Determine the valence of chemical elements

The level of knowledge of the structure of atoms and molecules in the XIX century did not allow us to explain the reason why atoms form a certain number of bonds with other particles. But the ideas of scientists have outstripped their time, and valency is still being studied as one of the basic principles of chemistry.

From the history of the concept of "the valence of chemical elements"

The eminent English chemist of the 19th century, Edward Frankland, introduced the term "connection" into scientific use to describe the interaction of atoms with each other. The scientist noticed that some chemical elements form compounds with the same number of other atoms. For example, nitrogen adds three hydrogen atoms to the ammonia molecule.

In May 1852, Frankland hypothesized that there is a specific number of chemical bonds that an atom can form with other tiny particles of matter. Frankland used the phrase "connective force" to describe what would later be called valency. The British chemist established how many chemical bonds form the atoms of individual elements known in the middle of the XIX century. The work of Frankland has become an important contribution to modern structural chemistry.

Development of views

German chemist F.A. Kekule proved in 1857 that carbon is a four-base. In its simplest connection - methane - there are connections with 4 hydrogen atoms. The term "basicity" the scientist used to identify the properties of elements to attach a strictly defined number of other particles. In Russia, data on the structure of matter was systematized by AM Butlerov (1861). Further development of the theory of chemical bonding was due to the theory of periodic changes in the properties of elements. Its author is another outstanding Russian chemist, DI Mendeleev. He proved that the valence of chemical elements in compounds and other properties are due to the position they occupy in the periodic system.

Graphical representation of valency and chemical bonding

The possibility of visualization of molecules is one of the undoubted merits of valence theory. The first models appeared in the 1860s, and since 1864 structural formulas have been used , representing circles with a chemical sign inside. Between the symbols of atoms a dash denotes a chemical bond, and the number of these lines is equal to the value of valence. In the same years, the first ball-and-rod models were produced (see the photo on the left). In 1866, Kekule proposed a stereochemical drawing of a carbon atom in the form of a tetrahedron, which he included in his textbook "Organic Chemistry."

The valence of chemical elements and the emergence of connections was studied by G. Lewis, who published his works in 1923 after the discovery of the electron. The so-called negatively charged tiny particles, which are part of the shells of atoms, are called. In his book, Lewis applied points around the four sides of the symbol of a chemical element to represent valence electrons.

Valence for hydrogen and oxygen

Before the creation of the periodic system, the valence of the chemical elements in the compounds was taken to be compared with those atoms for which it is known. As standards, hydrogen and oxygen were chosen. Another chemical element attracted or replaced a certain number of H and O atoms.

In this way, properties were determined in compounds with monovalent hydrogen (the valence of the second element is denoted by the Roman numeral):

• HCl - chloro (I):
• H ₂ O - oxygen (II);
• NH ₃ - nitrogen (III);
• CH ₄ - carbon (IV).

In the oxides K ₂ O, CO, N ₂ O ₃ , SiO ₂ , SO ₃ , the oxygen valence of metals and nonmetals was determined, doubling the number of O atoms added. The following values were obtained: K (I), C (II), N (III) , Si (IV), S (VI).

How to determine the valence of chemical elements

There are regularities in the formation of a chemical bond involving common electronic pairs:

• The typical hydrogen valence is I.
• The usual oxygen valence is II.
• For nonmetallic elements, the lower valence can be determined by the formula 8 - the number of the group in which they are in the periodic table. The higher, if it is possible, is determined by the group number.
• For the elements of the subgroups, the maximum possible valency is the same as the number of their group in the periodic table.

Determination of the valence of chemical elements by the formula of the compound is carried out using the following algorithm:

1. Write down above the chemical sign a known value for one of the elements. For example, in Mn ₂ O _{7, the} oxygen valence is II.
2. Calculate the total value, for which it is necessary to multiply the valence by the number of atoms of the same chemical element in the molecule: 2 * 7 = 14.
3. Determine the valence of the second element for which it is unknown. Divide the value obtained in subsection 2 by the number of Mn atoms in the molecule.
4. 14: 2 = 7. The valence of manganese in its higher oxide is VII.

Constant and variable valence

Values for hydrogen and oxygen are different. For example, sulfur in the compound H ₂ S is divalent, and in the formula SO ₃ - hexavalent. Carbon forms with oxygen CO monoxide and CO ₂ dioxide. In the first compound, the valence of C is II, and in the second, IV. The same value in methane CH ₄ .

Most elements show not a constant, but variable valence, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of theories of chemical bonding, ideas about the valence shell of electrons, molecular orbitals. The existence of different values of the same property has been explained in terms of the structure of atoms and molecules.

Modern concepts of valence

All atoms consist of a positive nucleus surrounded by negatively charged electrons. The outer shell, which they form, is unfinished. The completed structure is the most stable, it contains 8 electrons (octet). The appearance of a chemical bond due to common electron pairs leads to an energetically favorable state of the atoms.

The rule for the formation of compounds is the completion of the shell by taking electrons or giving out unpaired ones, depending on which process passes more easily. If an atom provides negative particles for the formation of a chemical bond, which do not have a pair, then it forms as many bonds as there are unpaired electrons. According to modern concepts, the valency of atoms of chemical elements is the ability to form a certain number of covalent bonds. For example, in a hydrogen sulfide H ₂ S molecule, sulfur acquires the valence II (-), since each atom takes part in the formation of two electron pairs. The sign "-" indicates the attraction of the electron pair to the more electronegative element. At less electronegative for the valence value, add "+".

With the donor-acceptor mechanism, electronic pairs of one element and free valence orbitals of the other take part in the process.

Dependence of the valence on the structure of the atom

Consider, for example, carbon and oxygen, how the valence of chemical elements depends on the structure of matter. Mendeleyev's table gives an idea of the main characteristics of a carbon atom:

• Chemical sign - C;
• Element number is 6;
• The charge of the nucleus is +6;
• Protons in the nucleus - 6;
• Electrons - 6, including 4 external ones, 2 of which form a pair, 2 - unpaired ones.

If a carbon atom in CO monooxide forms two bonds, then only 6 negative particles enter its use. To acquire an octet, it is necessary that the pairs form 4 external negative particles. Carbon has a valence of IV (+) in dioxide and IV (-) in methane.

The ordinal number of oxygen is 8, the valence shell consists of six electrons, two of which do not form a pair and take part in chemical bonding and interaction with other atoms. The typical oxygen valence is II (-).

Valence and oxidation state

In very many cases it is more convenient to use the term "oxidation state". This is the name for the charge of the atom, which it would acquire if all the binding electrons were transferred to an element that has a higher value of electronegativity (EO). The oxidizing number in the simple substance is zero. To the degree of oxidation more than the EO of the element is added the sign "-", less electronegative - "+". For example, for metals of main subgroups, oxidation degrees and ion charges are equivalent to the number of the group with the sign "+". In most cases, the valence and degree of oxidation of atoms in the same compound are numerically the same. Only when interacting with more electronegative atoms is the degree of oxidation positive, with elements for which EO is lower, is negative. The concept of "valency" is often applied only to substances of molecular structure.