The Oxidation Number: Definition, Rules, And Examples

What is meant by an oxidation number?

If striving to know what oxidation number is, let us inform you that it is the oxidation number representing the charge or gained and lost electrons of an atom, denoted in the positive, negative, or neutral expressions. It basically describes the degree of oxidation an atom has gone through in the chemical compound. The oxidation number helps one to keep track of electrons in the atom.

For example, the oxidation number of calcium is +2 and for oxygen, it is -2.

If the oxidation number is positive, it implies that the atom loses electrons, and if it’s negative, it implies that the atom has gained electrons. If the oxidation number of an atom is zero, it means that no electron is gained or lost by the same atom. 

If the electrons are lost by an atom and its oxidation number is positive, then it can be said that the same atom has gone through oxidation. But if the electrons are gained by an atom and its oxidation number is negative, then it can be said that the same atom has gone through reduction.

How to find the oxidation numbers?

If you are wondering how to find an oxidation number, then in chemistry, one needs to follow a particular set of rules to find or assign the oxidation number to an atom. Here mentioned are the most significant rules and exceptions that are to be kept in mind when finding or assigning the oxidation numbers:

Rule 1: When in its pure or free elemental form, the atom will have an oxidation number of zero.

What is meant by the pure element? A pure or free element is referred to the atom that is not combined with the other elements. For example, Ag, He, Cu, etc. The oxidation number for such atoms is zero.

Rule 2: The oxidation number for the monatomic ions is always the same as its charge.

An ion is referred to as an atom with a charge because of the gain or loss of the electrons. When talking about the charge, it is indicated as the superscript after an atom. If an atom has this, it implies that the same is an ion. 

For instance, the potassium ion is with the +1 charge. Therefore, the oxidation number for the same will also be +1.

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

Unless the metals are in their pure form, the oxidation number of a metal belonging to Group 1 will be +1, and Group 2 will be +2. 

It is to be remembered that although hydrogen belongs to Group 1, it is not a metal, and thus, hydrogen is an exception to this particular rule. 

Lithium is a metal and it belongs to Group 1. So, it can be said that the oxidation number for lithium is +1. On the other hand, magnesium belongs to Group 2 and is a metal. So, it can be said that the oxidation number for magnesium is +2.

Rule 4: There are two possible oxidation numbers for hydrogen- +1 and -1.

Whenever hydrogen is bonded with the nonmetals, the oxidation number for the same will be +1. For example, hydrogen is bonded with chlorine and chlorine is a nonmetal for hydrochloric acid. Therefore, it can be said that the oxidation number for hydrogen is +1.

Whenever hydrogen is bonded with the metals, the oxidation number for the same will be -1. For example, for the sodium hydride, hydrogen is bonded with sodium and it is a metal.

Therefore, it can be said that the oxidation number for hydrogen is -1.

Rule 5: There are three possible oxidation numbers for oxygen- +2, -2, and -1.

Usually, the oxidation number of oxygen is -2. The only exceptions in such cases are peroxides and it is where the oxidation state of oxygen is -1. And it is the compound of oxygen difluoride, where the oxidation number of oxygen is +2. 

Rule 6: The fluorine’s oxidation number in any compounds is -1.

Some examples of fluorine-containing compounds are hydrofluoric acid, hydrogen fluoride, sodium fluoride, and sulfur hexafluoride and the oxidation number of fluorine in all cases is -1.

Rule 7: For the other halogens like Cl, Br, l, usually the oxidation number is -1. An exception to these is when the halogens are combined with fluorine or oxygen.

The oxidation number for chlorine in the compound sodium chloride is -1, in the compound hypochlorous acid is +1, and in the perchloric compound, acid is +7. 

Rule 8: In a neutral compound, the sum of oxidation numbers equals zero.

For instance, the sum of oxidation numbers for the neutral compounds like HCI, NaOH, HF, etc., is always equal to zero.

Conclusion

The oxidation number basically represents the charge or the electrons gained or lost by the atom. The oxidation number for a particular atom can be positive, negative, or neutral. In simple terms the oxidation number can be described as the number allocated to the elements in a chemical combination. To know more about its meaning and how to find oxidation numbers consider going through the above-mentioned article.