In Chemistry, What Is the Oxidation Number?

Oxidation number is also called oxidation state. It is a chemical concept developed on the basis of valence theory and element electronegativity. It indicates the chemical state of an element in a compound to a certain extent. When balancing redox reaction equations based on valence rise and fall values and electron transfer conditions, it is often difficult to determine the valence number of an element for other substances except simple ionic compounds; for some compounds with complex structures or atomic groups, it is more difficult to determine The electron transfer in the reaction makes it difficult to indicate the valence of each element in the substance.

In the early 1970s, the International Union of Pure and Applied Chemistry (IUPAC)

The concept of oxidation number can be defined as follows: In a simple substance or compound, suppose that the electron in each chemical bond is assigned to one of the two electrons that are more electronegative, so the charge number of one atom of an element is The oxidation number of the element. It can be seen that the oxidation number is a certain artificial and empirical concept. It is a number specified according to certain rules and is used to characterize the element in the combined state.

Japanese chemistry professor Ryoichi Kiriyama (in 1952) and well-known American chemist Pauling (1975) and others published separate statements that set out some rules for the method of determining the oxidation number of an element. The generally accepted rules in the chemical industry are:

1. in

For a certain compound or element, the oxidation number of the elements can be determined as long as the above rules are followed, without having to consider the structure of the molecule and the type of bond. Therefore, it is much more convenient to use oxidation number for redox reaction than to use valence. Oxidation number has now become a basic concept in chemistry, used to define the concepts related to redox reactions and to balance the redox reaction equations.

However, in China's current middle school chemistry textbooks, positive and negative valences are still used to define related concepts of redox reactions. Comparing the concept of oxidation number and its application with the definition and application of valence in middle school chemistry textbooks, we can see that the valence defined in chemistry textbooks in middle school actually refers to oxidation number, which is different The concept of valence in modern chemistry (as applied in university chemistry courses).

Difference between oxidation number and valence

As mentioned earlier, the concept of oxidation number is derived from the development of the concept of positive and negative valence, which not only shows that they have a historical connection, but also shows that oxidation number and valence are two different concepts. The original intent of valence is a certain proportional relationship between the number of atoms of two elements when atoms of one element are combined with atoms of other elements, so the valence should not be non-integer. For example, in Fe ₃ O ₄ , Fe actually has two valence states: +2 and +3 valence, and its molecular composition is: 2Fe ³⁺ · Fe ²⁺ · 4O ^2- . The oxidation number is a formal charge number, so it can be a fraction. With the introduction of the concept of oxidation number, the concept of valence can maintain the meaning of the original number ratio of atoms, without having to use terms such as "average valence" that can easily obscure the concept of valence. This is one of the reasons why the concept of oxidation number is distinguished on the basis of the concept of positive and negative valence.

The significance and value of valence are related to the type of chemical bond in the molecule. For the same substance, the values of the valence and oxidation number of the same element are generally different. For ionic compounds, the charge of a simple ion formed by the gain and loss of an atom by an atom is exactly equal to the oxidation number of the element. The valence number of other ions is not necessarily equal to the oxidation number of the element. For covalent compounds, the oxidation number of an element is different from the covalent number. First, the oxidation number is positive or negative and can be a fraction; covalent numbers are not positive or negative and cannot be fractions. Second, the numbers of oxidation and covalent numbers of the same element in the same substance are not necessarily the same. For example, the oxidation numbers of H and N in both H ₂ and N ₂ molecules are 0, and their covalent numbers are 1 and 3, respectively. The covalent number of O in the H ₂ O ₂ molecule is 2 and its oxidation number is -1. In CH ₃ Cl, the covalent number of carbon is 4, the oxidation number of carbon is -2, and the number of covalent bonds between carbon and hydrogen atoms is 3. ^[1]

State of oxidation-reduction by oxidation number

For redox reactions of ionic compounds, electrons are completely lost or completely acquired. However, for covalent compounds, there are electron shifts in the redox reaction, but they have not been completely lost or obtained, so it is more reasonable to use the oxidation number to express it. E.g:

H ₂ + Cl ₂ = 2HCl

The product of this reaction is a covalent compound. The electrons of the hydrogen atom have not been completely lost, and the electrons of the chlorine atom have not been completely obtained, but the formed electron pair deviates from hydrogen and is biased toward chlorine. Expressed by the rise and fall of the oxidation number is chlorine from 0 to -1, hydrogen from 0 to +1. In this way, an increase in the oxidation number is oxidation, and a decrease in the oxidation number is reduction. In redox reactions, the increase in the oxidation number of one element is always equal to the decrease in the oxidation number of another element.