word "valence of the" Latin language («valēns») means "having power."For the first time it is mentioned in the early 15th century, but its value ("drug" or "extract") had nothing to do with a modern interpretation.The founder of this idea of valence is a famous English chemist E. Frankland.It is in 1852 published a paper in which were reinterpreted all the theories and assumptions that existed at that time.It Edward Frankland introduced the concept of "connecting the power," which became the basis of the doctrine of valence, but the answer to the question "How to find the valence of?" At that time had not yet been formulated.
further role in the development of the theory was played by the Friedrich August Kekule (1857), Archibald Scott Cooper (1858), AM Butlerov (1861), A. von Hoffmann (1865).A 1866 FA Kekule in his textbook cited stereochemical model molecules chemicals with the carbon atom tetrahedral configuration in the form of drawings, which became evident how to determine the valence of, for example, carbon.
modern theory of chemical bonding are quantum-mechanical concepts, proving that as a result of interaction between two atoms formed a common pair of electrons.The atoms with unpaired electrons with parallel spins, repel, and with antiparallel able to form common electron pair.The chemical bond formed between two atoms as they approach, a partial overlap of the electron clouds.As a result, between the two cores by the density of electric charge, which is attracted to the positively charged nuclei, and formation of a molecule.Such an understanding of the mechanism of interaction between different atoms formed the basis of the theory of chemical bonding and valence bond method.So after all, how to determine the valence?It is necessary to determine the number of links that can form atom.Otherwise, you can say that you 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 by the number of electrons in the outer shell of the atom.They are called valence.All the elements in each group (arranged in columns) in the outer shells have the same number of electrons.In the first group of elements (H, Li, Na, K, etc.) has one valence electrons.The second (Be, Mg, Ca, Sr, and so on) - two.A third (B, Al, Ga, and others) - three.In the fourth (C, Si, Ge, etc.) - with four valence electrons.In the fifth group element (N, P, As, etc.) by five valence electrons.You can continue further, since it is 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, it is observed for the first three groups of seven periods and even and odd rows (the periods and the rows are arranged in the rows of the table).Since the fourth period and the fourth group (e.g., Ti, Zr, Hf, Ku) by sub-elements that are in the even rows are in the outer shell of the number of electrons, other than the group number.
concept of "valence of" all this time has undergone significant changes.Currently, it is not a scientific or standardized interpretation.Therefore, the ability to answer the question "How to determine the valence?" Is usually used for methodological purposes.Valence is considered to be the ability of atoms react to form molecules with chemical bonds, called covalent.Therefore valence may be expressed in integer.
For example, how to determine the valence of the sulfur atom in compounds such as hydrogen sulfide or sulfuric acid.For a molecule where a sulfur atom is bonded to two hydrogen atoms, a sulfur valence of hydrogen is equal to two.In the molecule of sulfuric acid its valence of oxygen is six.And in fact, in both cases the valence numerically coincides with the absolute value of the oxidation state of the sulfur atom in these molecules.In a molecule of H2S its oxidation state is 2 (since the formation of the electron density is shifted due to a sulfur atom, it is increasingly electronegative).In the molecule of H2SO4 degree of oxidation of the sulfur atom is equal to six (as the electron density is shifted to the more electronegative oxygen atom).
