Valence is an attribute of an element. What is it, the nature of free atoms (that is, atoms that are not bound to a compound) or the nature of bound atoms (that is, atoms in a compound molecule)?
From the history briefly described above, it is not difficult to see that the concept of valence has two meanings and two development trends: one meaning, valence is the reaction ability of free atoms (or isolated atoms), and the other meaning, valence is the structural characteristics of atoms in molecules, that is, the properties of bonding atoms.
As mentioned earlier, the concept of valence theoretically comes from affinity. Frankland pointed out that no matter what the nature of binding atoms is, the binding force that attracts atoms always needs the same number of atoms or groups to satisfy. This shows that atoms have a chemical force to attract other atoms, which is the ability of atoms to combine to form compounds. Atoms rely on this chemical force to combine with each other and have chemical reactions. So it is a kind of reaction ability of atoms (specifically, free atoms). Kekule pointed out that atoms or bases of elements have certain affinity values. A certain number of affinity units or atoms, some elements are one atom, some are two atoms, etc. , essentially contains the same idea, that is, element atoms rely on this affinity to combine with a certain number of other atoms. This idea is more clearly expressed in the concept of chemical structure elaborated by butler Love. He pointed out: "Assuming that a chemical atom has a certain and limited chemical affinity value, the chemical atom will join forces to participate in the formation of matter ..." (1) The chemical affinity value mentioned here is also the nature of free atoms, which is the ability of chemical atoms to combine. He also directly said that the number of atoms is the property of free atoms and the value of free affinity units. In fact, in the19th century, valence was once regarded as a sign that one element was different from other elements. Each chemical element is different from other elements according to its chemical properties, and this difference can be attributed to the possibility of forming a certain valence. Elements rely on this possibility to interact chemically, turning the possibility into reality, that is, forming a chemical combination. At the end of 19, Werner explained the structure of the complex with principal valence and secondary valence, which also included the idea that valence is the atomic binding ability. At the beginning of the 20th century, in valence electron theory, based on the tendency of atoms to lose or gain electrons to form a stable electronic structure of inert gas elements, the formation of ionic valence and valence was explained. In essence, their combining ability (chemical reaction ability) is explained by the outermost electrons or unpaired electrons. The number of unpaired electrons is equal to the number of valence, and unpaired electrons combine with atoms to form compounds, which means that they are equivalent to unpaired electrons. This idea has been inherited and developed in the theory of quantum chemistry. For example, in the research work in Shanghai and London, it is assumed that what happens when oxygen atoms are close to each other and combine, which is essentially to regard an electron of hydrogen atom as a binding ability.
In the evolution of the concept of valence, another kind of understanding and concept has emerged, which regards valence as the nature of bound atoms (atoms in already formed molecules). The concept of valence in this sense has also gone through a series of evolution stages. ① In the first stage, the valence of atoms in a molecule is determined by the number of single chemical bonds formed by the atom and other atoms. In this case, chemical bonds are considered to be the result of mutual saturation of free affinity units. This concept came into being in 1950s and 1960s of19th century, and was later embodied in valence bond theory. In the second stage, due to the accumulation of information on the structure of complexes, especially the crystal structure of inorganic compounds, chemists have a new understanding of the relationship between atoms in compounds. For example, ZnS, according to previous knowledge or chemical formula, both S and Zn in this compound are divalent, but in the crystal structure, there are four S atoms around each Zn atom, and there are also four Zn atoms around each S atom. Therefore, the concepts of coordination number and valence are gradually distinguished. In a sense, a new concept-coordination number is differentiated from the concept of chemical valence, and coordination number is the property of binding atoms. In the third stage, it is found that the valence of bound atoms (determined according to the chemical formula) is different from the actual valence bond number. As pointed out just now, in the balance of redox reaction equation, it is often difficult to distinguish positive valence from negative valence for molecules or simple substances, so the concept of oxidation value is put forward.
In modern chemical literature, we can still see two different understandings of the concept of valence. For example, the former Soviet scholar Tatev (вмтатвскииии) proposed in the book Quantum Mechanics and Molecular Structure Theory that in the classical chemical structure theory, the concept of valence reflects atoms. Tatevsky believes that in valence electron theory, the situation is just the opposite. The valence represents the nature of a free atom, and its valence is equal to the number of unpaired electrons. If we want to determine the valence of atoms in a molecule (that is, the valence of integrated atoms), we should regard its valence as equal to that of free atoms. The author of the book History of Valence Theory, which is quoted many times in this book, disagrees with Tatev's analysis of the classical valence concept. The author of this book believes that in the classical theory of chemical structure, valence is the force that produces chemical phenomena, and it is an integer measure of the free affinity unit of free atoms, so it is the nature of free atoms. For another example, the definition of valence in the Encyclopedia Britannica (version 1973) is that the valence in chemistry (that is, the valence we say in this book) is the nature of an element, which determines the ability of one atom of the element to combine with other atoms. American chemists A·W· Greenstone and S·P· Harris pointed out when they talked about the concept of valence in the book "Chemical Summary": frankland believed that the binding capacity of mutually attractive elements was always satisfied by the same number of atoms, and "this binding capacity of different atoms is called valence". These two definitions are essentially the same. Valence is understood as the binding capacity of atoms, which is the reason for atomic binding, so it is the property of free atoms. L Pauling clearly pointed out in his book Chemistry that the valence of an element refers to the valence formed by an atom and other atoms. Xu Guangxian put forward the definition of * * valence: the * * valence Vj of the atom Aj in a molecule can be defined as the number of * * electrons it accepts when forming a molecule ... These two definitions are essentially the same, and the valence is understood as the nature of the atom in the formed molecule, which is the valence number of the atom in the molecule.
Whether as a free atom or a bonded atom, valence means that atoms are bonded to each other. Valence, as the nature of free atoms, shows the potential ability to combine with each other, while valence, as the nature of binding atoms, shows the combination of reality, that is, the combination that has been realized. Speaking of which, it naturally involves the relationship between valence and chemical bond.