What is the oxidation value?

According to the explanation of the relationship and difference between oxides and other valence states in the multimedia education resource library of China Education;

First, the concept of oxidation number and the rules for determining oxidation number

Oxidation number, also called oxidation state, is a chemical concept developed on the basis of valence theory and the concept of electronegativity of elements, which marks the combined state of elements in compounds to some extent. When balancing the redox reaction equation according to the fluctuation of valence and electron transfer, it is often difficult to determine the valence of elements for other substances except simple ionic compounds. For some compounds or atomic groups with complex structures, it is more difficult to determine their electron transfer in the reaction, so it is difficult to express the valence state of each element in the substance. 1948, Gladstone, an American chemistry professor, first proposed to use the word "oxidation number" to replace the valence of elements when balancing the redox reaction equation, thus simply indicating the transfer of electrons in the redox reaction, and then indicating the oxidation state of each element in the substance. He stipulated that the oxidation number should be expressed in Roman numerals to distinguish it from the valence expressed in Arabic numerals. After that, many chemical researchers put forward various views on oxidation number. Before the 1960s, the concepts of positive and negative valence and oxidation number were mixed in many cases.

In the early 1970s, IUPAC further defined the concept of oxidation number in the nomenclature of inorganic chemistry, and made some provisions on the solution of oxidation number. These regulations are very strict, but it is inconvenient to find the oxidation number of elements in a compound. For example, it is sometimes unclear how many bonding electrons are in a compound and how they are distributed. When the electronegativity of two elements in the compound is similar, such as NCl3 and S4N4, it is difficult to determine the oxidation number of these elements according to the above provisions.

The concept of oxidation number can be defined as follows: in a simple substance or compound, it is assumed that the electrons in each chemical bond are distributed to the atom with greater electronegativity among the two connected atoms, and the charge number of an element atom thus obtained is the oxidation number of the element. It can be seen that the oxidation number is an artificial and empirical concept, and it is a number specified according to certain rules, which is used to represent the formal charge number (or apparent charge number) of elements in the combined state. As the name implies, this form of charging has only formal significance.

Japanese chemistry professor Ryuichi Toyama (1952) and the famous American chemist Pauling (1975) respectively commented that some rules have been formulated for determining the oxidation number of elements. Now recognized chemical rules are:

1. In simple substance, the oxidation number of elements is zero.

2. In ionic compounds, the oxidation number of an element atom is equal to the charge number of a monatomic ion of the element.

3. In the valence compounds with known structure, when the electron pairs belonging to two atoms are allocated to the atoms with large electronegativity, the apparent charges left on the two atoms respectively are their oxidation numbers. For example, in H2O, the oxidation number of oxygen atom is-Ⅱ, and the oxidation number of hydrogen atom is+Ⅰ. For the valence bond between two atoms of the same element, the oxidation number of the element is zero. If an element in a compound has two or more valence bonds, the oxidation number of the element is the algebraic sum of the oxidation numbers represented by each bond.

4. In a valence compound with unknown structure, the oxidation number of one element can be calculated from the oxidation number of other elements in the compound according to the following provisions: the total charge number of molecules or complex ions is equal to the algebraic sum of the oxidation numbers of each element.

5. The oxidation numbers of several elements are as follows: (1) Except that the oxidation number of hydrogen in metal hydrides (such as LiH and CaH2) is-Ⅰ, the oxidation number of hydrogen in other hydrogen compounds is+Ⅰ. (2) In general, the oxidation number of oxygen is-Ⅱ, except that the oxidation number of oxygen in H2O2 and hydrogen peroxide is-Ⅰ; OF2 is+Ⅱ. (3) The oxidation number of fluorine in all its compounds should be-Ⅰ; Other halogens are positive oxidation numbers, except when they are combined with more electronegative halogens (such as ClF and ICl3) or when they are combined with oxygen, the oxidation number is-ⅰ.

For a compound or simple substance, the oxidation number of elements can be determined according to the above laws, regardless of molecular structure and bond type. Therefore, it is much more convenient to use oxidation number than valence for redox reaction. Now oxidation number has become a basic concept in chemistry, which is used to define concepts related to redox reaction and balance redox reaction equations.

However, in the current middle school chemistry textbooks in China, the concept of redox reaction is still defined by positive and negative valence. Comparing the concept and application of oxidation number mentioned above with the definition and application of valence concept in middle school chemistry textbooks, we can see that the valence defined in middle school chemistry textbooks actually refers to oxidation number, which is different from the valence concept in modern chemistry (such as applied in college chemistry courses).

Second, the difference between the two concepts of oxidation number and valence

As mentioned above, the concept of oxidation number is developed from the concept of positive and negative valence, which not only shows that they are historically related, but also shows that oxidation number and valence are two different concepts. The original intention of valence is a certain proportional relationship between the atoms of one element and those of other elements, so valence should not be non-integer. For example, in Fe3O4, Fe actually has two valence states of +2 and +3, and its molecular composition is: Fe+3 Fe+2 [Fe+3O4]. The oxidation number is a formal charge number, so it can be a fraction. After introducing the concept of oxidation number, the concept of valence can keep the original meaning of atomic number ratio, and it is unnecessary to use terms such as "average valence" which are easy to blur 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 types of chemical bonds in molecules. For the same substance, the valence and oxidation number of the same element are generally different. For ionic compounds, the electricity price of a simple ion formed by the gain and loss of an atom's electrons is exactly equal to the oxidation number of the element. The electrovalence number of other ions is not necessarily equal to the oxidation number of elements in them. For compounds with valence of * * *, the oxidation number of elements is different from that of * * *. First, there are positive and negative oxidation numbers, which can be fractions; * * * The price is neither positive nor negative, nor can it be a fraction. Second, the oxidation number and valence of the same element in the same substance are not necessarily the same. For example, the oxidation numbers of H and N in H2 and N2 molecules are both 0, and their valences are 1 and 3, respectively. In H2O2 molecule, the valence of O is 2 and its oxidation number is-1. In CH3Cl, the valence of carbon is 4, the oxidation number of carbon is +2, and the valence bond number between carbon and oxygen atom is 3.

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