The level of knowledge about the structure of atoms and molecules in the 19th century did not allow explaining the reason why atoms form a certain number of bonds with other particles. But the ideas of scientists were 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 outstanding English chemist of the 19th century Edward Frankland introduced the term "bond" into scientific use to describe the process of 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 attaches three hydrogen atoms in the ammonia molecule.
In May 1852, Frankland hypothesized that there was a specific number of chemical bonds that an atom could form with other tiny particles of matter. Frankland used the phrase "connecting force" to describe what would later be called valence. The British chemist determined how muchchemical bonds form atoms of individual elements known in the middle of the 19th century. Frankland's work was an important contribution to modern structural chemistry.
Developing attitudes
German chemist F. A. Kekule proved in 1857 that carbon is a tetrabasic one. In its simplest compound - methane - there are bonds with 4 hydrogen atoms. The scientist used the term "basicity" to denote the property of elements to attach a strictly defined amount of other particles. In Russia, data on the structure of matter were systematized by A. M. Butlerov (1861). The theory of chemical bonding received further development thanks to the doctrine of the periodic change in the properties of elements. Its author is another outstanding Russian chemist, D. I. Mendeleev. He proved that the valency of chemical elements in compounds and other properties are due to the position they occupy in the periodic system.
Graphic representation of valence and chemical bond
The possibility of a visual representation of molecules is one of the undoubted advantages of the theory of valency. The first models appeared in the 1860s, and since 1864 structural formulas have been used, which are circles with a chemical sign inside. Between the symbols of atoms, a dash indicates a chemical bond, and the number of these lines is equal to the value of the valency. In the same years, the first ball-and-stick models were made (see photo on the left). In 1866, Kekule proposed a stereochemical drawing of the atom.carbon in the form of a tetrahedron, which he included in his textbook Organic Chemistry.
The valency of chemical elements and the occurrence of bonds was studied by G. Lewis, who published his works in 1923 after the discovery of the electron. This is the name of the smallest negatively charged particles that are part of the shells of atoms. In his book, Lewis used the dots around the four sides of the chemical element symbol to represent valence electrons.
Valency for hydrogen and oxygen
Before the creation of the periodic system, the valency of chemical elements in compounds was usually 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, properties were determined in compounds with monovalent hydrogen (the valence of the second element is indicated by a Roman numeral):
- HCl - chlorine (I):
- H2O - oxygen (II);
- NH3 - nitrogen (III);
- CH4 - carbon (IV).
In oxides K2O, CO, N2O3, SiO 2, SO3 determined the oxygen valency of metals and non-metals by doubling the number of added 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 regularities in the formation of a chemical bond involving common electroniccouples:
- Typical hydrogen valency is I.
- Usual oxygen valency - II.
- For non-metal elements, the lowest valency can be determined by the formula 8 - the number of the group in which they are located 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 their group number in the periodic table.
Determination of the valence of chemical elements according to 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 Mn2O7 the oxygen valency is II.
- Calculate the total value, for which you need to multiply the valence by the number of atoms of the same chemical element in the molecule: 27=14.
- Determine the valence of the second element for which it is unknown. Divide the value obtained in step 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
Valence values for hydrogen and oxygen are different. For example, sulfur in the compound H2S is bivalent, and in the formula SO3 it is hexavalent. Carbon forms monoxide CO and dioxide CO2 with oxygen. In the first compound, the valency of C is II, and in the second, IV. Same value in methane CH4.
Mostelements exhibits not a constant, but a variable valence, for example, phosphorus, nitrogen, sulfur. The search for the main causes of this phenomenon led to the emergence of chemical bond theories, ideas about the valence shell of electrons, and molecular orbitals. The existence of different values of the same property was explained from the standpoint of the structure of atoms and molecules.
Modern ideas about valency
All atoms consist of a positive nucleus surrounded by negatively charged electrons. The outer shell that they form is unfinished. The completed structure is the most stable, containing 8 electrons (an octet). The emergence 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 accepting electrons or giving away unpaired ones - depending on which process is easier. If an atom provides for the formation of a chemical bond negative particles that do not have a pair, then it forms as many bonds as it has unpaired electrons. According to modern concepts, the valence of atoms of chemical elements is the ability to form a certain number of covalent bonds. For example, in a hydrogen sulfide molecule H2S, sulfur acquires valency 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 one, “+” is added to the valency value.
With the donor-acceptor mechanism, electron pairs of one element and free valence orbitals of another element take part in the process.
Dependence of valency on the structure of the atom
Let's look at the example of carbon and oxygen, how the valence of chemical elements depends on the structure of the substance. The periodic table gives an idea of the main characteristics of the carbon atom:
- chemical sign - C;
- element number - 6;
- core charge - +6;
- protons in the nucleus - 6;
- electrons - 6, including 4 external ones, of which 2 form a pair, 2 are unpaired.
If a carbon atom in CO monoxide forms two bonds, then only 6 negative particles come to its use. To acquire an octet, it is necessary that the pairs form 4 external negative particles. Carbon has valency IV (+) in dioxide and IV (–) in methane.
Ordinal number of oxygen 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. A typical oxygen valence is II (–).
Valency and oxidation state
In many cases it is more convenient to use the concept of "oxidation state". This is the name given to the charge of an atom that it would acquire if all the bonding electrons were transferred to an element that has a higher value of electronegativity (EO). The oxidation number in a simple substance iszero. The “–” sign is added to the oxidation state of the more EO element, the “+” sign is added to the less electronegative one. For example, for metals of the main subgroups, oxidation states and ion charges are typical, equal to the group number with a “+” sign. In most cases, the valency and oxidation state of atoms in the same compound are numerically the same. Only when interacting with more electronegative atoms, the oxidation state is positive, with elements in which the EO is lower, it is negative. The concept of "valency" is often applied only to substances of a molecular structure.
