(German) Valenz). In 1789, William Higgins published a paper in which he suggested the existence of bonds between the smallest particles of matter.

However, an accurate and later fully confirmed understanding of the phenomenon of valence was proposed in 1852 by the chemist Edward Frankland in a work in which he collected and reinterpreted all the theories and assumptions that existed at that time in this regard. . By observing the ability to saturate different metals and comparing the composition of organic derivatives of metals with the composition of non organic compounds, Frankland introduced the concept of " connecting force", thereby laying the foundation for the doctrine of valency. Although Frankland established some particular laws, his ideas were not developed.

Friedrich August Kekule played a decisive role in the creation of the theory of valence. In 1857, he showed that carbon is a tetrabasic (tetraatomic) element, and its simplest compound is methane CH 4. Confident in the truth of his ideas about the valency of atoms, Kekule introduced them into his textbook organic chemistry: basicity, according to the author, is a fundamental property of the atom, a property as constant and unchangeable as atomic weight. In 1858, views almost coinciding with the ideas of Kekule were expressed in the article “ About the new chemical theory» Archibald Scott Cooper.

Three years later, in September 1861, A. M. Butlerov made the most important additions to the theory of valency. He made a clear distinction between a free atom and an atom that has entered into combination with another when its affinity " binds and transforms into a new form" Butlerov introduced the concept of the complete use of the forces of affinity and the “ affinity tension", that is, the energetic nonequivalence of bonds, which is due to the mutual influence of atoms in the molecule. As a result of this mutual influence, atoms, depending on their structural environment, acquire different "chemical significance" Butlerov's theory made it possible to explain many experimental facts concerning the isomerism of organic compounds and their reactivity.

A huge advantage of the valency theory was the possibility of a visual representation of the molecule. In the 1860s. the first molecular models appeared. Already in 1864, A. Brown proposed using structural formulas in the form of circles with symbols of elements placed in them, connected by lines indicating the chemical bond between atoms; the number of lines corresponded to the valency of the atom. In 1865, A. von Hoffmann demonstrated the first ball-and-stick models, in which the role of atoms was played by croquet balls. In 1866, drawings of stereochemical models in which the carbon atom had a tetrahedral configuration appeared in Kekule's textbook.

Modern ideas about valency

Since the emergence of the theory of chemical bonding, the concept of “valency” has undergone significant evolution. Currently, it does not have a strict scientific interpretation, therefore it is almost completely crowded out of scientific vocabulary and is used mainly for methodological purposes.

Mainly under valency chemical elements is understood the ability of its free atoms to form a certain number of covalent bonds. In compounds with covalent bonds, the valence of atoms is determined by the number of two-electron two-center bonds formed. This is precisely the approach adopted in the theory of localized valence bonds, proposed in 1927 by W. Heitler and F. London in 1927. Obviously, if an atom has n unpaired electrons and m lone electron pairs, then this atom can form n+m covalent bonds with other atoms. When assessing the maximum valency, one should proceed from the electronic configuration of the hypothetical, so-called. “excited” (valence) state. For example, the maximum valence of a beryllium, boron and nitrogen atom is 4 (for example, in Be(OH) 4 2-, BF 4 - and NH 4 +), phosphorus - 5 (PCl 5), sulfur - 6 (H 2 SO 4) , chlorine - 7 (Cl 2 O 7).

In some cases, such characteristics of a molecular system as the oxidation state of an element, the effective charge on an atom, the coordination number of an atom, etc. are identified with valence. These characteristics may be close and even coincide quantitatively, but are in no way identical to each other. For example, in the isoelectronic molecules of nitrogen N 2, carbon monoxide CO and cyanide ion CN - a triple bond is realized (that is, the valency of each atom is 3), but the oxidation state of the elements is, respectively, 0, +2, −2, +2 and −3. In the ethane molecule (see figure), carbon is tetravalent, as in most organic compounds, while the oxidation state is formally equal to −3.

This is especially true for molecules with delocalized chemical bonds, for example, in nitric acid, the oxidation state of nitrogen is +5, while nitrogen cannot have a valency higher than 4. The rule known from many school textbooks is “Maximum valence element is numerically equal to the group number in the Periodic Table" - refers solely to the oxidation state. The concepts of “constant valency” and “variable valency” also primarily refer to the oxidation state.

See also

Notes

Links

• Ugay Ya. A. Valence, chemical bond and oxidation state are the most important concepts of chemistry // Soros educational journal. - 1997. - No. 3. - P. 53-57.
• / Levchenkov S.I. Brief outline of the history of chemistry

Literature

• L. Pawling The nature of the chemical bond. M., L.: State. NTI chem. literature, 1947.
• Cartmell, Foles. Valence and structure of molecules. M.: Chemistry, 1979. 360 pp.]
• Coulson Ch. Valence. M.: Mir, 1965.
• Murrell J., Kettle S., Tedder J. Valence theory. Per. from English M.: Peace. 1968.
• Development of the doctrine of valence. Ed. Kuznetsova V. I. M.: Chemistry, 1977. 248 p.
• Valence of atoms in molecules / Korolkov D.V. Fundamentals of inorganic chemistry. - M.: Education, 1982. - P. 126.

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See what “Valency” is in other dictionaries:

VALENCE, a measure of the “connecting power” of a chemical element, equal to the number of individual CHEMICAL BONDS that one ATOM can form. The valence of an atom is determined by the number of ELECTRONS at the highest (valence) level (external... ... Scientific and technical encyclopedic dictionary

VALENCE- (from the Latin valere to mean), or atomicity, the number of hydrogen atoms or equivalent atoms or radicals, a given atom or radical can join the swarm. V. is one of the basis for the distribution of elements in the periodic table D.I.... ... Great Medical Encyclopedia

Valence- * valence * valence the term comes from lat. having power. 1. In chemistry, this is the ability of atoms of chemical elements to form a certain number chemical bonds with atoms of other elements. In the light of the structure of the atom, V. is the ability of atoms... ... Genetics. Encyclopedic Dictionary

- (from Latin valentia force) in physics, a number showing how many hydrogen atoms a given atom can combine with or replace them. In psychology, valence is a designation coming from England for motivating ability. Philosophical... ... Philosophical Encyclopedia

Atomicity Dictionary of Russian synonyms. valency noun, number of synonyms: 1 atomicity (1) ASIS Dictionary of Synonyms. V.N. Trishin... Dictionary of synonyms

VALENCE- (from Latin valentia - strong, durable, influential). The ability of a word to grammatically combine with other words in a sentence (for example, for verbs, valency determines the ability to combine with the subject, direct or indirect object) ... New dictionary of methodological terms and concepts (theory and practice of language teaching)

- (from Latin valentia force), the ability of an atom of a chemical element to attach or replace a certain number of other atoms or atomic groups to form a chemical bond... Modern encyclopedia

- (from Latin valentia force) the ability of an atom of a chemical element (or atomic group) to form a certain number of chemical bonds with other atoms (or atomic groups). Instead of valency, narrower concepts are often used, for example... ... Big Encyclopedic Dictionary

VALENCE, valency, plural. no, female (from Latin valens having value, meaning) (chemical). Same as atomicity. Dictionary Ushakova. D.N. Ushakov. 1935 1940 ... Ushakov's Explanatory Dictionary

VALENCE, and, female. (specialist.). The ability of an atom (or atomic group) to form chemical bonds with other atoms (or atomic groups). | adj. valence, oh, oh. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

- (from Latin valentia force), the ability of atoms of elements to form chemical bonds; quantitatively characterized by number. V. can be considered as the ability of an atom to give or attach a definition. number of el new ext. electron shells... ... Physical encyclopedia

Books

• Stable comparisons in the system of Russian phraseology, V. M. Ogoltsev, The monograph reveals the structural and semantic diversity of stable comparisons, the specificity of their meanings, the relationship with phraseological units, their intra-systemic ones are revealed... Category: Language textbooks Publisher:

When considering chemical elements, you will notice that the number of atoms of the same element varies in different substances. How to write the formula correctly and not make a mistake in the index of the chemical element? This is easy to do if you have an idea of ​​what valence is.

What is valence needed for?

The valence of chemical elements is the ability of the atoms of an element to form chemical bonds, that is, to attach other atoms to themselves. A quantitative measure of valence is the number of bonds that a given atom forms with other atoms or atomic groups.

Currently, valence is the number of covalent bonds (including those arising through the donor-acceptor mechanism) by which a given atom is connected to others. In this case, the polarity of the bonds is not taken into account, which means that the valence has no sign and cannot be equal to zero.

A covalent chemical bond is a bond achieved through the formation of shared (bonding) electron pairs. If there is one common pair of electrons between two atoms, then such a bond is called a single bond; if there are two, it is called a double bond; if there are three, it is called a triple bond.

How to find valency?

The first question that concerns 8th grade students who have begun to study chemistry is how to determine the valence of chemical elements? The valency of a chemical element can be viewed in a special table of valency of chemical elements

Rice. 1. Table of valency of chemical elements

The valency of hydrogen is taken as one, since a hydrogen atom can form one bond with other atoms. The valence of other elements is expressed by a number that shows how many hydrogen atoms an atom of a given element can attach to itself. For example, the valency of chlorine in a hydrogen chloride molecule is equal to one. Therefore, the formula for hydrogen chloride will look like this: HCl. Since both chlorine and hydrogen have a valence of one, no index is used. Both chlorine and hydrogen are monovalent, since one hydrogen atom corresponds to one chlorine atom.

Let's consider another example: the valence of carbon in methane is four, the valence of hydrogen is always one. Therefore, the index 4 should be placed next to hydrogen. Thus, the formula of methane looks like this: CH 4.

Many elements form compounds with oxygen. Oxygen is always divalent. Therefore, in the formula of water H 2 O, where monovalent hydrogen and divalent oxygen always occur, the index 2 is placed next to the hydrogen. This means that the water molecule consists of two hydrogen atoms and one oxygen atom.

Rice. 2. Graphic formula of water

Not all chemical elements have a constant valency; for some it may vary depending on the compounds where the element is used. Elements with constant valency include hydrogen and oxygen, elements with variable valence include, for example, iron, sulfur, carbon.

How to determine valency using the formula?

If you do not have a valence table in front of you, but have a formula for a chemical compound, then it is possible to determine the valence using the formula. Let’s take as an example the formula manganese oxide – Mn 2 O 7

Rice. 3. Manganese oxide

As you know, oxygen is divalent. To find out what valence manganese has, it is necessary to multiply the valency of oxygen by the number of gas atoms in this compound:

We divide the resulting number by the number of manganese atoms in the compound. It turns out:

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Looking at formulas various connections, it is easy to see that number of atoms of the same element in the molecules of different substances is not identical. For example, HCl, NH 4 Cl, H 2 S, H 3 PO 4, etc. The number of hydrogen atoms in these compounds varies from 1 to 4. This is characteristic not only of hydrogen.

How can you guess which index to put next to the designation of a chemical element? How are the formulas of a substance made? This is easy to do when you know the valency of the elements that make up the molecule of this substance.

– is the property of an atom of a given element to attach, hold, or replace in chemical reactions a certain number of atoms of another element. The unit of valency is the valence of a hydrogen atom. Therefore, sometimes the definition of valence is formulated as follows: valence – This is the property of an atom of a given element to attach or replace a certain number of hydrogen atoms.

If one hydrogen atom is attached to one atom of a given element, then the element is monovalent, if two – divalent and etc. Hydrogen compounds are not known for all elements, but almost all elements form compounds with oxygen O. Oxygen is considered to be constantly divalent.

Constant valence:

I – H, Na, Li, K, Rb, Cs
II – O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd
III – B, Al, Ga, In

But what to do if the element does not combine with hydrogen? Then the valency of the required element is determined by the valency of the known element. Most often it is found using the valence of oxygen, because in compounds its valency is always 2. For example, it is not difficult to find the valence of elements in the following compounds: Na 2 O (valency of Na – 1, O – 2), Al 2 O 3 (valence of Al – 3, O – 2).

The chemical formula of a given substance can only be compiled by knowing the valency of the elements. For example, it is easy to create formulas for compounds such as CaO, BaO, CO, because the number of atoms in the molecules is the same, since the valences of the elements are equal.

What if the valences are different? When do we act in such a case? It is necessary to remember the following rule: in the formula of any chemical compound, the product of the valence of one element by the number of its atoms in the molecule is equal to the product of the valence by the number of atoms of another element. For example, if it is known that the valence of Mn in a compound is 7, and O – 2, then the formula of the compound will look like this: Mn 2 O 7.

How did we get the formula?

Let's consider an algorithm for compiling formulas by valence for compounds consisting of two chemical elements.

There is a rule that the number of valencies of one chemical element is equal to the number of valencies of another. Let us consider the example of the formation of a molecule consisting of manganese and oxygen.
We will compose in accordance with the algorithm:

1. We write down the symbols of chemical elements next to each other:

2. We put the numbers of their valency over the chemical elements (the valence of a chemical element can be found in the table of the periodic system of Mendelev, for manganese – 7, at oxygen – 2.

3. Find the least common multiple ( smallest number, which is divisible by 7 and 2 without a remainder). This number is 14. We divide it by the valences of the elements 14: 7 = 2, 14: 2 = 7, 2 and 7 will be the indices for phosphorus and oxygen, respectively. We substitute indices.

Knowing the valence of one chemical element, following the rule: valence of one element × the number of its atoms in the molecule = valency of another element × the number of atoms of this (other) element, you can determine the valence of another.

Mn 2 O 7 (7 2 = 2 7).

The concept of valency was introduced into chemistry before the structure of the atom became known. It has now been established that this property of an element is related to the number of external electrons. For many elements, the maximum valence follows from the position of these elements in the periodic table.

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In the world around us, individual atoms of chemical elements “by themselves” exist very rarely; as a rule, atoms various elements combine with each other to form molecules.

If several identical atoms are combined together, a simple substance is obtained ( modern science knows about 500 simple substances), but much more often different atoms are combined together to form complex substances (see Atomic-molecular theory).

Examples of simple substances: O 2 (oxygen), O 3 (ozone).

Examples of complex substances: NaCl ( table salt), H 2 SO 4 (sulfuric acid), H 2 O (water).

The composition and structure of molecules is described using chemical formulas, which show which chemical elements are included in the substance, as well as how many atoms of a particular chemical element are included in the molecule of the substance. For example, a molecule of sulfuric acid (H 2 SO 4) contains hydrogen (2 atoms), sulfur (1 atom), oxygen (4 atoms).

Using a chemical formula, it is very easy to determine the molecular mass of a substance, which is equal to the sum of the atomic masses.

The molecular weight of sulfuric acid is: H 2 SO 4 = 1 2 + 32 + 16 4 = 98.

Another very important quantitative characteristic of atoms interacting with each other is valence.

Valency is determined by the number of bonds an atom forms with other atoms. To write the correct formula of a substance, you need to know the valence of the atoms included in this substance.

IN structural formulas chemical bonds between atoms are indicated by a line (see formulas for covalent bonds), and each chemical bond is formed by two electrons of neighboring atoms (each atom allocates one electron of its own for this purpose, located in the outermost orbital). Thus, the valency of an atom (the number of bonds an atom can form with neighboring atoms) is determined by the number of its unpaired valence electrons.

Some chemical elements always exhibit constant valence:

Other elements have variable valency.

The valence of an unknown atom of a substance can be determined by other atoms with known valence that are part of this substance.

For example, sulfur can have valences of 2, 4, 6.

Let's determine what valency sulfur has in the compounds: H 2 S, SO 2, SO 3?

It is known that the valency of hydrogen = 1, and the valency of oxygen = 2. To solve the problem, it is necessary to multiply the known valence of an atom by the number of these atoms included in the substance: H 2 = 2; O 2 = 4; O 3 = 6. Since in all formulas there is only one sulfur atom, the resulting numbers will indicate the valence of sulfur in these formulas.

Knowing the valences of all elements included in a substance, you can create the correct chemical formula of the substance. To do this, you must first find the least common multiple, and then, to determine the number of atoms of a particular element, divide the least common multiple by the valency of each atom included in the formula.

For example, phosphorus oxide contains phosphorus (valency 5) and oxygen (2). The least common multiple would be 5 2 = 10. 10/5 = 2; 10/2 = 5. We get the formula P 2 O 5.

Why can some atoms have only one valence, while others have several? For an answer to this question, see

Different chemical elements differ in their ability to form chemical bonds, that is, to combine with other atoms. Therefore, in complex substances they can only be present in certain proportions. Let's figure out how to determine valency using the periodic table.

What is valence?

There is such a definition of valence: this is the ability of an atom to form a certain number of chemical bonds. Unlike , this quantity is always only positive and is denoted by Roman numerals.

This characteristic for hydrogen is used as a unit, which is taken equal to I. This property shows how many monovalent atoms a given element can combine with. For oxygen, this value is always equal to II.

It is necessary to know this characteristic in order to record correctly chemical formulas substances and equations. Knowing this quantity will help establish the relationship between the number of atoms various types in a molecule.

This concept originated in chemistry in the 19th century. Frankland started a theory explaining the combination of atoms in various proportions, but his ideas about the “binding force” were not very widespread. The decisive role in the development of the theory belonged to Kekula. He called the property of forming a certain number of bonds basicity. Kekulé believed that this was a fundamental and unchanging property of every type of atom. Butlerov made important additions to the theory. With the development of this theory, it became possible to visually depict molecules. This was very helpful in studying the structure of various substances.

How can the periodic table help?

You can find valence by looking at the group number in the short-period version. For most elements for which this characteristic is constant (takes only one value), it coincides with the group number.

Such properties have main subgroups. Why? The group number corresponds to the number of electrons in the outer shell. These electrons are called valence electrons. They are responsible for the ability to connect with other atoms.

The group consists of elements with a similar electronic shell structure, and the nuclear charge increases from top to bottom. In short-term form, each group is divided into main and secondary subgroups. Representatives of the main subgroups are s and p elements, representatives of the side subgroups have electrons in d and f orbitals.

How to determine the valency of chemical elements if it changes? It can coincide with the group number or be equal to the group number minus eight, and also take other values.

Important! The higher and to the right the element, the less its ability to form relationships. The more it is shifted down and to the left, the larger it is.

The way valency changes in the periodic table for a particular type of atom depends on the structure of its electron shell. Sulfur, for example, can be di-, tetra- and hexavalent.

In the ground (unexcited) state of sulfur, two unpaired electrons are located at the 3p sublevel. In this state, it can combine with two hydrogen atoms and form hydrogen sulfide. If sulfur goes into a more excited state, then one electron will move to the free 3d sublevel, and there will be 4 unpaired electrons.

Sulfur will become tetravalent. If you give it even more energy, then another electron will move from the 3s sublevel to 3d. Sulfur will go into an even more excited state and become hexavalent.

Constant and variable

Sometimes the ability to form chemical bonds may change. It depends on which compound the element is included in. For example, sulfur in H2S is divalent, in SO2 it is tetravalent, and in SO3 it is hexavalent. The largest of these values ​​is called the highest, and the smallest is called the lowest. The highest and lowest valencies according to the periodic table can be established as follows: the highest coincides with the group number, and the lowest is equal to 8 minus the group number.

How to determine the valence of chemical elements and whether it changes? We need to establish whether we are dealing with a metal or a non-metal. If it is a metal, you need to establish whether it belongs to the main or secondary subgroup.

• Metals of the main subgroups have a constant ability to form chemical bonds.
• For metals of secondary subgroups - variable.
• For non-metals it is also variable. In most cases, it takes on two meanings - higher and lower, but sometimes it can be larger number options. Examples are sulfur, chlorine, bromine, iodine, chromium and others.

In compounds, the lowest valence is shown by the element that is higher and to the right in the periodic table, respectively, the highest is the one that is to the left and lower.

Often the ability to form chemical bonds takes on more than two meanings. Then you won’t be able to recognize them from the table, but you will need to learn them. Examples of such substances:

• carbon;
• sulfur;
• chlorine;
• bromine.

How to determine the valence of an element in the formula of a compound? If it is known for other components of the substance, this is not difficult. For example, you need to calculate this property for chlorine in NaCl. Sodium is an element of the main subgroup of the first group, so it is monovalent. Consequently, chlorine in this substance can also create only one bond and is also monovalent.

Important! However, it is not always possible to find out this property for all atoms in complex substance. Let's take HClO4 as an example. Knowing the properties of hydrogen, we can only establish that ClO4 is a monovalent residue.

How else can you find out this value?

The ability to form a certain number of connections does not always coincide with the group number, and in some cases it will simply have to be learned. Here the table of valency of chemical elements will come to the rescue, which shows the values ​​of this value. The 8th grade chemistry textbook provides values ​​for the ability to combine with other atoms of the most common types of atoms.

Application

It is worth saying that chemists currently hardly use the concept of valency according to the periodic table. Instead, the concept of oxidation degree is used for the ability of a substance to form a certain number of relationships, for substances with a structure - covalence, and for substances with an ionic structure - the charge of the ion.

However, the concept under consideration is used for methodological purposes. With its help it is easy to explain why atoms different types combine in the ratios that we observe, and why these ratios are different for different compounds.

At the moment, the approach according to which the combination of elements into new substances was always explained using valency according to the periodic table, regardless of the type of bond in the compound, is outdated. Now we know that for ionic, covalent, metal bonds There are different mechanisms for combining atoms into molecules.

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Let's sum it up

Using the periodic table, it is not possible to determine the ability to form chemical bonds for all elements. For those that exhibit one valency according to the periodic table, in most cases it is equal to the group number. If there are two options for this value, then it can be equal to the group number or eight minus the group number. There are also special tables from which you can find out this characteristic.