How to determine the valence

The word "valence" from the Latin language ("valēns") is translated as "having power". It was first mentioned in the beginning of the 15th century, but its meaning ("drug" or "extract") had nothing to do with modern interpretation. The founder of the current idea of valence is the famous English chemist E. Frankland. In 1852 he published a paper in which all theories and assumptions that existed at that time were rethought. It was Edward Frankland who introduced the notion of a "connecting force", which became the basis for the doctrine of valence, but the answer to the question "How to find valence?" At that time had not yet been formulated.

A further role in the development of the theory was played by the works of Friedrich August Kekule (1857), Archibald Scott Cooper (1858), A. Butlerov (1861), A. von Hofmann (1865). And in 1866 FA Kekule in his textbook led stereochemical models of molecules of chemical substances with the carbon atom of the tetrahedral configuration in the form of figures, according to which it became obvious how to determine the valence, for example, of carbon.

The basis of the modern theory of chemical bonding is the quantum mechanical representation, which proves that as a result of the interaction of two atoms, a common pair of electrons is formed. Atoms with unpaired electrons having parallel spins repel, and with antiparallel ones they can form a common electron pair. The chemical bond formed between two atoms as they approach each other is a partially overlapping electron cloud. As a result, between the two nuclei an electric charge density is formed, to which positively charged nuclei are attracted, and a molecule is formed. This idea of the mechanism of interaction of different atoms formed the basis of the theory of chemical bonding or the valence bond method. So all the same, how to determine valence? It is necessary to determine the number of bonds that an atom is capable of forming. Otherwise, we can say that we need to find the number of valence electrons.

If we use the periodic table, it is easy to understand how to determine the valence of the element in terms of the number of electrons in the outer shell of the atom. They are called valence. All the elements in each group (located in columns) have the same number of electrons in the outer shells. The elements of the first group (H, Li, Na, K and others) have one valence electron. The second (Be, Mg, Ca, Sr, and so on) has two. The third (B, Al, Ga and others) - three. The fourth (C, Si, Ge and others) has four valence electrons. The elements of the fifth group (N, P, As and others) have five valence electrons. We can continue further, since it is quite obvious that the number of electrons in the outer shell of the electron cloud will be equal to the number of the periodic table group. However, this is observed for the first three groups of all seven periods and their even and odd series (the periods and series are located in the rows of the table). Beginning with the fourth period and the fourth group (for example, Ti, Zr, Hf, Ku), the elements of the subgroups in even rows have in the outer shell a number of electrons different from the group number.

The concept of "valence" for all this time has undergone significant changes. At present there is no scientific or standardized interpretation of it. Therefore, the ability to answer the question "How to determine valence?" Is usually used for methodological purposes. Valence is considered to be the ability of atoms to react, to form molecules with chemical bonds, which are called covalent. Therefore, the valence can be expressed only by an integer.

For example, how to determine the valence of a sulfur atom in compounds such as hydrogen sulphide or sulfuric acid. For a molecule where the sulfur atom is bound to two hydrogen atoms, the hydrogen valence for hydrogen will be two. In the sulfuric acid molecule, its oxygen valence is six. In both cases, the valence coincides numerically with the absolute value of the degree of oxidation of the sulfur atom in these molecules. In the H2S molecule, its oxidation state will be -2 (since the electron density in the formation of the bond is shifted to a sulfur atom, which is electronegative to a greater extent). In the H2SO4 molecule, the degree of oxidation of the sulfur atom is +6 (since the electron density is shifted to the more electronegative oxygen atom).