The level of knowledge about the structure of atoms and molecules in the 19th 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 are ahead of their time, and valency is still being studied as one of the basic principles of chemistry.
From the history of the concept of “valency of chemical elements”
The prominent English chemist of the 19th century, Edward Frankland, introduced the term “connection” into scientific use to describe the process of interaction of atoms with each other. The scientist noted that some chemical elements form compounds with the same number of other atoms. For example, nitrogen attaches three hydrogen atoms in an 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 “connecting power” to describe what would later be called valency. The British chemist established how many chemical bonds form the atoms of the individual elements known in the mid-19th century. Frankland's work has made an important contribution to modern structural chemistry.
Development of attitudes
German chemist F.A. Kekule proved in 1857 that carbon is tetrabasic. In its simplest compound, methane, bonds arise with 4 hydrogen atoms. The term "basicity" the scientist used to denote the property of elements to attach a strictly defined number of other particles. In Russia, data on the structure of matter were systematized by A. M. Butlerov (1861). The theory of chemical bonding was further developed through the study of the periodic change in the properties of elements. Its author is another prominent Russian chemist, D.I. Mendeleev. He proved that the valency of chemical elements in compounds and other properties are due to the position that they occupy in the periodic system.
Graphic representation of valency and chemical bonding
The ability to visualize molecules is one of the undoubted advantages of the valence theory. The first models appeared in the 1860s, and since 1864 structural formulas have been used , which are circles with a chemical sign inside. A chemical bond is indicated by a dash between the symbols of atoms , and the number of these lines is equal to the valency value. In the same years, the first ball-rod models were made (see 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 valency of chemical elements and the appearance of bonds were studied by G. Lewis, who published his works in 1923 after the discovery of the electron. So called negatively charged tiny particles that are part of the shells of atoms. In his book, Lewis applied dots around the four sides of a chemical element symbol to display valence electrons.
Hydrogen and oxygen valency
Before the creation of the periodic system, the valency of chemical elements in compounds was customarily compared with those atoms for which it is known. Hydrogen and oxygen were chosen as standards. Another chemical element attracted or replaced a certain number of H and O atoms.
In this way, the properties in compounds with monovalent hydrogen were determined (the valency of the second element is indicated by a Roman numeral):
- HCl - chlorine (I):
- H 2 O is oxygen (II);
- NH 3 - nitrogen (III);
- CH 4 is carbon (IV).
In oxides K 2 O, CO, N 2 O 3 , SiO 2 , SO 3 , the oxygen valency of metals and nonmetals was determined, doubling the number of attached O atoms. The following values ​​were obtained: K (I), C (II), N (III) , Si (IV), S (VI).
How to determine the valency of chemical elements
There are patterns of the formation of a chemical bond with the participation of common electron pairs:
- Typical hydrogen valency is I.
- The usual oxygen valency is II.
- For non-metal elements, the lowest valency can be determined by the formula 8 - No. of the group in which they are in the periodic system. The highest, if possible, is determined by the group number.
- For elements of secondary subgroups, the maximum possible valency is the same as the number of their group in the periodic table.
Determination of the valency of chemical elements by the formula of the compound is carried out using the following algorithm:
- Write the known value for one of the elements above the chemical sign. For example, in Mn 2 O 7, the oxygen valency is II.
- Calculate the total value, for which it is necessary to multiply the valency by the number of atoms of the same chemical element in the molecule: 2 * 7 = 14.
- Determine the valency of the second element for which it is unknown. Divide the value obtained in Section 2 by the number of Mn atoms in the molecule.
- 14: 2 = 7. The valency of manganese in its higher oxide is VII.
Constant and variable valency
The valency values ​​for hydrogen and oxygen are different. For example, sulfur in the H 2 S compound is divalent, and in the SO 3 formula it is hexavalent. Carbon forms CO monoxide and CO 2 dioxide with oxygen. In the first compound, valency C is II, and in the second, IV. The same value in methane CH 4 .
Most elements do not exhibit a constant, but a variable valency, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of theories of chemical bonds, 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 valency
All atoms are composed of a positive nucleus surrounded by negatively charged electrons. The outer shell that 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 atoms.
The rule for the formation of compounds is the completion of the shell by receiving electrons or recoiling unpaired - depending on which process is easier to go through. If an atom provides negative particles without a pair for the formation of a chemical bond, then it forms as many bonds as it has 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 the hydrogen sulfide molecule H 2 S, sulfur acquires valence II (-), since each atom takes part in the formation of two electron pairs. The “-" sign indicates the attraction of an electron pair to a more electronegative element. For a less electronegative to the value of valency add "+".
With the donor – acceptor mechanism, the process involves electron pairs of one element and free valence orbitals of another.
The dependence of valency on the structure of the atom
Consider the example of carbon and oxygen, how the valency of chemical elements depends on the structure of the substance. The periodic table gives an idea of ​​the main characteristics of a carbon atom:
- chemical sign - C;
- item number - 6;
- core charge - +6;
- protons in the nucleus - 6;
- 6 electrons, including 4 external, of which 2 form a pair, 2 - unpaired.
If the carbon atom in CO monoooxide forms two bonds, then only 6 negative particles come into its use. To acquire an octet, it is necessary that the pairs form 4 external negative particles. Carbon has a valency of IV (+) in dioxide and IV (-) in methane.
The oxygen serial number is 8, the valence shell consists of six electrons, 2 of them do not form pairs and take part in chemical bonding and interaction with other atoms. Typical oxygen valency is II (-).
Valence and oxidation state
In many cases, it is more convenient to use the concept of “oxidation state”. This is the name of the atomic charge, which he would have acquired if all the binding electrons passed to the element that has a higher electronegativity (EO) value. The oxidizing number in a simple substance is zero. The sign “-” is added to the oxidation state of more EO elements, and the “+” sign is less electronegative. For example, for metals of the main subgroups, oxidation states and ion charges equal to the group number with the “+” sign are typical. In most cases, the valency and oxidation state of atoms in the same compound numerically coincide. Only when interacting with more electronegative atoms, the oxidation state is positive, with elements whose EO is lower, negative. The concept of "valency" is often applied only to substances of molecular structure.