The word “valency” from the Latin language (“valēns”) is translated as “having power”. It was first mentioned at the beginning of the 15th century, but its meaning (“drug” or “extract”) had nothing to do with modern interpretation. The founder of the current concept of valency is the famous English chemist E. Frankland. In 1852 he published a work in which all theories and assumptions that existed at that time were rethought. It was Eduard Frankland who introduced the concept of “connecting force”, which became the basis of the doctrine of valency, but the answer to the question “How to find valency?” at that time was not yet formulated.
The further role in the development of the theory was played by the works of Friedrich August Kekule (1857), Archibald Scott Cooper (1858), A. M. Butlerov (1861), A. von Hoffmann (1865). And in 1866, F. A. Kekule in his textbook cited stereochemical models of chemical molecules with a carbon atom of a tetrahedral configuration in the form of drawings, from which it became obvious how to determine the valency, for example, of carbon.
The foundations of the modern theory of chemical bonding are quantum-mechanical concepts, proving 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 are able to form a common electron pair. The chemical bond formed between two atoms as they approach each other is partially overlapped electron clouds. As a result, between two nuclei an electric charge density is formed, to which positively charged nuclei are attracted, and a molecule is formed. Such an idea of the mechanism of interaction of different atoms formed the basis of the theory of chemical bonds or the valence bond method. So still, how to determine the valency? 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 you use the periodic table, it is easy to understand how to determine the valency of an element by the number of electrons in the outer shell of the atom. They are called valence. All elements in each group (located in columns) have the same number of electrons in the outer shells. Elements of the first group (H, Li, Na, K and others) have one valence electron each. The second (Be, Mg, Ca, Sr and so on) - two. The third (B, Al, Ga and others) - three each. The fourth (C, Si, Ge, and others) has four valence electrons each. Elements of the fifth group (N, P, As and others) each 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 group number of the periodic table. However, this is observed for the first three groups of all seven periods and their even and odd rows (periods and rows are located in the rows of the table). Starting from the fourth period and the fourth group (for example, Ti, Zr, Hf, Ku), the elements of the side subgroups located in even rows have in the outer shell the number of electrons different from the group number.
The concept of “valency” has undergone significant changes throughout this time. There is currently no scientific or standardized interpretation of it. Therefore, the ability to answer the question "How to determine the valency?" Usually used for methodological purposes. The valency is considered to be the ability of atoms, entering into reactions, to form molecules with chemical bonds, which are called covalent. Therefore, valency can only be expressed as an integer.
For example, how to determine the valency of a sulfur atom in compounds such as hydrogen sulfide or sulfuric acid. For a molecule where a sulfur atom is bonded to two hydrogen atoms, the hydrogen valence of sulfur will be two. In a sulfuric acid molecule, its oxygen valency is six. In both cases, the valency numerically coincides 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 during the formation of the bond is shifted to the sulfur atom, which is more electronegative). In the H2SO4 molecule, the oxidation state of the sulfur atom is +6 (since the electron density is shifted to a more electronegative oxygen atom).