The rules and exceptions which determine the correct oxidation number of an atom are:

1. In its pure elemental form, an atom has an oxidation number of zero.
2. The oxidation number of an ion is the same as its charge.
3. The oxidation number of metals is +1 in Group 1 and +2 in Group 2.
4. Hydrogen has two possible oxidation numbers: +1 and -1.
5. Oxygen has three possible oxidation numbers: +2, -2, and -1.
6. The oxidation number of fluorine in any compound is -1.
7. For the other halogens (Cl, Br, I), the usual oxidation number is -1, except when they are combined with oxygen or fluorine.
8. The sum of the oxidation numbers in a neutral compound is equal to zero.
9. The sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion.

Key Terms

Oxidation number - a number that is assigned to an atom in a substance to help keep track of electrons in an atom

Positive oxidation number - denotes that the atom will lose electrons

Negative oxidation number - denotes that the atom will gain electrons

Zero oxidation number - denotes that the atom will neither gain nor lose electrons

Oxidation - loss of electrons

Reduction - gain of electrons

OIL RIG - a mnemonic for remembering oxidation and reduction; Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons)

Pure element - an atom that is not combined with any other elements

Ion - an atom with a charge due to the gain or loss of electrons

Polyatomic ion - a substance made of more than one atom

Learning Outcomes

After viewing this lesson, check to see if you can:

• State the meaning of positive, negative, and zero oxidation numbers
• Differentiate between the processes of oxidation and reduction
• Use the 9 rules for assigning oxidation numbers to appropriately apply an oxidation number to a variety of elements, both pure and in compounds
• Define pure elements, ions, and polyatomic ions