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Oxidation Number: Definition, Rules & Examples

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  • 0:00 What is an Oxidation Number?
  • 1:30 Oxidation Number Rules
  • 8:30 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Knowing the ion form of an atom is important when putting together atoms to form a compound. An oxidation number is assigned to an atom to know its ion form. In this lesson, we will go over the rules to assign oxidation numbers to atoms.

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.

Oxidation Number

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:

Oxidation Number of Pure Elements

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.

Oxidation Number Rule 2

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.

Oxidation Number Rule 2 Example

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.

Group 1 and 2 Metals

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.

Metals and Nonmetals

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:

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:

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